.USCS LI8RARY.R£STON
WATER RESOURCES OF THE SABANA SECA
TO VEGA BAJA AREA, PUERTO RICO
By Arturo Torres-Gonza'lez and Jose R. Di'az
.... , . ..,«inn mil mil »in inn ulu w) lm
3 Ifilfl DDDnflTb fl
U.S. Geological SurveyWater-Resources Investigations Report 82-4115
Prepared in cooperation with the Puerto Rico Department of Agriculture
San Juan, Puerto Rico 1984
UNITED STATES DEPARTMENT OF THE INTERIOR
WILLIAM P. CLARK, Secretary
GEOLOGICAL SURVEY
V
Dallas L. Peck, Director
For additional information write to:
Chief, Caribbean District, WRDU.S. Geological SurveyGPO Box 4424San Juan, Puerto Rico 00936(Telephone: (809) 753-4414)
Copies of this report can be purchased from:
Open-File Services Section Western Distribution Branch U.S. Geological Survey Box 25425, Federal Center Denver.Colorado 80225 (Telephone: (303) 234-5888)
CONTENTS
Page
Abstract............................................................. 11.0 Introduction.................................................... 2
1.1 Objective.................................................. 21.2 Description of the area.................................... 4
2.0 General features.....................\ .......................... 62.1 Geology.................................................... 62.2 Landforms ................................................. 82.3 Land use................................................... 102.4 Climate.................................................... 12
3.0 Surface water................................................... 143.1 Streamflow................................................. 143.2 Floods .................................................... 163.3 Low flow................................................... 183.4 Flow duration.............................................. 20
4.0 Ground water.................................................... 224.1 Occurrence................................................. 224.2 Aquifer characteristics.................................... 244.3 Ground-water flow.......................................... 264.4 Yields to wells............................................ 284.5 Water levels............................................... 30
5.0 Quality of water................................................ 325.1 Surface water.............................................. 32
5.1-1 Chemical, physical and bacteriologicalcharacteristics.............................. 32
5.1-2 Suspended sediment ................................. 345.1-3 Saltwater intrusion in streams and canals........... 36
5.2 Ground water............................................... 405.2-1 Chemical and physical characteristics............... 405.2-2 Seawater intrusion in coastal aquifers.............. 425.2-3 Classification for irrigation....................... 48
6.0 Water availability and management............................... 507.0 List of References .............................................. 51Appendix ........................................................... 53
ILLUSTRATIONS
Figure 1.1-1 Map showing the location of study area and other areasunder investigation designated for rice cultivation......-................................. 3
1.2-1 Map showing description of the area...................... 52.1-1 Sketch showing the general geology of Sabana Seca to
Vega Baj a area.................................... 72.1-2 Sketch showing typical geologic section A-A 1 in the
Sabana Seca to Vega Baja area..................... 72.2-1 Map showing landforms of the Sabana Seca to Vega Baja
area.............................................. 92.3-1 Map showing land use in the Sabana Seca to Vega Baja
area, 1978........................................ 11
III
ILLUSTRATIONS (Continued)Page
2.4-1 Map showing mean annual precipitation in and nearthe study area.................................... 13
2.4-2 Diagrams showing monthly rainfall in and near thestudy area ....................................... 13
3.1-1 Hydrograph showing typical mean daily discharge atRio Cibuco at Vega Baja........................... 15
3.1-2 Hydrograph showing typical mean daily discharge atRio de la Plata at Toa Alta....................... 15
3.2-1 Map showing maximum known flooded areas in the RioCibuco-Rio de la Plata area....................... 17
3.3-1 Diagram showing 7-Day low-flow frequency at Rio Cibucoat Vega Baja...................................... 19
3.4-1 Diagram showing flow-duration curves for Rio Cibucoat Vega Baja and Rio de la Plata at Toa Alta...... 21
4.1-1 Sketch showing water movement in alluvial aquifers....... 234.1-2 Sketch showing water movement in cavernous aquifers...... 234.2-1 Map showing location of Cibuco aquifer test and
specific capacity values for selected wells...... 254.2-2 Sketch showing vertical section of observation wells
and pumping well of the Cibuco aquifer test....... 254.2-3 Diagrams showing drawdown curves at Ceiba pumping well
and observation wells 1, 1A, 2, and 3............. 254.3-1 Map showing computed ground-water flow through sections
of the Sabana Seca to Vega Baja area............. 274.3-2 Sketch showing generalized ground-water flow in water-
table aquifers.................................... 274.4-1 Map showing location of selected wells in the Sabana
Seca to Vega Baja area............................ 294.5-1 Map showing generalized water-table configuration
in the Sabana Seca to Vega Baja area.............. 314.5-2 Diagram showing hydrographs of observation wells at
Sabana Hoyos and Campanilla, and precipitationat Dorado (Dorado rainfall station)............... 31
5.1-1 Sketch showing location of two quality-of-waterstations at Rio Cibuco and Rio de la Plata........ 33
5.1-2-1 Diagram showing the graphical correlation between instantaneous water and suspended-sediment discharges at Rio de la Plata, Toa Alta, 1974-81........................................... 35
5.1-3-1 Map showing saltwater intrusion in streams andcanals in the Sabana Seca to Vega Baja area....... 37
5.1-3-2 Map showing location of salt-water wedges in Rio Cibuco and Rio de la Plata, and sketches showing cross sections of streams with specific conductance data.................... 39
5.2-1 Map showing location of selected wells in theSabana Seca to Vega Baja area..................... 41
5.2-1-1 Diagram showing typical analyses of water fromblanket dune deposits near coast.................. 41
5.2-1-2 Diagram showing typical analyses of water from theAymamon Limestone................................. 41
IV
ILLUSTRATIONS (Continued)Page
5.2-1-3 Diagram showing typical analyses of water fromalluvium.......................................... 41
5.2-2-1 Sketch showing boundaries between fresh and saltyground water...................................... 43
5.2-2-2 Sketch showing circulation of saltwater from the sea to the zone of diffusion and back to the sea............ v............................... 43
5.2-2-3 Sketch showing two aspects of saltwater intrusion..... 435.2-2-4 Locations in the Sabana Seca to Vega Baja area where
the depths to the freshwater-saltwater zone has been determined............................... 45
5.2-2-5 Area map showing suspected depth of the freshwater- saltwater zone in the Rio Cibuco valley, based on surface resistivity profiles and test-hole data.................................... 47
5.2-2-6 Diagram showing depth to the freshwater-saltwaterzone at aquifer test site......................... 47
5.2-2-7 Section G-G' showing possible saltwater intrusion,in the Campanilla area ........................... 47
5.2-3-1 Diagram for the classification of irrigationwaters............................................ 49
5.2-3-2 Map showing classification of water for irrigationin the Sabana Seca to Vega Baja area.............. 49
TABLES
Table 3.1-1 Monthly mean discharge of Rio Cibuco at Vega Baja....... 153.1-2 Monthly-mean discharge of Rio de la Plata at
Toa Alta......................................... 153.2-1 Log-Pearson Type III flood-frequency discharge at
Rio Cibuco at Vega Baja and Rio de la Plata at Toa Alta...................................... 17
3.2-2 Rio Cibuco at Vega Baja floods.......................... 173.2-3 Rio de la Plata at Toa Alta floods...................... 173.3-1 Minimum flows at Rio Cibuco of Vega Baja, 1973-78....... 193.3-2 Minimum flows at Rio de la Plata of Toa Alta,
1974-78 (regulated).............................. 193.4-1 Flow duration at Rio Cibuco at Vega Baja (for period
of record and by individual months, 1973-78)..... 213.4-2 Flow duration at Rio de la Plata at Toa Alta after
regulation began, 1974-78 (for indicated period and by individual months)................. 21
4.3-1 Aquifer characteristic's and computed ground-waterflows through sectors between Sabana Seca toVega Baja........................................ 27
4.4-1 Depth, diameter, yield, and general water- quality characteristics of wells in the Sabana Seca to Vega Baja area........................... 29
5.1-1-1 Chemical, physical, and bacteriological characteristicsof waters at Rio Cibuco at Vega Baja............. 33
5.1-1-2 Chemical, physical, and bacteriological characteristicsof waters at Rio De La Plata at Toa Alta......... 33
FACTORS FOR CONVERTING INCH-POUND UNITS TO INTERNATIONAL SYSTEM OF UNITS (SI)
For the convenience of readers who may want of Units (SI), the data may be converted by
Multiply inch-pound units
inches (in) feet (ft) 2 square feet (ft )
miles (mi) .2,square miles (mi )
acresacre-feet (acre-ft)
gallons per minute (gal/min) million gallons per day (Mgal/d)
3 cubic feet per second (ft /s)
pounds (lb)pounds per acre (Ib/acre)
tons, short (tons)tons per square mile (tons/mi )
25.40.30480.09290
1.6092.590
40471233
0.063090.04381
0.02832
453.60.1121
0.90720.03753
to use International System using the following factors:
To obtain SI units
millimeters (mm) meters (m) _ square meters (m )
kilometers (km) square kilometers (km )
2 square meters (m )cubic meters (m )
liters per second (L/s) cubic meters per day
(in /d) cubic meters per second
(mJ /s)
grams (g)grams per_square meter
(g/nT)Megagrams (mg) Megagrams per square
kilometer (tkm )
VI
WATER RESOURCES OF THE SABANA SECA TO VEGA BAJA AREA, PUERTO RICO
by
Arturo Torres-Gonz^lez and Jose' R. Diaz
ABSTRACT
An assessment of the water resources of the Sabana Seca to Vega Baja was made from 1978-80 in cooperation with the Puerto Rico Department of Agriculture. The area is under intense agricultural development with plans for eventual planting of nearly 10,000 acres in rice.
The geology of the area consists of a sequence of limestone formation over lain by alluvium and blanket-sand deposits. The surficial deposits and the Ay- mamon and Aguada Limestones form a water-table aquifer, which supplies most of the water in the area. The alluvial valleys of Rio de la Plata and Rio Cibuco, the two principal streams, comprise 50 percent of the area. Karst solution features are abundant to the south, with lagoons, marshes, and swamps adjacent to the coast.
Streamflow of Rio de la Plata and Rio Cibuco, varies seasonally with preci pitation. The climate of the area is marine-tropical, with an average annual rainfall of 65 inches. The Rio de la Plata, flow is regulated at de la Plata reservoir, upstream from Toa Alta. Although the average annual flow of Rio de la Plata at Toa Alta is about 279 cubic feet per second, regulation reduces flows to as low as 3.4 cubic feet per second. Flow at the site exceeds 12 cubic feet per second 90 percent of the time. Rio Cibuco flow at Vega Baja exceeds 18 cubic feet per second 90 percent of the time. The flow at Rio Cibuco is more sustained than at Rio de la Plata, with a 7-day, 10-year minimum flow of 7.5 cubic feet per second.
A water-table aquifer system, composed of Aymamon Limestone and underlying Aguada Limestone, and alluvial and sand deposits, is present throughout the study area. Depth to water below the land surface ranges from 16 feet near the coast to 120 feet near Highway 2. Ground-water flows in a general northerly direction to the ocean. Storage coefficients vary from 0.01 to 0.05 in the limestone, and 0.1 and higher in the alluvium. Transmissivities in the lime stone are as high as 100,000 feet squared per day, due to local cavernous condi tions. In the alluvium, transmissivities range from 100 to 10,000 feet squared per day.
Ground-water flows were estimated at a minimum of 1.6 million gallons per day per mile. Yields to wells range from 10 to 4,000 gallons per minute, gen erally exceeding 300 gallons per minute. Ground-water levels fluctuate season ally with precipitation and pumpage, but declined about 7 feet from 1973 to 80.
The surface waters from Rio Cibuco and Rio de la Plata are suitable for most uses including irrigation. Calcium and bicarbonate are the principal ions, with dissolved-solids concentration ranging from 135 to 325 milligrams per li ter. Suspended-sediment yields from Rio Cibuco and Rio de la Plata basins are less than 85 tons per square mile per year. Seawater is a major problem in canals and the lower reaches of the rivers. The salt-water wedge was detected 1.75 river miles upstream from the mouth of Rio Cibuco and 3.0 river miles up stream from the mouth of Rio de la Plata.
1
ABSTRACT (Continued)
The quality of ground water varies from excellent to very poor. Water from shallow wells tapping the limestone and alluvial aquifers is mostly of a cal cium-bicarbonate type, with dissolved-solids concentrations ranging from 250 to 350 milligrams per liter. The water is suitable for most uses. Seawater in trusion is a major problem toward the coast due to the cavernous nature of the limestone aquifer. In the vicinity of Highway 2, near the southern boundary of the study area, the depth to the fresh-salt water mixing zone is about 210 feet, Overpumping of wells in the Campanilla area has resulted in sea-water encroachment into the aquifer.
Although abundant water supplies are available in the study area, better management practices are essential to optimize their use and prevent further sea-water encroachment.
1.0 INTRODUCTION
1.1 Objective
SABANA SECA TO VEGA BAJA AREA WATER RESOURCES ASSESSMENT
A study of the water resourcesin the Sabana Seca to Vega Baja area was conducted
in support of the Puerto Rico Department of Agriculture rice program.
This report describes the general hydrology of the Sabana Seca to Vega Baja area, in north-central Puerto Rico (fig. 1.1-1). It summarizes the findings of a 2-year study of the water resources of the area conducted from 1978 to 1980, The study was conducted in cooperation with the Puerto Rico Department of Agri culture (PRDOA) as part of the cooperative water resources program between the U.S. Geological Survey, and agencies of the Commonwealth of Puerto Rico.
The Sabana Seca to Vega Baja area is one of several coastal valleys selec ted by the Commonwealth government for the production of rice on a commercial scale. Rice, one of the staple foods in Puerto Rico, is imported mostly from California and Louisiana. The Commonwealth's program envisions the substitution of the imported rice by locally produced crops. The climate of Puerto Rico will allow for higher yields than at most other rice-growing areas in the world (Vicente-Chandler and others, 1977). Vicente-Chandler estimated yields as high as 5,000 (lb/acre)/crop. From 2 to 2% crops can be obtained per year. This production however, will require large amounts of water. The PRDOA, in charge of the Commonwealth's rice program, v requested an assessment to determine the availability of water from the proposed rice production areas, including the Sabana Seca to Vega Baja valleys. Other areas under investigation are also shown in figure 1.1-1.
The main objectives of the study were to determine the quantity and qual ity of surface and ground-water supplies, their occurrence, and movement. The extent of saline-water intrusion was also investigated.
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1.0 INTRODUCTION (Continued)
1.2 Description of the Area
STUDY AREA LOCATED IN NORTH COAST LIMESTONE REGION
The area includes about 57 square miles between Sabana Seca and Vega Baja. Most of the land in the area is used for sugarcane and pastures.
2 The area includes about 57 mi along the coastal plain, between the towns
of Sabana Seca and Vega Baja, bordered by Highway 2 along most of its south edge and the Atlantic Ocean to the north (fig. 1.2-1). Two main rivers, Rio Cibuco and Rio de la Plata, flow into the area from the limestone hills to the south. Both rivers cross the area in a nearly south-to-north direction, dis charging into the Atlantic Ocean.
The principal urban areas are the towns of Toa Baja, Dorado, Vega Alta, and Vega Baja. Other communities in the area are Sabana Seca, Campanilla, Higuillar, Espinosa, and Monserrate. The population of the area is about 115,000 (1980 Census).
Most of the land, in rural areas, is used for sugarcane and pastures. The cultivation of rice will reduce significantly the sugarcane acreage. The Cons- tancia and San Vicente sugar mills, now closed, operated until recently. Light industry, such as the manufacture of pharmaceuticals, food products, clothing, and furniture are now the principal industrial and employment sources. Water demands for these activities are small when compared to prior sugarcane irriga tion and milling requirements.
Water in the area is provided from a combination of shallow wells (about 40) and streams. The most important well fields are near Campanilla and Vega Baja. Domestic supplies for Vega Baja are augmented by pumpage from Rio Indio, a tributary of Rio Cibuco.
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2.0 GENERAL FEATURES
2.1 Geology
LIMESTONE FORMATIONS AND ALLUVIUM
FORM GEOLOGIC FRAMEWORK
The geology of the area consists of a thick sequenceof predominately limestone formations: the Lares Limestone,
the Cibao Formation, the Aguada Limestone, and theAymamon Limestone; overlain in places
by alluvium and other surficial deposits.
The surface and pertinent subsurface geology in the area of investigation consists of predominently limestone formations of Oligocene to Miocene age. The formations dip gently to the north, and are overlain by unconsolidated to semi- consolidated Quaternary deposits on the coastal plain and in stream valleys (figs. 2.1-1, 2.1-2). The following description of the geologic formations from oldest to youngest was adapted from Briggs and Akers (1965). Thickness of the formations was determined from Monroe (1980).
The Lares Limestone is composed of thick-bedded to massive dense limestone and calcarenite. Its thickness ranges from near 0 at Toa Alta to about 500 ft in the western part of the study area. The Cibao Formation is composed of inter- bedded marl, chalk, and limestone, some thin sand and clay beds, and occasional conglomerate lenses. It is 500-600 ft thick. The Aguada Limestone is composed mostly of hard thick-bedded to massive calcarenite and dense limestone interbed- ded with chalky limestone and marl. It commonly contains some quartz grains. It is about 300 ft thick. The Aymamon Limestone is composed of thick-bedded and massive dense limestone, calcarenite, and some dolomite beds. It is 600-700 ft thick.
Quaternary deposits are subdivided into three types. Blanket deposits of quartz sand, clayey sand, and sandy clay occur principally in the coastal plan and in areas of karst topography. Beach and dune deposits are largely sand, composed of calcite, quartz, volcanic rock fragments, and some magnetite, and locally include cemented sand (beachrock) in bands parallel to the shore. Allu vial deposits are composed of sand, silt, clay, gravel, and flood-plain and ter race deposits.
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EXPLANATION
BEACH. RIVER TERRRACE AND ANCIENT DELTAIC DEPOSITS
SWAMP DEPOSITS
ALLUVIUM
BLANKET SANDS
CAMUY LIMESTONE
AYMAMON LIMESTONE
AGUADA LIMESTONE
CIBAO FORMATION (Subsurface only)
> QUATERNARY
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300 i
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DISTANCE, IN THOUSAND FEET FROM SHORELINE
Figure 2.1-2. Typical geologic section through A-A (See fig. 2.1-1 for section location.)
2.0 GENERAL FEATURES 2.1 Geology
2.0 GENERAL FEATURES (Continued)
2.2 Landforms
AREA IS MARGINAL TO THE KARST BELT GEOGRAPHIC REGION
The alluvial valleys of Rio Cibuco and Rio De La Plata comprise 50 percent of the area. Karst solution
features are abundant; lagoons, mangrove swamps and marshes are common along the coastline.
Puerto Rico can be divided into three main geographic regions (Monroe, 1976 p. 6); a mountainous area that constitutes most of the southern two thirds of the island, a belt of rugged karst topography in the north-central and northwestern parts of the island, and a discontinued fringe of relatively flat coastal plains (Fig. 2.2-1).
The karst region in north-central and northwestern Puerto Rico is an area underlain by limestone, in which the topography is formed chiefly by solution. The topography varies from extremely rugged karst terrain with a relief of as much as 1,000 ft to gently rolling hills.
The coastal plain slopes gently from the shore to an altitude of about 50 ft at the foothills. Depressions are common; several lagoons, marshes, and man grove areas have developed. A relatively continuous line of marshes, mangrove swamps, and small lagoons lie just inland from the shoreline.
The Sabana Seca-Vega Baja area lies in the lowlands of the north-coastal plain. The plain is about 3 mi wide in the study area. It has been built up by surficial deposits consisting of sand, silt, clay, and muck overlying a dissect ed older surface. Isolated limestone hills called "mogotes" project out of the plain. Round limestone hills of the Aguada and Aymamon Limestones, bordered by patches of blanket deposits, are conspicuous in the southern part of the area. The alluvial valleys of Rio Cibuco and Rio de la Plata on the west and east boundaries of the area, respectively," are incised through the foothills and ex tend to the coast. The valleys comprise about 50 percent of the study area.
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2.0 GENERAL FEATURES
2.2 Landforms
2.0 GENERAL FEATURES
2.3 Land Use
AGRICULTURAL LANDS COVER MOST OF THE
SABANA SECA TO VEGA BAJA AREA
Nearly half of the study area, 16,000 acres, is used for agriculture. The remaining land, 22,000 acres,
is used for urban and rural housing, hotels, and recreational facilities.
Most of the coastal plain and alluvial valleys have been utilized by agri culture for many years. Sugarcane, pastures, and ornamental plant nurseries predominate. Urban and industrial development are centered in the towns of Toa Baja, Dorado and Vega Alta. Numerous villages are scattered among the agricul tural lands. The northern section includes two large hotels, three golf courses, recreational beach areas, and several housing projects.
Land use in the study area as of 1978 is shown in figure 2.3-1. Agricul tural areas included 15,000 acres. The remaining 22,000 acres included commun ities, industry, scattered housing, hotels and coastal recreational areas. About 10,000 acres have been proposed by PRDOA for the rice program. Most of this land was originally planted in sugarcane. Dairy farms in the central part of the area account for about 6,000 acres.
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2.0 GENERAL FEATURES
2.3 Land Use
2.0 GENERAL FEATURES (Continued)
2.4 Climate
AREA CHARACTERIZED BY A TROPICAL MARINE CLIMATE
Temperature ranges from 23 to 27"Centigrade. Precipitation ranges from 60 to 70 inches per year.
The climate of the Sabana Seca to Vega Baja area is tropical marine with mean daily temperatures ranging from 23° during the winter to 27°C during the summer. Temperature is nearly uniform and very similar to the San Juan Metropo litan area.
Precipitation over the area ranges from 60 to 70 in/year (Calvesbert, 1970) (fig. 2.4-1). The persistent easterly and southeasterly tradewinds contribute to produce precipitation throughout the year, although seasonal variations occur, Precipitation records at 3 sites in or near the study area (fig. 2.4-2) show monthly fluctuations and seasonal patterns. The most intense rain storms occur early in the spring (April), and during the rainy season from August to Decem ber. Normally, about two thirds of the annual precipitation occurs during the rainy season. The driest period is from January to March.
Average actual evapotranspiration in the study area is about 45 inches/year. This value was estimated from data collected along the north coast by Giusti (1978). Giusti concluded that the ratio of pan evaporation (EP) to actual eva- potranspiration (ET), is about 0.76. Wind effects in the Sabana Seca to Vega Baja area may contribute to higher than average actual evapotranspiration val ues. The need to conduct studies to measure ET is evident. This data will be essential to planners and developers involved in the irrigation projects now under contruction for the rice program.
12
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A
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ER
TO
N
UE
VO
PU
NT
A
CO
RO
S
CA
BO
C
AR
IBE
DO
RA
DO
A
IRF
IEL
D
HIG
UIL
LA
AL
MIR
AN
TE
N
OR
TE
2 M
ILE
S
3 K
ILO
ME
TE
RS
Bat*
lro« U
8Q
8 «o
po
m
«p
i »c«l«
1
:30
.00
0 B
.y.m
oi
EX
PL
AN
AT
ION
LIN
E
OF
E
QU
AL
ME
AN
-AN
NU
AL
_
_8
0 -
R
AIN
FA
LL
(CO
NT
OU
R
INT
ER
VA
L
5.1
0
AN
D 5
0
INC
HE
S)
34
09
P
RE
CIP
ITA
TIO
N
ST
AT
ION
A
ND
N
UM
BE
R
(NO
AA
)
Fig
ure
2.4
-1. M
ean
-an
nu
al
pre
cip
itatio
n 1
931-6
0
in a
nd
n
ea
r the
stu
dy
a
rea
. (D
ata
fro
m
NO
AA
)it
D
'}
0RA
f
D0
34OS
1972
C »1C rf-
>J 1973
C {'r
i-
'J 1974
DJ ilk I
-
1975
D m-]
TJ
1976
D tr-
J 1977
TO
A B>
fl-T
\v IA\^
12
1972
D
1if 1 r-
-,iJ
1973
D
-1} r -i- -i-i
J 1
97
4
C Hk
r- -
r-1
-
>J
1975
C1 J
r-iI19
76
D j|J NT-
-
1977
'
16
14
COm
i .
112
oZ1
0z_r 8_i<
6
u.Z<
4DC
20
CA
ND
EL
AR
IA 1
536
~_---~ --
"~
[1969
D
IT
-
-
J 1970
°
-
J 1971
.-
_C
_
_I'J
1972
D ^I
_
____-
n
r
fJ
1976
D Jltt
-]
J 1977
°
Fig
ure
2.4
-2. M
on
thly
ra
infa
ll in and
ne
ar th
e stu
dy a
rea.(D
ata
from
NO
AA
).
2.0
G
EN
ER
AL
F
EA
TU
RE
S
2.4
C
limate
3.0 SURFACE WATER
3.1 Streamflow
STREAMFLOW VARIES SEASONALLY
Streamflow at Rio Cibuco and Rio de la Plata fluctuate seasonally with precipitation. Base flows at Rio de la Plata are
affected by regulation.
The two principal streams in the study area, Rio Cibuco and Rio de la Plata, flow abundantly in response to seasonal precipitation. Most of the high er flows occur during the rainy season (September to December), but equally high discharges can occur in almost any month (figs. 3.1-1 and 3.1-2).
Rio de la Plata is affected by significant regulation. Flow from about 8.2 mi of drainage in the basin is diverted to the south coast at Lago Carite. In 1974, a water-supply reservoir was built upstream from Toa Alta. About 40 Mgal/d (62 ft /s) are diverted from the reservoir to the San Juan metropoli tan area. This has resulted in lower^base flows at the Toa Alta gaging station, In the Rio Cibuco basin, about 24 mi of drainage are partly undefined contri buting area in the limestone zone. Low flows in the Rio Cibuco are probably a- ffected by the undefined area. Runoff from this area is probably minimal during periods of scarce rainfall.
The average-annual discharge of Rio Cibuco at the Vega Baja gaging station (50039500) is about 85 ft /s (5 years of record). At Rio de la Plata at Toa Alta gaging station (50046000), with a drainage area twice as large as the Rio Cibuco at Vega Baja (200 versus 90 mi ), the average-annual discharge is 279 ft /s (17 years of record). Prior to the construction of the reservoir the annual average flow at this site was about 319 ft /s.
Monthly-mean discharges at Rio Cibuco (table 3.1-1) range from 12.8 ft /s (June 1977) to 382 ft /s (Oct 1975). At Rio de la Plata, the monthly means (table 3.2-2) ranges from 11.4 to 4,813 ft /s. The monthly means, together with flow-duration and low-flow data (sections 3.3 and 3.4), can be used to es timate maximum volume of water that can 6e withdrawn from the streams. A long- term record of monthly means can also provide an insight into the effects of withdrawals at a stream site. This is evident for Rio de la Plata (table 3.1-2) after 1974. Monthly means have declined dramatically after the reservoir for water supply was built.
14
1000 f=
RE
AM
FL
OW
VA
RIE
S S
EA
SO
NA
LL
Y
Cibuco and
Rio de
la Plata
fluctuate seasonally
ation. Base
flows at
Rio de
la Plata
are affected
by regulation.
streams in
the study
area, Rio
Cibuco and
Rio de
la response
to seasonal
precipitation. Most
of the
high-2
rainy season
(September to
December), but
equally in
almost any
month (figs.
3.1-1 and
3.1-2).
affected by
significant regulation.
Flow from
about ; basin
is diverted
to the
south coast
at Lago
Carite. iservoir was
built upstream
from Toa
Alta. About
40 iverted
from the
reservoir to
the San
Juan metropoli-
ed in
lower^base flows
at the
Toa Alta
gaging station,
about 24
mi of
drainage are
partly undefined
contri- ne
zone. Low
flows in
the Rio
Cibuco are
probably a-
rea. Runoff
from this
area is
probably minimal
during*"harge
of Rio
Cibuco at
the Vega
Baja gaging
station /s
(5 years
of record).
At Rio
de la
Plata at
Toa JOOO),
with a
drainage area
twice as
large as
the !00
versus 90
mi ),
the average-annual
discharge is
:ord). Prior
to the
construction of
the reservoir
the : site
was about
319 ft
/s.
3 s
at Rio
Cibuco (table
3.1-1) range
from 12.8
ft /s
(Oct 1975).
At Rio
de la
Plata, the
monthly means
11.4 to
4,813 ft
/s. The
monthly means,
together flow
data (sections
3.3 and
3.4), can
be used
to es
ter that
can be
withdrawn from
the streams.
A long-
ins can
also provide
an insight
into the
effects of
*. This
is evident
for Rio
de la
Plata (table
3.1-2) have
declined dramatically
after the
reservoir for
RIO CIBUCO AT VEGA BAJA
OCT
NOV
DEC
JAN
FEB
MAR
APRIL
MAY
JUNE
JULY
AUG
SEPT
Fig
ure
3
.1-1
. Ty
pic
al
mean
d
isc
ha
rge
h
yd
rog
rap
h
of
Rfo
C
ibu
co
at
Veg
a
Ba
ja,
1978
Tab
le 3
.1-1
. Mo
nth
ly
mean
d
isc
ha
rge
o
f R
fo
Cib
uc
o
at
Veg
a
Baja
(5
0039500)
DIS
CH
AR
GE
IN
C
UB
IC
FE
ET
P
ER
S
EC
ON
D
YE
AR
19
73
1974
19
75
1976
1977
1978
19
79
1980
19
81
1982
OC
T*4
5.9
03
82
.00
10
3.0
01
62
.00
77
.70
46
.20
16
8.0
01
15
.00
23
1.0
0
NO
V*4
0.0
03
39
.00
195.0
06
7.7
0223.0
04
6.4
0523.0
04
5.6
04
09
.00
DE
C*5
2.1
01
74
.00
38
1.0
04
8.5
06
5.0
03
0.5
02
25
.00
15
0.0
01
31
6.0
JA
N
59
.00
65
.50
12
9.0
09
2.5
07
8.8
03
8.2
0148.0
01
12
.00
92
.20
110.0
0
FE
B
12
4.0
07
6.7
07
6.3
01
20
.00
32
.60
43
.70
101.0
01
15
.00
73
.80
12
0.0
0
MA
R
49
.50
43
.40
61
.00
12
6.0
02
7.3
06
3.2
01
05
.00
92
.60
29
8.0
05
4.9
0
AP
R
36
9.0
04
2.2
03
4.1
01
32
.00
34
.60
17
5.0
01
52
.00
75
.70
22
5.0
06
6.2
0
MA
Y
95.0
029.3
031.3
077.6
024.7
068.4
0361.0
0104.0
05
66
.00
23
2.0
0
JUN
E
58
.00
18
.20
21
.10
38
.60
12
.80
34
.60
15
1.0
07
7.7
01
56
.00
72
.30
JU
LY
38.9
01
9.9
03
1.7
02
6.0
01
5.5
02
7.9
01
62
,00
42
.70
85
.30
64
.60
AU
G
46
.00
47
.60
35
.70
24
.90
15
4.0
02
1.2
04
61
.00
79
.00
12
1.0
05
5.0
0
SE
PT
48
.40
11
4.0
08
2.9
04
5.8
04
5.5
03
6.3
04
50
.00
10
9.0
08
3.9
08
5.1
0
*
IND
ICA
TE
S
A
NO
-VA
LU
E
MO
NT
H
14
AJA
DR
AIN
AG
E
AR
EA
-AB
OU
T
90
MI
DR
AIN
AG
E
AR
EA
-AB
OU
T 2
00
M
lR
IO
DE
L
A
PL
AT
A
AT
T
OA
A
LT
A
ES
TIM
AT
ED
R
EC
OR
D
ES
TIM
AT
ED
R
EC
OR
D
AP
RIL
M
AY
J
UN
E
JU
LY
A
UG
rog
rap
h
of
Rio
C
ibuco
at
Ve
ga
B
aja
, 1978
SE
PT
OCT
NOV
DEC
JAN
FEB
MAR
APRIL
MAY
JUNE
JULY
AUG
Fig
ure
3
.1-2
. Typ
ica
l m
ea
n-d
aily
dis
charg
e h
yd
rog
rap
h
of
Rfo
d
e
la
Pla
ta
at
To
a A
lta,
1978.
SE
PT
io
Cib
uco
at
Ve
ga
B
aja
(5
00
39
50
0)
Table
3.I-2
. Mo
nth
ly m
ean disch
arg
e o
f Rio de la
Pla
ta a
t Toa A
lta (5
0046000).
BIC
F
EE
T
PE
R
SE
CO
ND
IAR
9.5
03.4
01
.00
6.0
07
.30
3.2
05
.00
2.6
0?.ooi.9
0
AP
R
369.0
04
2.2
03
4.1
0132.0
03
4.6
01
75
.00
15
2.0
075.7
02
25
.00
66
.20
MA
Y
95
.00
29,3
031.3
07
7.6
'024.7
068.4
03
61
.00
10
4.0
0566.0
0232.0
0
JU
NE
58
.00
18
.20
21.1
03
3.6
01
2.8
034.6
01
51
.00
77
.70
15
6.0
072.3
0
JU
LY
38
.90
19
.90
31
.70
26
.00
15
.50
27
.90
162.0
04
2.7
08
5.3
06
4.6
0
AU
G
46.0
047.6
035.7
02
4.9
01
54
.00
21
.20
461.0
07
9.0
0121.0
055.0
0
SE
PT
48.4
01
14
.00
82.9
04
5.8
04
5.5
03
6.3
04
50
.00
10
9.0
08
3.9
085.1
0
DIS
CH
AR
GE
IN
C
UB
IC
FE
ET
P
ER
S
EC
ON
D
YE
AR
19
60
19611962
1963
1964
1965
19
66
19
67
1968
19
69
19
70
19
71
1972
1973
19
74
19
75
19
76
19
77
19
78
1979
19
80
19
81
1982
OC
T*555.0
05
46
.00
46Z
-00
300.0
01
51
.00
22
2.0
0555.0
0181.0
04
7.9
03
01
.00
48
13
.02
18
.00
28
3.0
035.1
08
56
.00
27
0.0
074.4
01
77
.00
499.0
0401.0
01
42
.00
267.0
0
NO
V
*5
08
.00
488.0
0158.0
0249.0
03
5.2
03
08
.00
29
5.0
02
20
.00
641.0
01
14
5.0
11
61
.0155.0
01
21
.00
74
.30
1144.0
483.0
04
7.4
01137.0
207.0
0592.0
03
1.0
02
57
.00
DE
C*1135.0
775.0
0154.0
01
25
.00
68
.40
11
84
.0382.0
05
7.0
0308.0
03
28
.00
1352.0
13
1.0
0271.0
02
9.5
04
21
.00
848.0
03
6.6
0283.0
054.1
03
78
.00
88
.10
1071.0
JAN
90.1
0362.0
0362.0
0197.0
01
12
.00
83
.40
25
4.0
01
66
.00
12
5.0
05
18
.00
26
6.0
0472.0
01
54
.00
92
.10
89
.50
24
7.0
01
23
.00
40.3
094.2
01
53
.00
13
7.0
06
8.5
01
37
.00
FE
B
10
9.0
01
61
.00
15
6.0
0220.0
078.9
04
1.0
01
33
.00
16
0.0
01
15
.00
34
3.0
09
8.1
0371.0
01
11
.00
151.0
01
17
.00
73
.20
13
0.0
01
6.9
095.2
01
48
.00
144.0
03
3.8
01
67
.00
MA
R
84
.40
14
6.0
01
12
.00
15
7.0
04
8.7
031.5
02
83
.00
11
3.0
065.2
04
68
.00
72
.80
18
7.0
02
82
.00
78.7
04
1.0
052.2
08
5.5
01
9.0
01
67
.00
10
1.0
01
34
.00
12
7.0
03
4.5
0
APR
243.0
0208.0
0270.0
0336.0
08
9.7
067.8
03
72
.00
79.6
061.8
0462.0
066.1
05
47
.00
164.0
0349.0
03
4.2
027.8
01
11
.00
15
.40
371.0
08
8.4
0109.0
0291. CO
27.0
0
MA
Y
177.0
0149.0
05
62
.00
54
8.0
060.1
0899.0
0390.0
08
3,5
0151.0
0881.0
05
60
.00
256.0
08
0.8
07
1.3
03
9.1
03
8.6
08
7.0
012.7
01
14
.00
55
2.0
0156.0
08
38
.00
379.0
0
JUN
E
11
4.0
0113.0
04
08
.00
22
4.0
01
01
.00
46
1.0
0176, CO
50
.50
119. CO
18
9.0
0847.0
0156.0
040.9
07
6.0
033.7
01
6.2
03
2.6
01
1.4
04
1.7
0620. CO
77
.20
460. CO
35
.50
JUL
Y
162.0
069C
.OO
25
0.0
0211.0
0132.0
0258.0
0136.0
061.6
0146.0
0278.0
0446.0
014
6.0
04
9.5
03
9.3
035.4
03
2.8
01
3,9
021.7
04
3.7
0592.0
060.3
020
1.0
073.1
0
AU
G
401.0
08
99
.00
41
4.0
03
61
.00
26
2.0
06
73
.00
27
0.0
01
05
.00
34
1.0
03
00
.00
71
6.0
02
25
.00
66
.80
12
9.0
01
38
.00
51.3
016.5
03
0.9
06
9.9
01
67
7.0
49.0
0164.0
054.5
0
SE
PT
1691.0
269.0
0501.0
0383.0
01
50
.00
34
3.0
0494.0
064.0
0134.0
03
99
.00
36
5.0
01
15
.00
91.1
07
8.2
0237.0
0*7
6.6
056.1
05
4.4
01574.0
12
1.0
01
24
.00
33
0.0
0
*
IND
ICA
TE
S
A
NO
-VA
LU
E
MO
NT
H3
.0
SU
RF
AC
E
WA
TE
R
3.1
S
trea
mflo
w
3.0 SURFACE WATER (Continued)
3.2 Floods
FLOODS ARE FREQUENT AND SEVERE
Rio Cibuco and Rio de la Plata flood significant parts of the study area. At Vega Baja, the maximum known flood of Rio Cibuco occurred
in 1965, with a peak discharge of 28,000 cubic feet per second. At Toa Alta, Rio de la Plata had a peak discharge 120,000 cubic feet
per second in 1928.
Effective planning for the development and management of flood plains re quires studies and analyses of their flooding potential and characteristics. The two principal streams in the study area, Rio Cibuco and Rio de la Plata, produce severe and frequent floods that inundate large tracts of land.
The floods of Rio Cibuco in the Vega Alta and Vega Baja areas were de scribed by Hickenlooper (1968). The most severe flood of record occurred on December 11, 1965 when a peak discharge of 28,000 ft /s was recorded at the Highway 2 bridge (tables 3.2-1 and 3.2-2). This discharge corresponds to a re currence interval of 25 years (4 percent chance of occurring in any given year). The area inundated was about 10 mi (fig. 3.2-1). However, a levee was built in 1966 in the vicinity of Vega Baja, with the intention of reducing the area inundated by future floods.
The largest known flood of Rio de la Plata in the study area occurred in 1928. A peak discharge of 120,000 ft /s at Hwy 165 near the Toa Alta bridge was estimated from historical data (Lopez, 1964). The flood of 1899 was proba bly higher, but the data are insufficient to estimate the discharge. The flood of 1960 was the second largest known, with a peak discharge of 95,500 ft /s (table 3.2-3) and a recurrence interval of about 32 years (a 3 percent chance of occurrence in any given year). An area of about 18 mi was flooded in 1960, including the towns of Dorado, Toa Baja, and agricultural areas in the vicinity.
The flooding potential of Rio Cibuco and Rio de la Plata is an important factor in the selection of areas for agricultural development. Most of the flood plain north and east of Vega Baja, planned for intense rice cultivation, would be subject to flooding. Depth of flood waters in this area could be as much as 6 to 8 ft. Areas considered for rice cultivation south of Dorado would also be subject to similar flooding.
16
3.0 SURFACE WATER (Continued)
3.3 Low Flow
MINIMUM FLOWS OF RIO CIBUCO AT VEGA BAJA AND RIO DE LA
PLATA AT TOA ALTA OCCUR FROM JUNE THROUGH AUGUST
Monthly minimum 7-day consecutive flows of Rio de la Plata are as low as6.5 cubic feet per second and of Rio Cibuco are as low as
7.4 cubic feet per second.
The minimum flow at a selected site of a stream is an important factor for water resources planning and management. The 7-day, 10-year minimum flow is used as an index to determine the capacity of streams to assimilate wastes. Minimum flows also are essential for the proper design of water supply facili ties. The frequency of occurrence of selected minimum flows, such as the 1,3,7, 14, and 30-consecutive-day minimum flows are often criteria for design of pro jects. In agriculture, the design of irrigation facilities must include minimum flow data.
The 7-day, 10-year minimum flow at a specific stream site in Puerto Rico can be obtained from a frequency analysis of 7-day consecutive minimum flows (Cobb, 1978). The technique normally applies to streamsites with a minimum of 10 years of record and with no significant regulation.
The minimum-flow data for Rio Cibuco at Vega Baja include only six years of record (1973-78). A preliminary analysis of the data indicates that the 7- day, 10-year minimum flow is about 7.5 ft /s (fig. 3.3-1). At Rio de la Plata at Toa Alta, the 7-day, 10-year minimum was not determined due to the signifi cant regulation from the La Plata reservoir.
A further insight into the low-flow regimes at the two sites in the study area can be obtained from the monthly variations in the 7-day consecutive mini mum flows. Lowest mean daily flows for 1, 3, 7, 14, and 30-consecutive days of Rio Cibuco and Rio de la Plata are shown in tables 3.3-1 and 3.3-2. At Rio Cibuco at Vega Baja the lowest flow during 1, 3, and 7-consecutive days oc curred during July. The 14 and 30-days minimum flows were recorded in June. The 1 and 3-consecutive-day minimum flows of Rio de la Plata at Toa Alta occur red in August whereas the 7, 14, and 30-consecutive day minimum flows occur red in June (table 3.3-2).
The data from both tables show that low flows are more sustained in the Rio Cibuco than in the Rio de la Plata. ^Further reductions in the low flows of Rio de la Plata may occur upon completion of the ongoing enlargement of the Rio de la Plata reservoir and additional withdrawals for the San Juan metropolitan area. However, low flows possibly could be augmented by increasing releases from the reservoir.
18
18°3
0'
27'3
0'
25
' _
18°2
4'
PU
NT
A P
UE
RT
O
NU
EV
O
OC
EA
NO
A
TL
AN
TIC
O
PU
NT
A
CO
RO
SO
DO
RA
DO
A
IRF
IEL
D
FL
OO
D
OF
D
EC
EM
BE
R
11
, 1965
(HA
-28
9,
1968)
FL
OO
D
OF
S
EP
TE
MB
ER
6
, 1960
(HA
-128,
19
74
)
5S
CA
ND
EL
AR
IAA
LM
IRA
NT
E
NO
RT
EV
EG
A
AL
TA
Ba
se
from
U
SG
S to
po
ma
ps
sca
le
1:3
0,0
00
Ba
yam
o'n
, V
eg
a
AJta
, a
nd
M
anati
quads.
Fig
ure
3
.2-1
. Maxim
um
kn
ow
n
floo
ds
in
the
Rio
C
ibuco
and
Rio
de
la
Pla
ta
are
as.
(Re
fer
to ta
ble
s:
3.2
-1,
3.2
-2,
an
d 3.3
-3)
D .5
Ir
iD
1
1 ii2
2II 3
I 2 M
ILE
S
3 K
ILO
ME
TE
RS
" "able 3.2-». tog Pearsoo Type HI flood t equency discharge at Ri'o Cibuco
af Vega Baia and *f\> de la Plata at Tea Alta.
*RECURRENCE INTERVAL
(Rl), IN YEARS
2
10
25
50
100
PEAK DISCHARGE, IN CUBIC FEET PER SECOND
RIO CIBUCO AT VEGA BAJA (50039500)
6600
19,200
28,300
36,500
45,800
RIO DE LA PLATA AT TOA ALTA (50046000)
13.500
58,300
101,000
145,000
202,000
* TO OBTAIN THE PERCENTAGE CHANGE OF OCCURRENCE IN ANY GIVEN YEAR. OBTAIN THE INVERSE OF THE Rl AND MULTIPLY BY 100.
Table 3.2-2. Rio Cibuco floods
DATE OF FLOOD
ELEVATION ABOVE MEAN SEA LEVEL AT THE DOWN STREAM SIDE OF HIGHWAY 2 , INMETERS
DISCHARGE, IN CUBIC FEET
PER SECOND
APR. 8, 1915
MAY 4,
1959
7.40
6800
SEPT. 6,
1960
7.26
4600
DEC. 6, 1961
7.52
9100
OCT. 18, 1962
MAY 23, 1963
7.34
6000
APR 1964
<6.50
<2600
DEC.11,
1965
8.00
28,000
APR. 20,
1966
7.83
19,000
Table 3.2-3. Rio de la Plata floods
FLOOD ELEVATION REACHED AT HIGHWAY 165 AT TOA ALTA. OVERFLOW LIMITS FOR ONLY THE 1899 AND 1960 FLOODS ARE SHOWN ON THE MAP.
DATE OF FLOOD
ELEVATION ABOVEMEAN SEA LEVEL,rN METERS
DISCHARGE, IN CUBICFtET PER SECOND
AUG8.1899
13.6
AUG22.1916
12.8
70,000
SEPT13,1928
14.0
120,000
JUNE16,1943
13.1
82,000
OCT14,1943
12.1
52,000
SEPT6,1960
13.5
95,000
AUG27,1961
12.7
68.000
3.0 SURFACE WATER 3.2 Floods
3.0 SURFACE WATER (Continued)
3.3 Low Flow
MINIMUM FLOWS OF RIO CIBUCO AT VEGA BAJA AND RIO DE LA
PLATA AT TOA ALTA OCCUR FROM JUNE THROUGH AUGUST
Monthly minimum 7-day consecutive flbws of Rio de la Plata are as low as6.5 cubic feet per second and of Rio Cibuco are as low as
7.4 cubic feet per second.
The minimum flow at a selected site of a stream is an important factor for water resources planning and management. The 7-day, 10-year minimum flow is used as an index to determine the capacity of streams to assimilate wastes. Minimum flows also are essential for the proper design of water supply facili ties. The frequency of occurrence of selected minimum flows, such as the 1,3,7, 14, and 30-consecutive-day minimum flows are often criteria for design of pro jects. In agriculture, the design of irrigation facilities must include minimum flow data.
The 7-day, 10-year minimum flow at a specific stream site in Puerto Rico can be obtained from a frequency analysis of 7-day consecutive minimum flows (Cobb, 1978). The technique normally applies to streamsites with a minimum of 10 years of record and with no significant regulation.
The minimum-flow data for Rio Cibuco at Vega Baja include only six years of record (1973-78). A preliminary analysis of the data indicates that the 7- day, 10-year minimum flow is about 7.5 ft /s (fig. 3.3-1). At Rio de la Plata at Toa Alta, the 7-day, 10-year minimum was not determined due to the signifi cant regulation from the La Plata reservoir.
A further insight into the low-flow regimes at the two sites in the study area can be obtained from the monthly variations in the 7-day consecutive mini mum flows. Lowest mean daily flows for 1, 3, 7, 14, and 30-consecutive days of Rio Cibuco and Rio de la Plata are shown in tables 3.3-1 and 3.3-2. At Rio Cibuco at Vega Baja the lowest flow during 1, 3, and 7-consecutive days oc curred during July. The 14 and 30-days minimum flows were recorded in June. The 1 and 3-consecutive-day minimum flows of Rio de la Plata at Toa Alta occur red in August whereas the 7, 14, and 30-consecutive day minimum flows occur red in June (table 3.3-2).
The data from both tables show that low flows are more sustained in the Rio Cibuco than in the Rio de la Plata. v Further reductions in the low flows of Rio de la Plata may occur upon completion of the ongoing enlargement of the Rio de la Plata reservoir and additional withdrawals for the San Juan metropolitan area. However, low flows possibly could be augmented by increasing releases from the reservoir.
18
50
g 40
8 30UJ COa: 20UJa.K LUff 10
oCQ
3 52
UJ O CC
I O CO
i i r i i i
I I I II I 1 I
1.01 2 5 10
RECURRENCE INTERVAL , IN YEARS
Figure 3.3-1. 7-Day low-flow frecuency of Rfo Cibuco at Vega Baja.
50 100
Table 3.3-1. Minimum flows of Rfo Cibuco at Vega Baja.
Table 3.3-2. Minimum flows of Rfode la Plata at Toa Alta.
LOWEST MEAN DAILY DISCHARGE FOR THE INDICATED NUMBER OF CONSECUTIVE DAYS. IN FT 3/s
MONTH
JAN
FEB
MAR
APR
MAY
JUNE
JULY
AUG
SEPT
OCT
NOV
DEC
DAYS
1
27
23
13
18
11
93
7.4
9.2
8.8
22
29
28
3
28
24
13
19
11
9.8
7.6
9.5
11
24
29
32
7
28
26
13
22
13
10
8.5
10
15
29
30
33
14
29
29
23
23
15
1 1
13
12
21
34
33
35
30
30
27
34
25
13
15
21
36
46
40
48
LOWEST MEAN DAILY DISCHARGE FOR THE INDICATED NUMBER OF CONSECUTIVE DAYS. IN FT 3 /s
MONTH
JAN
FEB
MAR
APR
MAY
JUNE
JULY
AUG
SEPT
OCT
NOV
DEC
DAYS
1
11
12
8.4
9.2
8.0
60
5.2
3.4
83
15
13
14
3
11
12
9.3
9.5
8.1
60
6.0
3.8
8.9
15
13
14
7
11
14
13
10
8.3
6.5
7.0
8.4
13
17
15
14
14
14
15
15
1 1
9.3
7.6
92
15
23
31
21
16
30
41
19
15
13
11
14
16
54
64
47
36
3.0 SURFACE WATER 3.3 Low Flow
3.0 SURFACE WATER (Continued)
3.4 Flow Duration
FLOWS AT RIO CIBUCO AND RIO DE LA PLATA EXCEED 10 CUBIC
FEET PER SECOND ALMOST 95 PERCENT OF THE TIME
Flows of Rio Cibuco near Vega Baja are fairly well sustained, with a 90-percent duration of 18 cubic feet per second. Flow regulation and withdrawals have reduced low flows of Rio de la Plata at Toa Alta, by two thirds to a 90 percent duration of 12 cubic feet per second.
The streamflow at a given point in a basin is an integration of the effects of climate, topography, and geology. Streamflow provides a distribution of run off in time and magnitude. This distribution can be expressed by means of a flow duration curve. The duration curve is a cummulative frequency that indi cates the percent of the time that a particular value (in this case, streamflow) has been equaled or exceeded. The curve shows no chronological order and ap plies only to the period of record for which the data were collected. Flow-du ration data can be used for comparing flow characteristics of streams. The slope of the curve is a measure of the variability of flow. A steep slope indi cates highly variable flow, while a flat slope indicates more sustained flow possibly from ground and surface-water storage.
A flow-duration curve for a particular station usually is based on mean daily discharges for a period of record. This curve, referred as the "period of record curve", provides no insight into seasonal effects on streamflow nor other changes such as flow regulation. Partial-record curves can be derived to study regulation effects. In this type of analysis, the periods before and after regulation are studied independently. The seasonal variation of stream- flow can be studied from partial monthly curves, in which the data for indivi dual months for all the years of record are analyzed.
The flow duration data for Rio Cibuco at Vega Baja (fig. 3.4-1 and table 3.4-1) indicate fairly sustained but highly variable flows. A discharge of at least 10 ft~/s is exceeded 99 percent of the time. The 90 percent duration is about 18 ft Is. The monthly data show a significant variability in middle flows between the dry and wet seasons. The 50- and 20-percent duration lines (table 3.4-1) illustrate this variability. The importance of the monthly analyses is more significant if the 50 percent duration for the period of record (1973-78) is compared with that for the driest and wettest months. For the period of re cord the 50 percent duration is about 40 ft /s. During November (wet) it is about 110 ft /s, while during July (dry) it is about 25 ft /s.
The flows of Rio de la Plata at Toa Alta are affected by regulation at the Puerto Rico Aqueduct and Sewer Authority reservoir upstream from Toa Alta. Flow duration curves before and after regulation began in 1974 (fig. 3.4-1) show that low and middle flows at the gaging station decreased about two thirds. The after-regulation data of Rio de la Plata (table 3.4-2) show that for the period of record (1974-78), 10 ft /s is exceeded about 94_percent of the time. However, during the dry June-July months, only about 8 ft /s flowed by the gage 90 percent of the time. The variability in the middle flows is more severe than that of Rio Cibuco. The 50 percent duration of Rio de la Plata during July (dry) was about 23 ft /s. During November (wet) the same duration was about 460 ft /s.
20
Table 3.4-1. Flow-duration at Rio Cibuco at Vega Baja (for period of record and by individual months,1973-78).
FLOW, IN CUBIC FEET PER SECOND
7.4
10 21
30
42
50 60
85 100 140
200
290
410
580
820 1200
*PERCENT OF TIME 1973-78
100
99.3
85.6 70.9
51.4
43.3
35.1 22.6 18.6 12.4
7.9 4.5
2.3
1.3
0.6 0.2
PERCENTAGE OF TIME FLOW WAS EQUALED OR EXCEEDED FOR THE MONTH INDICATED
JAN
100
100 100
° 89 L
73
60
-fs-12120 10
3.0
1.6
0.5
0.0
.0
.0
FEB
100
100 100
92 I
67
55 f
21
17 8.32.5
0.6
0.5
0.5
0.0
.0
.0
MAR
100
100 96
87
57 f
44
3016 136.5
3.0
1.1
0.0
.0
.0
.0
APR
100
10098
69
48 I
44
37
27 2318
10
8.3
5.0
3.5
2.5 2,2
MAY
100
10077
62
53
42
3516 9.1
4.8
1.4 0.5
0.0
.0
.0
.0
JUNE
too96
58
45 20-^
13 6.71.0
0.6
0.0
.0
.0
.0
.0
.0
.0
JULY
100
93
62
37
6.2
3.5 2.7
0.5 0.5 0.0
.0
.0
.0
.0
.0
.0
AUG
100
97
6845 |31
23
18 [13 12
5.4
5.4
2.7
2.1
1.0
0.0 .0
SEPT
100
99
84 I
66
^127 *
2314
138.3
5.2
3,3 1.6
0.5
0.0 .0
OCT
100
100 100
86 |
55 50-
43 26
2319
131 1
7.7
5.0
4.2 1.9
NOV
100
100 100
98
79
70
65
58 54
42
2618
7.3
3.0
2.0 0.6
DEC
100
100
100
9977 °°
6755 -o45 ~°
4433
2018 12
6.4
5.2
1.0 0.0
NOTE: 90, 50 AND 20 PERCENTAGE DURATIONS INDICATED BY LINES ON MONTHLY DATA.
Table 3.4-2. Flow-duration at Rfo de la Plata at Toa Alta after regulation began, 1974-78 (for indicated period and by individual months).
FLOW, IN CUBIC FEET PER SECOND
3.4
4.5
5.9 7.8
10
13
18
23
31
41
53 70
93 120
160 280
480 840 1500
2200
*PERCENT OF TIME 1974-78
100
99.7
99.3 97.6
94.2 89.0
81.0
73.2
63.0 53.0
43.5 36.5
30.0 25.0
20.5 13.0
7.6 4.3 1.6
0.7
PERCENTAGE OF TIME FLOW WAS EQUALED OR EXCEEDED THE MONTH INDICATED F°H
JAN
100
100
100 100
100 95
93 f
78
74
70
60 [ou 47
33 20-20 I"
7.7
3.2 0.0
.0
.0
FEB
100
100
100 100
100 98
87
81 78 65
55 | 39
30
21 f 14 3.5
0.7
0.0
.0
.0
MAR
100
100
100 100
99 91 |
85
76 69 62 (
48
36 24 12 [
5.2 2.6
2.0 0.0
.0
.0
APR
100
100
100 100
94 | 85
77
66 53 r
44
37
34
29
27
19 7.3
4.6 3.0 1.2
0.0
MAY
100
100
100
100 f
87
77
66
56 50-«
46
38 21
14
10
7.7
2.5
0.6
0.0 .0
.0
JUNE
100
100
100 88 *
76 71
59
543818
8.7
4.0 2.0
0.0
.0
.0
.0
.0
.0
.0
JULY
100
100
99 -90-
79
72
58
52
2415
8.6 6.0 3.3
1.3
0.0 .0
.0
.0
.0
.0
AUG
100
98
96 95
94 92
74 I
664634
30 21
15
12
7.1 3.9
2.5 1.2
0.0
.0
SEPT
100
100
100 100
98
97
92 i
72
61
44
3629
25
19 10
5.8 2.5 0.8
0.0
OCT
100
100 100 100
100 100
96
9384 80
75 64
47
40
31 21
16 103.5
3.0
NOV
100
100
100
100
100
100
95 i
92 |83 78
78 73
70
68
6558 |
4833 [12
4.0
DEC
100
100 100
100
100 100 rQO go
87 85 82
80 79
78
75
6848 5°
25 _10 204.0
2.0
: 90, 50 AND 20 PERCENTAGE DURATIONS INDICATED BY LINES ON MONTHLY DATA.
# FLOW WAS EQUALED OR EXCEEDED IN PERIOD OF RECORD
1000
Qzo o$ 100QC UJ Q.I- UJ UJ
o moz
UJoQC
XoCO Q
10
1.0
I I ! ! i EXPLANATION
-\
- VEGA BAJA (1973-78)
© RIO DE LA PLATA AT TOA ALTA
RIO CIBUCO AT VEGA BAJA
RIO DE LA PLATA BEFORE REGULATION (1960-73)
RIO DE LA PLATA PERIOD OF RECORD (1960-73)
RIO DE LA PLATA AFTER REGULATION (1974-78)
0 5 10 20 30 40 50 60 70 80 90 95
PERCENTAGE OF THE TIME INDICATED DISCHARGE
WAS EQUALED OR EXCEEDED
Figure 3.4-1. Flow-duration curves for Rio Cibuco at Vega Baja and Rio de la Plata at Toa Alta.
100
3.0 SURFACE WATER 3.4 Flow Duration
4.0 GROUND WATER
4.1 Occurrence
ALLUVIAL AND CAVERNOUS LIMESTONE AQUIFER
SYSTEM UNDERLIES STUDY AREA
A water-table aquifer system, possibly underlain by a deep artesian system, is present in the Sabana Seca to Vega Baja area. The northward
movement of water to the sea is controlled locally by orientation of solution cavities in the limestone.
An unconfined or water-table aquifer system, composed primarily of the Aguada Limestone, Aymamon Limestone, and alluvial, dune, and blanket deposits, is present throughout the study area. Locally confined (artesian) conditions exist in the Aymamon Limestone in the coastal part of the Rio Cibuco valley. This is due to overlying alluvial silt and clay deposits. Similar conditions may exist elsewhere in the area.
Although unverified by drilling, water is believed to occur in the deeper Lares Limestone and Cibao Formation. Such water may be in hydraulic continuity with the overlying water-table system, in which case it is probably saline. It may also be a separate freshwater confined system under high artesian pressure as in the Barceloneta area. At present no information on yield, pressure, or aquifer characteristics is available for the deeper formations.
In general, water in the alluvium, dune, and blanket deposits occurs in intergranular pore spaces, and water in the limestones occurs in fractures and solution channels (fig. 4.1-1; 4.1-2). Although ground-water movement in cav ernous limestones is locally controlled by orientation of solution-formed channels, its general direction is north from the mountains to the Atlantic Ocean (Giusti, 1978).
Depth to water in the water-table system ranges from 16 to 20 ft below land surface in wells near Toa Baja and Higuillar to about 100 to 120 ft in wells drilled in the limestone near Highway 2. At present only the water-table aqui fer and the locally confined aquifer in the Rio Cibuco valley are developed.
22
AIR-FILLED OPENING
WATER TABLE
'// ' ' / ' / / ' 'n'/'f (' i '.'/", //' '// '/ WATER TABLE
' '' ' ' '*i' STREAM >f
Figure 4.1-1. Water movement in alluvial aquifers.
PERENNIAL 'STREAM /''///// t ,, ' ' r i ' ! ' > . .' , I
EXPLANATIONWATER MOVEMENT
Figure 4.1-2. Water movement in cavernous aquifers.
4.0 GROUND WATER 4.1 Occurrence
4.0 GROUND WATER (Continued)
4.2 Aquifer Characteristics
AQUIFER CHARACTERISTICS VARY WITH LOCATION
Specific capacities of wells within the study area indicate that cavernous conditions in the limestone
aquifer are localized.
Aquifer characteristics useful in understanding how an aquifer responds to stresses are transmissivity ( T_ ) and storage coefficient ( ^ ). The transmis- sivity of an aquifer is defined as the rate of flow of water at the prevailing kinematic viscosity ( 11 ) through a unit width of the aquifer and extending the full saturated thickness of the aquifer under a unit hydraulic gradient. The storage coefficient of an aquifer is the volume of water that is released from or taken into storage per unit change in head per unit surface area of the a- quifer. These characteristics can be computed from mathematical analysis of a- quifer tests and from laboratory tests of aquifer materials. Transmissivity can also be estimated from the specific capacity (well discharge per unit drawdown) (Heath, 1980).
Storage coefficients in the water-table aquifer of the study area range from about 0.01 to 0.05 in the limestone, and 0.1 or higher in the alluvium. Where local artesian conditions exist in the limestone, storage coefficients could be in the order of 0.001. Transmissivities higher than 100,0002ft /d are not unusual in cavernous limestone and range from 100 to 10,000 ft /d in the alluvium of Rio Cibuco and Rio de la Plata valleys.
An aquifer test was conducted in a locally confined, limestone artesian aquifer near the Rio Cibuco with one pumping well and 4 observation wells (fig. 4.2-1 and 4.2-2). The pumping and observation wells were open to the sand, gravel, and limestone deposits in the area (fig. 4.2-2). The test lasted 16 days. Drawdown curves corrected for tidal changes are shown in figure 4.2-3. Drawdown in the test well, pumped at 1,800 gal/min, reached 4.3 ft in the first 2 minutes and remained essentially constant. Data from the observation wells indicate a shallow, nearly flat, cone of depression with a maximum drawdown of 0.45 ft at 80 ft from the pumped well,xand 0.3 ft at a distance of 860 ft. Af ter 16 days of continuous pumping, the water level throughout the cone of de pression was dropping at a rate of less than 0.01 ft/d. The extremely low rate of water-level decline is due to cavernous conditions of the limestone at that specific site. The transmissivity obtained from the test, 200,000 ft /d, is not representative of the aquifer in general. Specific capacities for other wells in the area (fig. 4.2-1) indicate that such cavernous conditions are lo calized. The Cibuco test does indicate, however, the high production potential of the aquifer in cavernous areas. Individual aquifer tests at other locations in the study area are needed to define local aquifer characteristics.
24
SOUTH
.PR NO. 160
ALLUVIUM, SAND AND GRAVEL-
CEIBA WELL
NORTH
WELL NO. 3
SEA LEVEL
WELL NO. 1A WELL NO. 1
WELL NO. 2
I
LIMESTONE. CAVERNOUS AT SITE-I
100'
150
200' -
250'
200 400
66°25'
18°3O
600 800 1000
DISTANCE. IN FEET
1200 1400 1600 1800
Figure 4.2-2. Vertical section of observation wells and pumping well of the Cibuco aquifer test.
22'30" 20' 17'30" 66°12'30"
27'30'
18°24
DORADO AIRFIELD
.10 £LAR^_y r
-< .
ALMIRANTE NORTE
2 MILES
3 KILOMETERS
Figure 4.2-1. Location of Cibuco aquifer test and specific capacity values, in gallons per
minute per feet of drawdown, for selected wells ( ).
10.0
1.0
0.1
0.01
I I I I I I l| I I I Mill) \ I | I I I I L
CEIBA WELL i Q-1800 GALLONS PER MINUTE
OBSERVATION WELL NO. 1A DISTANCE =80 FEET
OBSERVATION WELL NO. 1 DISTANCE=1O4 FEET
OBSERVATION WELL NO. 3 DISTANCE = 860 FEET
OBSERVATION WELL NO. 2 DISTANCE3230 FEET
J I ' I''" ' i I I I I I I ' ' ' I I I I I I ' I I I I I I I ' i i I i I I I
0.01 1.0 10 100 TIME, IN MINUTES
1000 10.000
Figure 4.2-3. Drawdown curves at Ceiba well and observation wells 1A, 1, 2, and 3.
4.0 GROUND WATER4.2 Aquifer Characteristics
4.0 GROUND WATER (Continued)
4.3 Ground-water flow
AT LEAST 19 MILLION GALLONS PER DAY OF GROUND WATER
FLOW SEAWARD THROUGH STUDY AREA
Aquifer characteristics based on non-cavernousconditions are used to compute volume of underflow
in the water-table aquifer between Sabana Seca and Vega Baja.
Estimates of ground-water flow through the water-table aquifer in the study area (figure 4.3-1) were made from Darcy's law using the potentiometric gradi ent near Highway 2 and the transmissivity computed from hydraulic conductivi ties (Giusti and Bennett, 1976). Transmissivity values are for noncavernous limestones and are considered conservative. A thickness of 200 ft was assumed for the fresh-water zone. The discharges for different sectors of the area (table 4.3-1) were estimated as follows:
Q = TIL
Where Q = Flow, in cubic feet per dayT = Transmissivity, in feet squared per dayI = Gradient of potentiometric surface (dimensionless)L = Width of aquifer under consideration, in feet
Dorado-Sabana Seca sector, 3 mi wide, gradient /0.003
Q = 13,400 x 0.003 x 5280 = 212,250 (ft3/d)/mi (1.6 (Mgal/d)/mi) QD-SS =1.6 (Mgal/d)/mi x 3 mi = 4.8 Mgal/d
Vega Alta-Dorado sector, 5 mi wide, gradient ~-y/0.003Q = 13,400 x 0.003 x 5280 = 212,250 (ft 'd)/mi (1.6 (Mgal/d)/mi)
QVA-D = K6 (M8al/d>/mi x 5 mi = 7.9 Mgal/d
2 Vega Baja-Vega Alta sector, 3.5 mi wide, gradient /0.00076
Q = 54,000 x 0.00076 x 5280 = 216,700 (ft /d)/mi (1.6 (Mgal/d)/mi) QVB-VA * 1>6 (M8al/d>/mi x 3.5 = 5.7 Mgal/d
The average ground-water flow through a section one mile wide is^about 213,000 ft /d. Total flow in the 11.5 mile section is about 2.5 x 10 ft /d or 18.3 Mgal/d (Fig. 4.3-2).
These estimates are based on flow in a non-cavernous limestone, and are therefore considered conservative. Actual flow may be greater than the above estimates.
,/From Anderson (1976), gradient in Aguada Limestone east of Dorado.From Giusti and Bennett (1976), gradient in Aymamon Limestone in Vega Baja-
Vega Alta area.
26
66°2
5'
22'3
0"
20'
17'3
0"
15'
66
°12
'30
"
18°3
0
27
'30
18
°24
ALM
IRA
NT
E
NO
RT
E
2 M
ILE
SVE
GA
B
AJA
-VE
GA
A
LT
A
5.7
M
ILL
ION
G
AL
LO
NS
PE
R
DA
Y
VE
GA
A
LT
A-D
OR
AD
O
7.9
M
ILLIO
N
GA
LL
ON
S
PE
R
DA
Y
DO
RA
DO
-SA
BA
NA
S
EC
A
4.8
M
ILLIO
N
GA
LLO
NS
PE
R
DA
Y
Fig
ure
4.3
-1. C
om
pu
ted
g
rou
nd
-wa
ter
flow
th
rough s
ectio
ns,
Sabana
Se
ca
to
V
eg
a
Baja
.
ff F
RE
SH
W
AT
ER
F
LO
W -
FR
ES
HW
AT
ER
-SA
LT
WA
TE
WH
'I &
JIV
llkl/>
T
^N
fclr
- .
. .
....... ...... m
il
Fig
ure
4.3
-2. G
en
era
lized
g
rou
nd
-wa
ter
flow
in w
ate
r-table
aquife
r.
Ta
ble
4.3
-1. A
quife
r ch
ara
cte
ristic
s
an
d
com
pu
ted
gro
und-w
ate
r flo
ws
thro
ug
h sectio
ns
be
twe
en
S
ab
an
a S
eca
a
nd
V
eg
a B
aja
.
SE
CT
ION
DO
RA
DO
-SA
BA
NA
S
EC
A
VE
GA
A
LT
A-D
OR
AD
O
VE
GA
B
AJA
-VE
GA
A
LT
A
HY
DR
AU
LIC
C
ON
DU
CT
I V
ITY
, IN
F
EE
T
PE
R
DA
Y
(K)
67
67
27
0
TH
ICK
N
ES
S
OF
F
RE
SH
WA
TE
R
ZO
NE
, IN
FE
ET
20
0
20
0
20
0
TR
AN
SM
ISS
I- V
ITY
, IN
F
EE
T
SQ
UA
RE
D
PE
R D
AY
(T
)
13,4
00
13,4
00
54,0
00
HY
DR
AU
LIC
G
RA
D
IEN
T
(I)
0.0
03
0,0
03
0.0
00
76
WID
TH
O
F
SE
CT
ION
, IN
F
EE
T
(L)
15,8
40
26,4
00
18,4
80
FLO
W,
IN
MIL
LIO
N
GA
LS
. P
ER
D
AY
(Q
)
4.8
7.9
5.7
4.0
G
RO
UN
D
WA
TE
R4.3
G
rou
nd
-wa
ter
Flo
w
4.0 GROUND WATER (Continued)
4.4 Yields to wells
HIGHER YIELDS OBTAINED IN WESTERN PART
OF THE STUDY AREA
Yields to wells in the Sabana Seca to Vega Baja Area are highly variableand range from 10 to 4,000 gallons per minute. Yields from
alluvial deposits are less than those from cavernous limestones.
Yields to wells tapping the Aymamon and underlying Aguada Limestones any where in the north coast area are variable because water in the limestone oc curs in fractures and solution cavities. Yields between Vega Baja and Toa Baja range from 10 to 4,000 gal/min. Yields of 600 to 1,000 gal/min can be obtained between Campanilla and Sabana Seca.
The yields of alluvium and terrace deposits are also variable, depending on the thickness and grain size. Wells drilled in coarse alluvium of Rio Cibuco valley yield as much as 300 gal/min, whereas the finer alluvium in the Rio de la Plata valley yield less than 150 gal/min.
Beach and dune deposits usually contain saline water, but in some areas the wells in these deposits yield small quantities of potable water. The blanket de posits where underlain by impermeable beds may contain perched fresh water.
Data on yields of drilled wells are commonly obtained from well drillers whose figures often are based on short pumping tests. The tests are usually run for only a few hours, so long-term yields and water-level changes are not known. The reported yield of a well frequently is not a good measure of its capacity, nor of the characteristics of the aquifer. The reported capacity of a well may be affected by well construction and pump characteristics. Yields to wells must be accepted and used with understanding of the potential possible limitations involved. Yields to wells in the study area (fig. 4.4-1) are shown in table 4.4-1.
28
66°2
5'
22'3
0"
20
'17*3
0'
15'
18
°30
'
27*3
0'
25'
-
18
°24
'
PU
NT
A P
UE
RT
O
NU
EV
O
EX
PL
AN
AT
ION
OC
EA
NO
A
TL
AN
TIC
OW
EL
L
AN
D
NU
MB
ER
PU
NT
A
CO
RO
SO
DO
RA
DO
A
IRF
IEL
D
A 2
8
SA
BA
NA
SE
CA
24^
25
BA
JU
RA
1
^4 «
E
SP
INO
SA
VE
GA
A
LT
AA
LM
IRA
NT
E
NO
RT
EC
AN
DE
LA
RIA
Ba
se
fro
m
US
GS
top
o
map
s scale
1:3
0,0
00
Ba
ya
md
n,
Ve
ga
A
Jta, an
d
Ma
na
tf qu
ad
s.
Fig
ure
4
.4-1
. Lo
ca
tion
o
f w
ells
in
th
e
Sabana
Se
ca
to
V
ega
Baja
are
a.
2 M
ILE
S
Jl3 K
ILO
ME
TE
RS
Table 4.4-1. Depth, diameter, yield, and general water-quality characteristics of wells in the Sabana Seca-Vega Baja area.
WELL NO. ON MAP
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
DEPTH (FEET BELOW LAND
SURFACE)140
150
175
168
160
120
70
80
11194
148
210
350
124
85
84
240
250
400
180
200
350
800
115
98
86
200
85
303
165
>200
160
155
70
DIAMETER (INCHES)
16
16
12
12
16
12
8
7
12
8
12
12
12
6
6
6
10
10
12
10
10
10
12
14
8
12
10
8
10
12
12
16:12
16:12
6
YIELD (GALS PER MINUTE)
1000
1200
1500-4000
1500
1000
950-1500
1500
60
300 .
600
360
325
450
350
500
350
600
8
8
8
500
100
650
300
400
500
300
300
75
800
1000
150
150
225
1200
800
1040
1040
600
500
WATER-QUALITY
FRESHWATER
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
SALTYWATER
DO
DO
DO
FRESHWATER
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
DO
FRESH WATER-SALTY WATER
DO
DO
FRESHWATER
DO
4.0 GROUND WATER 4.4 Yields to Wells
4.0 GROUND WATER (Continued)
4.5 Water Levels
WATER LEVELS HAVE DECLINED ABOUT 7 FEET
IN THE LAST 8 YEARS
The decline is probably in response to ground- water withdrawals. Water levels are
also affected by recharge from rainfall.
Water levels are important where a stream channel is in direct contact with an aquifer. The stream may either lose water to, or gain water from, the aquifer depending on the relative elevations of the surface- and ground-water levels. Ground-water levels may also be used to determine flow direction and changes in the amount of water in storage in an aquifer. In coastal aquifers a knowledge of the ground-water surface elevation relative to mean sea level can help define the depths of the fresh-salt water mixing zone (see section 5.2-2).
Water levels in the study area range from 0 to 200 ft above mean sea level (fig. 4.5-1). In the western part along Rio Cibuco Valley, the nearly flat water-level gradients indicate the high permeability of the Aymamon Limestone. The gradient, however, steepens to the east because the water-bearing forma tion in this part of the area (Aguada Limestone) is much less permeable. Water levels in between reflect the transitional change in geology from the Aymamon Limestone to the Aguada Limestone (fig. 2.1-1).
Water levels in the study area are affected by recharge from rainfall and ground-water withdrawals (fig. 4.5-2). Water levels in observation wells fluc tuated as much as 10 ft at Campanillas and 6 ft at Sabana Hoyos. In 1979, as a result of excessive rainfall from hurricane David and tropical storm Frederick, ground-water levels increased from 5 to 10 ft. However, within the following year most of the gain had been lost. Water levels declined about 7 ft at both sites from 1971 to 1978, probably in response to regional pumpage.
30
EX
PL
AN
AT
ION
PU
NT
A P
UE
RT
O
NU
EV
O 2
0
WA
TE
R-T
AB
LE
C
ON
TO
UR
- S
how
s a
ltitud
e
of
wa
ter-
tab
le,
in fe
et
above
me
an
se
a le
vel.
Conto
ur
Inte
rva
ls
4,
5,
10
. 30.
an
d
15
0 fe
et.
OC
EA
NO
A
TL
AN
TIC
O5
OB
SE
RV
AT
ION
W
ELL-
Nu
mb
er
is w
ate
r le
vel,
in fe
et
above
me
an
sea le
ve
l.
RE
CO
RD
ING
W
ELL-
Refe
r to
F
ig.4
.5-2
DO
RA
DO
W
EA
TH
ER
S
TA
TIO
NP
UN
TA
C
OR
OS
O
DO
RA
DO
A
IRF
IEL
D
t5
SA
BA
NA
H
OY
OS
SA
BA
NA
SE
CA
CA
MP
AN
UL
AS
CA
ND
EL
AR
IA
20
3A
LM
IRA
NT
E
NO
RT
EV
EG
A
AL
TA
25'
_
18°2
4B
ase
fro
m
US
GS
to
po
ma
ps
sca
le
1:3
0.0
00
Ba
yam
o'n
, V
ega
AJta
, a
nd
M
anatf q
uads.
Fig
ure
4
.5-1
. Ge
ne
raliz
ed
w
ate
r-tab
le config
ura
tion
in th
e
Sa
ba
na
S
eca
to
V
ega
Baja
are
a.
.5 I
2
MIL
ES
J3
KIL
OM
ET
ER
S
CAMPANILLA WELL
SABANA HOYOS WELL
I <DORADO RAINFALL STATION
1971 1972 1973 1974 1975 1976 1977 1978 1979 1980
Figure 4.5-2. Hydrographs of observation wells Campanilfa and Sabana Hoyos and Rainfall at Dorado station.
4.0 GROUND WATER 4.5 Water Levels
5.0 QUALITY OF WATER
5.1 Surface Water
5.1-1 Chemical, Physical and Bacteriological Characteristics
QUALITY OF WATER FROM RIO CIBUCO AND
RIO DE LA PLATA IS VERY SIMILAR
Geology is the major natural influence affecting the chemical characteristics of surface water in the area.
The geology upstream from the study area is the principal factor in the chemical characteristics of waters in the major streams. Water in drainage canals is affected by sea-water intrusion. Streams flowing into the area show physical and chemical characteristics typical of waters from the limestone for mations to the south. These waters contain large amounts of dissolved carbonate salts; chiefly calcium carbonate, and high alkalinity and hardness values.
The chemical and physical characteristics of waters of Rio Cibuco at Vega Baja and Rio de la Plata at Toa Alta are very similar (Table 5.1-1-1, 5.1-1-2 and fig. 5.1-1-1). Calcium and bicarbonate are the principal ions. Dissolved solids range from about 130 to 325 mg/L at both streams. Sodium concentrations range from about 6 to 40 mg/L. Chloride concentrations, although usually not exceeding 40 mg/L, occasionally are as great as 70 mg/L in the Rio De La Plata. The waters range from hard to very hard, are slightly alkaline, and are suit able for most uses, including irrigation.
Fecal coliform concentrations at Rio Cibuco at Vega Baja range from 200 to 9,000, with an average of about 3,600 colonies/100 mL. At Rio de la Plata, the range is from 53 to 26,000, with an average of 8,000 colonies/100 mL sample. These bacteria concentrations are typical of streams receiving human and animal fecal wastes.
32
Tab
le 5.1
-1-1
. Ch
em
ical
and
p
hysic
al
ch
ara
cte
ristic
s
of
Rfo
C
ibu
co
at
Ve
ga
B
aja
P
.R.
50039500 (1
97
2-7
9)
CO
LO
NIE
S
PE
R 1
00
M
ILL
ILIT
ER
O
F S
AM
PL
E.
CO
NS
.TIT
UE
NT
pH
. F
IELD
SP
EC
IFIC
C
ON
DU
CT
AN
CE
. IN
M
ICR
O-
MH
OS
P
ER
C
EN
TIM
ET
ER
A
T
25°C
TE
MP
ER
AT
UR
E.°C
RE
SID
UE
S
US
PE
ND
ED
,<®
180°C
CA
LC
IUM
, T
OT
AL
MA
GN
ES
IUM
, T
OT
AL
SO
DIU
M,
DIS
SO
LV
ED
BIC
AR
BO
NA
TE
CH
LO
RID
E,
DIS
SO
LV
ED
FLU
OR
IDE
, D
ISS
OLV
ED
SU
LF
AT
E,
DIS
SO
LV
ED
NIT
RO
GE
N,
TO
TA
L
AS
N
o3
PH
OS
PH
OR
US
. T
OT
AL
AS
P
IMM
ED
IAT
E
CO
LIF
OR
M
FE
CA
L
CO
LIF
OR
M
FE
CA
L
ST
RE
PT
OC
OC
CI
MIN
IMU
M
6.5
302
22
22
2
447
.9
1 1
15
0
160
.10
991
.5
0.1
0
28
00
20
0
50
0
MA
XIM
UM
7.9
55
0
31
26
6
64
1 1
21
215
310
.8
273
.7
0.6
1.7
00,0
00
9000
31
00
ME
AN7.4
40425.6
25
056
.1
9.1
15.7
20122.3
0.1
8
15
.9
2.7
0.3
77
,00
0
36
00
1800
*
90th
PE
RC
EN
TIL
E
7.7
46
0
28
26361
.2
9.8
19
22
4
270
.2
213
.5
0.4
160.0
00
61
80
29
00
*
PE
RC
EN
TA
GE
O
F
TH
E
TIM
E
IND
ICA
TE
D
VA
LU
E
WA
S
EQ
UA
L
OR
L
ES
S.
Ta
ble
5.1
-1-2
. Ch
em
ical
and
p
hy
sic
al
ch
ara
cte
ristic
s
of
Rio
d
e la
P
lata
at T
oa A
lta,P
.R.
50
04
60
00
(1
958-7
9)
AL
L
AN
AL
YS
ES
IN
M
ILL
IGR
AM
S
PE
R
LIT
ER
E
XC
EP
T
AS
IN
DIC
AT
ED
. C
OL
IFO
RM
S,
IN
CO
LO
NIE
S
PE
R
10
0
MIL
LIL
ITE
RS
O
F
SA
MP
LE
CO
NS
TIT
UE
NT
pH
, F
IELD
SP
EC
IFIC
C
ON
DU
CT
AN
CE
, IN
M
ICR
O-
MH
OS
P
ER
C
EN
TIM
ET
ER
AT
25°C
TE
MP
ER
AT
UR
E, °C
RE
SID
UE
S
US
PE
ND
ED
^)1
80°C
CA
LC
IUM
, T
OT
AL
MA
GN
ES
IUM
, T
OT
AL
SO
DIU
M,
DIS
SO
LV
ED
BIC
AR
BO
NA
TE
CH
LO
RID
E.
DIS
SO
LV
ED
FL
UO
RID
E,
DIS
SO
LV
ED
SIL
ICA
, D
ISS
OL
VE
D
SU
LF
AT
E,
DIS
SO
LV
ED
NIT
RO
GE
N,
TO
TA
L
AS
N
o3
PH
OS
PH
OR
US
. T
OT
AL
A
S
P
IMM
ED
IAT
E
CO
LIF
OR
M
FE
CA
L
CO
LIF
OR
M
FE
CA
L
ST
RE
PT
OC
OC
CI
MIN
IMU
M
6.5
195
20
120
14
176
.2
20
100
.0
8.3
8.0
00.0
2
200
53
32
MA
XIM
UM
8.5
14
00
32
4100
87
20
40
298
961
.3
30
323
.2
0.7
92
0.0
00
26
,00
0
42
,00
0
ME
AN7
.5
889
26.7
23
0
38
12
20
16
8
26
.7
0.2
21
15.5
0.6.1
3
54
.00
0
8000
41
00
*
90th
PE
RC
EN
TIL
E
8.0
550
29
30
00
55
15
27
22439
.7
0.3
26
182
.3
0.2
7
120,0
00
20
,00
0
61
00
15
'6
6°1
2'3
0'
27'3
0
PU
NT
A
PU
ER
TO
N
UE
VO
BA
JU
RA
AL
MIR
AN
TE
N
OR
TE
2
MIL
ES
25
' -
18
°24
'
Fig
ure
5.1
-1. T
wo
q
ua
lity-o
f-wa
ter
sta
tion
s
loc
ate
d
at R
fo C
ibu
co
and
R
fo d
e la
Pla
ta.
5.0
QU
AL
ITY
OF
WA
TE
R
5.1 S
urfa
ce
Wate
r5.1
-1
Ch
emical, P
hysical, and
Bacterio
log
ical Ch
aracteristics
* P
ER
CC
'JT
\GE
O
F
TH
E
TIM
E
IND
ICA
TE
D
VA
LU
E
WA
S
EQ
UA
L
OR
L
ES
S.
5.0 QUALITY OF WATER (Continued)
5.1 Surface Water (Continued)
5.1-2 Suspended Sediment
SUSPENDED-SEDIMENT LOADS AND YIELDS
FOR BOTH RIVERS ARE ABOUT EQUAL
Suspended-sediment and water discharge data from 1973-78 indicate that the average annual sediment yield of Rio Cibuco atVega Baja is about 85 tons per square mile, while for
Rio de la Plata at Toa Alta it is about 82 tons per square mile.
The concentration of sediment affects the suitability of water for domes tic, industrial, and agricultural uses. For irrigation purposes, a high sed iment concentration is undesirable, more so if most of the sediment is in the silt-clay particle-size range (less than 0.062 mm).
Sediment data are very scarce for the two principal streams in the study area (Rio Cibuco and Rio de la Plata). Miscellaneous suspended-sediment samples have been collected at sites on both streams since 1973. There are no data on the bedload component of the transported sediment. Preliminary size analyses of medium to low-flow suspended-sediment samples indicate that about 90 percent of the sediment is silt and clay.
The miscellaneous suspended-sediment samples . collected from Rio Cibuco at Vega Baja and Rio de la Plata at Toa Alta can be correlated against the instan taneous water discharge (figure 5.1-2-1. An approximate graphical correlation of the data points can be used to estimate the instantaneous suspended-sediment load (in tons per day). Seasonal effects in the transport of suspended sedi ment, which have been documented in other basins in Puerto Rico, are not con sidered in the correlation. The data available are inadequate for seasonal correlations. The estimate can be used as a guide to determine the amount of suspended sediment transported to irrigation areas if waters from the two rivers are utilized.
The mean annual suspended-sediment yield for both streams is about the same. These were computed using flow-concentration duration techniques described by Miller (19511. The effective drainage area of Rio de la Plata at^Toa Alta is about 200 mi , while for Rio Cibuco at Vega Baja it is about 90 mi (although about 24 mi are entirely or partly non-contributing, mostly during periods of low flows)* The average annual suspended-sediment yield for Rio Cibuco is about 85 tons/mi while for Rio de la Plata it is about 82 tons/mi . The computations for Rio de la Plata are based on the 1974-78 period, after regulation at the La Plata reservoir began. It is probable that prior to the construction of the reservoir, the yields from Rio de la Plata basin were higher.
34
CC UJ Q.COz o
IUo cc<XoCOoh-zIU
1000
500
200
100
50
oIU COi 20 UlozIU
CO 10
CO
CODO 5iuzh-
h-CO 2Z
1.01.0 2 5 10 20 50 100 200 500 1000
INSTANTANEOUS WATER DISCHARGE, IN CUBIC FEET PER SECOND
Figure 5.1-2-1. Graphical correlation between instantaneous water and suspended-sediment discharges at Rfo Cibuco at Vega Baja, 1973-1981, and at Rfo de la Plata at Toa Alta, 1974-1981.
5.0 QUALITY OF WATER 5.1 Surface Water
5.1-2 Suspended Sediment
5.0 QUALITY OF WATER (Continued)
5.1 Surface Water (Continued)
5.1-3 Saltwater Intrusion in Streams and Canals.
SEA-WATER ENTERS THE AREA THROUGH THE MOUTHS
OF THE RIVERS AND COASTAL OPENINGS
The saltwater wedge was located 1.75 river miles upstream from the mouth of Rio Cibuco and 3.0 river miles upstream from
the mouth of Rio de la Plata in 1978 surveys.
Seawater enters the study area through the mouths of Rio Cibuco, Rio de la Plata, Rio Cocal, plus several other minor openings to the sea. The areas affec ted by seawater intrusion include the estuaries, and mangrove swampy areas bordering the beaches from Vega Baja to Sabana Seca (fig. 5.1-3-1).
Water from many drainage canals is relatively high in chloride (200 to 600 mg/L), sulfate (35 to 90 mg/L) and, total dissolved solids (850 to 1800 mg/L) concentrations. Most of the waters are of the sodium-chloride type, indicating seawater intrusion. This condition prevails in drainage canals at the lower ends of Rio Cibuco and Rio de la Plata valleys. A maximum-chloride concentration of 20,000 mg/L was determined from samples collected at Finca La Julia, in the Rio de la Plata valley.
36
17*3
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66
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"
18°3
0'
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NT
A P
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Fig
ure
5.1
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.--Are
as o
f salt-w
ate
r in
trusio
n.
2 M
ILE
SJ3
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5.0 QUALITY OF WATER (Continued) ^
5.1 Surface Water (Continued)
5.1-3 Saltwater Intrusion in Streams and Canals. (Continued)
SEA-WATER ENTERS THE AREA THROUGH THE MOUTHS
OF THE RIVERS AND COASTAL OPENINGS
The saltwater wedge was located 1.75 river miles upstream from the mouth of Rio Cibuco and 3.0 river miles upstream from
the mouth of Rio de la Plata in 1978 surveys.
Specific conductance surveys were made in the Rio Cibuco and Rio de la Plata to determine the inland tidal effect and the extent of the salt-water wedge in the rivers during low and high tides. Two separate surveys at Rio Cibuco, conducted on August 29 (high tide) and October 17, 1978 (low tide), showed that the wedge was located 1.75 river miles upstream from the mouth (fig. 5.1-3-2). In the Rio de la Plata during a low tide on September 28, 1978, the wedge was located 2.75 river miles upstream from the mouth. During the storm-wave swash of December 20, 1978, the salt-water wedge at Rio de la Plata was about 3.0 river miles upstream from the mouth (fig. 5.1-3-2). The maximum potential inland movement of sea water through both streams was not determined. Altitudes of the channel bottom of the Rio Cibuco indicate that the salt-water wedge could advance about 3 miles upstream from the mouth. Reports from local residents suggest that the maximum potential inland seawater intrusion at Rio de la Plata is about 3.5 miles upstream from the mouth.
38
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Fig
ure
5.1
-3-2
. Locatio
n
of
sa
lt-wa
ter
wedges
in R
fo
Cib
uco
a
nd
R
io
de
la P
lata
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Gro
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5.2
-1
Ch
em
ical
and
P
hysic
al
Ch
ara
cte
ristic
s
GR
OU
ND
W
AT
ER
IN
S
TU
DY
A
RE
A
IS
MO
ST
LY
O
F
A
CA
LC
IUM
-BIC
AR
BO
NA
TE
T
YP
E
Ground water in
the limestone and
alluvium is
predominantly of
a calcium-bicarbonate type.
In blanket and
dune deposits it
is usually
a calcium-chloride or
sodium-chloride type,
depending upon proximity to
the sea
and the
composition of
the sand.
Water-quality data
of wells
in study area
(fig 5.2-1)
indicate that
ground
water is
mostly of
a calcium-bicarbonate type.
The water
from wells drilled
in blanket and
dune deposits
is usually a
calcium-chloride or
a sodium-chloride
type depending upon the
proximity to
the sea
(fig. 5.2-1-1).
The relative
order
of abundance of
anions is
chloride, bicarbonate,
and sulfate.
Chloride concen
trations usually range
from 50
to 800
mg/L. Sulfate
concentrations
are nor
mally less
than 25
mg/L, but
in wells affected by
sea water,
can be
as much as
100 mg/L or
more. Temperature of
the water ranges
from 25
to 27°C.
Water from wells
tapping the
limestone aquifers
is predominately of
a cal
cium-bicarbonate type
(fig. 5.2-1-2).
Dissolved-solids
concentration
ranges
from 250
to 300
mg/L, of
which silica
represents 3 percent,
about 8
to 10
mg/L. Calcium and
magnesium concentrations
range from 70
to 150
mg/L, about
90 per
cent of
the total
cations in
solution. The
order of
abundance of
anions is:
bicarbonate, chloride,
and sulfate,
of which bicarbonate is
about 80
percent. Chloride concentrations are
relatively low,
ranging from 10
to 20
mg/L. How
ever, chloride
concentrations of
100 mg/L or
more have been measured
in wells
near the
coast tapping
the Aguada
Limestone. Nitrates usually are
low, averag
ing from 5
to 15
mg/L. Ground-water
temperature from
pumping wells
averages
26°C. Values
of pH
are on
the order of
6.5 to
7.5 units.
Water in
alluvial deposits of
the Rio
Cibuco and
Rio de
la Plata
valleys
is also
of a
calcium-bicarbonate type
(fig. 5.2-1-3).
Generally, the
dissolved
solids concentration ranges
from 300
to 450
mg/L, of
which silica
is less
than 25
mg/L. The
water is
very hard,
usually greater
than 200
mg/L hardness as
CaCO .
Bicarbonate is
the predominant anion,
about 80
percent of
the total
anions. The
chloride concentration of
the water ranges
from 10
mg/L at
wells in
the southern part
of the
Rio Cibuco valley to
about 250
mg/L at
a well
near the
coast. Sulfate
and chloride
concentrations usually
are about
equal ranging
from
less than
10 to
25 mg/L.
Nitrate concentrations range
from 3
to 20
mg/L. The
waters are
slightly alkaline,
with pH
values ranging
from 7.2
to 7.5
units, si
milar to
those of
river water.
40
COMPOSITION, IN PERCENT MILLIEQUIVALENTS
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Fig
ure
5.2
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. Ty
pic
al
an
aly
seso
f wate
r from
b
lan
ke
t an
dd
un
e de
po
sits
near th
e c
oa
st.
27'3
0*
Fig
ure 5
.2-1
. Sele
cte
d w
ells used
for ch
emical
an
aly
ses.
25'
18°2
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COMPOSITION, IN PERCENT MILLIEQUIVALENT!
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5.0 QUALITY OF WATER (Continued)
5.2- Ground Water (Continued)
5.2-2 Sea-water Intrusion in coastal Aquifers
SEA-WATER INTRUSION IS A MAJOR PROBLEM
IN COASTAL AQUIFERS
Horizontal and vertical migration of the fresh-sea water mixing zone can occur as a result of uncontrolled pumpage.
In coastal aquifers, ground water usually discharges into the sea. When ever pumpage exceeds recharge in these aquifers, seawater intrusion may result. If the seawater moves inland, potable ground-water supplies become useless. Contamination with saltwater may take years to remove even when fresh ground water is available to flush out the saline water.
The relative position between the freshwater and the seawater is controlled primarily by their differences in density. A boundary surface or zone of dif fusion is formed whenever the fluids are in contact (figures 5.2-2-1, 5-2-2-2). The shape and movement of the zone of diffusion are governed by a hydrodynamic balance of the freshwater and the saltwater. The relative position of seawater migrates horizontally or vertically when uncontrolled pumpage of freshwater oc curs. Upconing (vertical migration) of seawater beneath pumping wells (fig ure 5.2-2-3) is a more imminent problem in most areas than is lateral intrusion (Heath, 1980). A much larger volume of freshwater has to be displaced in lat eral intrusion than in upconing.
In the Campanilla area the Puerto Rico Aqueduct and Sewer Authority (PRASA) has reported high chloride concentrations in some of the wells. A ser ious saltwater hazard to the ground water was indicated by results obtained from a quality of water survey made during 1973 (Diaz, oral commun., 1980). The chloride concentration in water from two wells (wells 88 and 89, fig. 4.4-1) in creased from 16 to 720 mg/L between 1964 and 1973. The increase of more than 700 mg/L in the chloride concentration in these pumping wells was considered as evidence of seawater intrusion in the area. Water from the wells originally showed physical and chemical characteristics typical of freshwater from the Ay- mamon and Aguada Limestones.
Most of the wells in the Campanilla area affected by saline water intru sion (chlorides greater than 250 mg/L) were drilled to depths greater than 130 ft below mean sea level. Continuous uncontrolled pumpage has lowered the water surface substantially reducing the thickness of the freshwater lens. The wells are now withdrawing water from the fresh-saltwater mixing zone (fig. 5.2-2-3).
42
SEA LEVEL DISCHARGE
DISCHARGE ZONE
^<^^m ffi^^l^fco^-iiii^S^:!£:£££::£ Is AT^
Figure 5.2-2-1. Boundaries between fresh and salty-ground water.(From Cohen and others, 1968).
^SALTWATER*
UMPING WELL
LAND SURFAC^
UNCONFINED AQUIFER
CONFINED AQUIFER
WATER-TAlJLE~
FRESHWATER f
-CONFINING BED-" ~^^^-Tl-n^^
LATERAL INTRUSION
^ CAI TVIA/ATCTD - SALTWATER
Figure 5.2-2-2. Circulation of salt water from sea to the zone of diffussion and return.
(From Cooper, Jr., 1964).
Figure 5.2-2-3. Two aspects of salt-waterintrusion. (From Heat. 1980).
5.0 QUALITY OF WATER 5.2 Ground Water
5.2-2 Sea-water Intrusion in Coastal Aquifers
5.0 QUALITY OF WATER (Continued)
5.2 Ground Water (Continued)
5.2-2 Sea-water Intrusion in Coastal Aquifers (Continued)
SEA-WATER INTRUSION IS A MAJOR PROBLEM
IN COASTAL AQUIFERS
Intrusion potential in study area is high due to the cavernous natureof the limestone aquifer and the shallowness of the
fresh-salt water mixing zone.
In the design of water-supply wells in coastal areas, consideration must be given to the possibility of seawater intrusion. This may involve construc tion of shallow wells or low pumping rates to avoid upconing. The relocation of wells inland may be necessary to avoid lateral intrusion.
Seawater intrusion has been detected in the Cibuco and the Campanilla sec tors of the Vega Baja to Sabana Seca area (fig. 5.2-2-4). The possibility of in trusion in other areas is relatively high due to the cavernous nature of the limestone aquifer and the shallowness of the fresh-salt water mixing zone.
44
66°25'
22'30'20'
17'30'15'
ee°i2'30"18°30'
27'30'
EXPLANATION
OCEANO ATLANTICO
DE
PT
H
TO
F
RE
SH
-WA
TE
R S
AL
T-W
AT
ER
M
IXIN
G
ZO
NE
HA
S
BE
EN
D
ET
ER
MIN
ED
.
25
' -
18°2
4'
T<>
FIG
. 5
.2-
2_
7_
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J P
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PUNTA PUERTO NUEVO
RE
FE
R
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5.2
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DO
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aya
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Fig
ure
5.2
-2-4
. Are
as w
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5.0 QUALITY OF WATER (Continued)
5.2 Ground Water (Continued)
5.2-2 Sea-water Intrusion in Coastal Aquifers (Continued)
SEA-WATER INTRUSION IS A MAJOR PROBLEM
IN COASTAL AQUIFERS
Intrusion potential in study area is high due to the cavernous natureof the limestone aquifer and the shallowness of the
fresh-salt water mixing zone.
In the Rio Cibuco valley, a surface geophysical technique was employed to define the depth to the fresh-salt water mixing zone (fig. 5.2-2-5). Electrical resistivity tests using the Schlumberger electrode array (Zohdy, and others, 1974) were conducted. The results were correlated with test holes, well logs, and ground-water quality data.
The top of the fresh-saltwater mixing zone (chloride concentrations greater than 250 mg/L) was found to range from about 50 to 210 ft below the land sur face. At Highway 2 (station 2) the mixing zone was encountered at a depth of about 210 ft (fig. 5.2-2-5 and 5.2-2-6). North of Highway 688 (station 6), the zone of diffusion was encountered at a depth of about 100 ft (fig. 5.2-2-5).
In the Campanilla area, data from existing wells was used to estimate the location of the fresh-saltwater zone of diffusion (fig. 5.2-2-7). In the vici nity of the Campanilla well field, the mixing zone is encountered at a depth of about 150 ft below the land surface. Seawater can also infiltrate into the aquifer through rivers and canals hydraulically connected to the sea.
46
AL
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ure
5.2
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. Estim
ate
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Fig
ure
5
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-6. D
ep
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e fre
sh
-sa
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at
aq
uife
r test
site
in
the
R
fo C
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y.
(See
figu
re 4.2
-1
for
loca
tion
o
f aquife
r te
st
site
.)
EX
PL
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tivity
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rofile
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. D
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SIS
TIV
ITY
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- 2
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Fig
ure
5
.2-2
-7, P
os
sib
le
sa
lt-wa
ter
intru
sio
nin
the
C
am
pa
nu
la
are
a.
(See
figure
5
.2-2
-4 fo
r lo
ca
tion
o
f sectio
n G
-G'.)
5.0
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UA
LIT
Y
OF
W
AT
ER
5
.2
Gro
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d W
ate
r5.2
-2 S
ea
-wa
ter In
trusio
n in C
oastal A
qu
ifers
5.0 QUALITY OF WATER (Continued)
5.2 Ground Water (Continued)
5.2-3 Classification for Irrigation
AREA CHARACTERIZED BY A MEDIUM-TO
HIGH-SALINITY WATER
Waters classified according to specific conductance values.
The suitability of water for irrigation can be determined from the follow ing characteristics: The total salinity of the water (salinity hazard), the ratio of sodium to other dissolved cations (sodium hazard), and the concentra tion of toxic elements. Some of the indices used to describe these character istics include total dissolved solids, soluble sodium, sodium-adsorption ratio (SAR), residual sodium carbonate and soluble boron.
The concentration of soluble salts in irrigation waters can be expressed in terms of specific conductance, which is related to the total concentration of ions in solution. The percent of soluble sodium indicates the proportion of sodium ions in solution in relation to the total cation concentration. The so dium-adsorption ratio (SAR) expresses the relative activity of exchange reac tions between sodium ions and the soil. High values of SAR indicate that a hazard exists for the replacement of calcium and magnesium by sodium. This ac cumulation of sodium damages soil structures and inhibits plant growth.
A correlation between the SAR, specific conductance, and suitability of waters for irrigation (fig. 5.2-3-1) was prepared by the U.S. Department of Agriculture (Methods of U.S. Salinity Laboratory Staff, 1954).
Groundwater in the area of investigation is suitable for irrigation al though it is mostly classified as a medium to high-salinity water (fig. 5.2-3-2) For comparison purposes, surface-water sites other than the two main streamflow stations in the study area, are also shown in figure 5.2-3-2. Additional data for the sites are summarized in the appendix.
48
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:30
,00
0
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ya
mo
'n,
Veg
a A
jta.
an
d
Ma
na
tf qu
ad
s.
Fig
ure
5.2
-3-2. C
lassific
atio
n
of
wate
r fo
r irrig
atio
n,
in
the
S
abana
Se
ca
to
V
eg
a
Ba
ja
are
a,
.5
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MIL
ES
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OM
ET
ER
S
5.0
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UA
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Y
OF
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AT
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round W
ate
rC
lassific
atio
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r Irrig
atio
n
6.0 WATER AVAILABILITY AND MANAGEMENT
ABUNDANT WATER SUPPLIES EXIST IN THE AREA
Management practices to augment the available water and to preventsaltwater intrusion will be required. Further studies to
define the ground-water system are needed.
How much water is available in the Sabana Seca to Vega Baja area? Rio Cibuco is the principal source of surface water. It flows more than 18 ft /s 90 percent of.,the time. However, during the dry season, its flow may be as low as 7.5 ft /s. Rio de la Plata, the other major stream, is affected by re gulation and withdrawal of more than 40 Mgal/d (62 ft /s). Minimum flows are now as low as 3.4 ft /s, but will probably be reduced even more as additional water is withdrawn to supply metropolitan San Juan. Ground water from the water-table aquifer is relatively abundant, with a flow toward the coast of a minimum of about 19 Mgal/d. The most productive known areas appear to be in the lower Rio Cibuco valley. In the overall water balance in the area, rainfall is also an important source of water, contributing as much as 0.25 in/d during the rainy season.
The estimated minimum ground-water flow throughout the area (19 Mgal/d) may not be available for pumpage. At least 10 Mgal/d are currently (1982) withdrawn for industrial, domestic and agricultural uses. Also seaward flow of freshwater must be maintained to prevent lateral saltwater encroachment. In some areas Campanilla, Dorado and Cibuco), evidence of seawater encroachment indicates that withdrawals are exceeding the available ground-water supplies.
What are some of the problems for the management of the water resources in the area? The lower reaches of Rio Cibuco and Rio de la Plata are affected by saltwater encroachment. The salt-water wedge can reach as far as 3 river miles upstream from the coast. Flow depletion could induce further encroachment. Per haps as important are the severe floods produced by both streams, inundating most of the lower valleys. Additional water may be available from the Rio de la Plata, if storage at La Plata Reservoir is optimized against seasonal runoff and releases.
The full potential of the ground-water resources in the area has not been determined. The limestone aquifer is not homogeneous and flow occurs through caverns and solution channels. These are localized conditions that will require specific site evaluations. The possible existence of a deep artesian aquifer throughout the area may mean an additional source of water is available.
Seawater intrusion is a major threat in the potential development of the coastal aquifers. Uncontrolled pumpage or poorly designed wells could induce additional saltwater intrusion either by lateral movement or by upconing such as has occurred in the Campanilla well field.
50
7.0 LIST OF REFERENCES
Anderson, H.R., 1976, Ground water in the San Juan metropolitan area, PuertoRico: U.S. Geological Survey Water-Resources Investigations 41-75, 34 p.
Briggs, R.P., and Akers, J.P., 1965, Hydrogeologic map of Puerto Rico andadjacent islands: U.S. Geological Survey Hydrologic Investigations Atlas HA-197.
Calvesbert, R.J., 1970, Climate of Puerto Rico and U.S. Virgin Islands: U.S. Department of Commerce Environmental Science Services Administrative Publication 60-52, Silver Spring, Md., 29 p.
Cobb, E.D., 1978, Estimates of 7-day, 10-year minimum flows at selected stream sites in Puerto Rico: U.S. Geological Survey Open-File Report 78-583, 47 p.
Cohen, P., Franke, O.L., and Foxworthy, B.L., 1968, An atlas of Long Island's water resources: New York Water Resources Comm. Bull. 62, 117 p.
Cooper, H.H., Jr., Kohout, F.A., Henry, H.R., and Glover, R.E., 1964, Sea water in coastal aquifers: U.S. Geological Survey Water-Supply Paper 1613-C, 84 p.
Giusti, E.V., 1978, Hydrogeology of the Karst of Puerto Rico: U.S. Geological Survey Professional Paper 1012, 68 p.
Giusti, E.V., and Bennett, G.D., 1976, Water resources of the north coast limestone area, Puerto Rico: U.S. Geological Survey, Water Resources Investigations 42-75, 42 p.
Heath, R.L., 1980, Basic elements of ground-water hydrology with reference to conditions in North Carolina: U.S. Geological Survey Water-Resources Investigations Open-File Report 80-44, 86 p.
Hickenlooper, I.J., 1968, Floods in the area of Vega Alta and Vega Baja, Puerto Rico: U.S. Geological Survey Hydrologic Investigations Atlas HA-289.
Lopez, M.A., 1964, Floods at Toa Alta, Toa Baja and Dorado, Puerto Rico: U.S. Geological Survey Hydrologic Investigations Atlas HA-128.
Lopez, M.A., Colon-Dieppa, Eloy, and Cobb, E.D., 1979, Floods in Puerto Rico, magnitude and frequency: U.S. Geological Survey Water-Resources Inves tigations 78-141, 66 p.
McGuiness, C.L., 1946, Records of wells in Puerto Rico: San Juan, Puerto Rico Aqueduct and Sewer Service, Mimeographed report, 267 p.
_____, 1948, Ground-water resources of Puerto Rico: San Juan, Puerto RicoAqueduct and Sewer Service, Mimeographed report, 617 p.
51
7.0 LIST OF REFERENCES (Continued)
Miller, C.R., 1951, Analysis of flow-duration, sediment rating curve method of computing sediment yield: Denver, U.S. Bureau of Reclamation, 15 p.
Monroe, W.H. 1973a, Geologic map of the Bayamon quadrangle, Puerto Rico: U.S. Geological Survey, Miscellaneous Geologic Investigations Map 1-751.
____1973b, Geologic map of the Vega Alta quadrangle, Puerto Rico: U.S.Geological Survey Miscellaneous Geologic Investigations Map GQ-191.
____1976, The Karst landforms of Puerto Rico: U.S. Geological Survey Professional Paper 899D., 69 p.
____1980, Geology of the Middle Tertiary formation of Puerto Rico: U.S. Geological Survey Professional Paper 953, 93 p.
U.S. Department of Agriculture, 1954, Diagnosis and improvement of saline and alkaline soils: U.S. Department of Agriculture Handbook No. 60, Chap. 5, p. 69-81.
U.S. Geological Survey, 1981, Water resources data for Puerto Rico, Water Year 1978: U.S. Geological Survey Water Data Report PR-78-1, 255 p.
Vicente-Chandler, Jose, and others, 1977, Cultivo intensive y perspectivas del arroz en Puerto Rico: Univ. of Puerto Rico, Rio Piedras, Puerto Rico Estacion Experimental Agrlcola Bulletin 250, 71 p.
Zohdy, A.A.R., 1973, A computer program for the automatic interpretation of Schlumberger sounding curves over horizontally stratified media: U.S. Geological Survey NTIS.PB-232-703, 11 p.
Zohdy, A.A.R., Eaton, G.P., and Mabey, D.R., 1974, Application of surfacegeophysics to ground-water investigations: U.S. Geological Survey TWRI, Book 2, Chapter Dl, 116 p.
52
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