Ground Water Flow in Aquifer Systems: Floridan Aquifer Case Study

Envi 518September 10, 2002

Approximately 29% of the world’s fresh water resources exists in aquifers

Global Water Supply

Definition: A geological unit which can store and supply significant quantities of water.

Principal aquifers by rock type:

Unconsolidated

Sandstone

Sandstone and Carbonate

Semiconsolidated

Carbonate-rock

Volcanic

Other rocks

Aquifers

Ground water occurs when water penetrates the subsurface through cracks and pores in soil and rock

Ground Water

Natural Precipitation

melting snow

Infiltration by streams and lakes

Transpiration by plants

Artificial Recharge wells

Spread water over land in pits, furrows, ditches, or erect small dams in stream channels to detain and deflect water

Recharge

Hydrologic Cycle – Rainfall in becomes Recharge to the water table

Saturated zonebelow the water table

Water table

Soil zone

Unsaturated zone

Precipitation

Recharge to water table

Evapotranspiration

Infiltration

Runoff

Over Pumping

Cone of Depression

Drawdown

Unhealthy vs. Healthy Lake

PumpingWell

Section 21 Wells

Northern Tampa Bay (NTB)

PASCO

HILLSBOROUGH

PINELLAS

HERNANDO

POLK

N

0 10 Miles

NTB Overpumping IssueInterconnected Regional Well Fields, Southwest Florida Water Management District

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

500000

1930 1940 1950 1960 1970 1980 1990

m3 /day

Cross Bar

Cypress Creek

South Pasco

Section 21

Eldridge Wilde

Cosme Odessa

CROSS BAR RANCH

COSME

SOUTH PASCOCYPRESS CREEK

ELDRIDGE WILDE SECTION 21

NTB Overpumping ImpactsExcessive Groundwater

Pumping has Caused:

Decline in aquifer water levels

Lowered water levels in lakes,wetlands & springs

Formation of sinkholes

Reduced flow in river systems

Seawater intrusion

Dock on Florida Lake in 1970’s Same Dock in 1990

Surface Water Issues

Over 50,000 homeowners in South Pasco and North Hillsborough counties have been hit with massive land subsidence, as a result of over pumping.

Negative Impacts: Sinkholes

Sinkhole Formation• dissolution of soluble carbonate rocks by weakly

acidic water• the process starts in the atmosphere, where rain

falls on the ground and percolates through the soil

• dissolves carbon dioxide gas from the air and soil, forming carbonic acid (H2CO3), a weak acid

• carbonic acid percolates through the ground cover down to the bedrock

• carbonic acid reacts with limestone and dolomite and dissolves these carbonate rocks into component ions of calcium (Ca2+), magnesium (Mg2+) and bicarbonate (HCO3

-).

CO2

H2O

CaCO3

Atmosphere

CoverSediment

CarbonateBedrock

CO2

H2O

H2CO3CaMg(CO3)3

Mg++ Ca++ HCO3-

CO2 + H20 H2CO3

Negative Impacts: Sinkholes#

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0 10 20 Miles N

Sinkholes by year# 1960-1969# 1970-1979# 1980-1984# 1985-1989# 1990-1994# 1995-2000

Wetlands and Lakes in the NTB Area

5 0 5 10 MilesN

NTB CountiesLakesWetlands

Overpumping has negative effects on surface waters as well – wetlands in the area continue to dry out

Negative Impacts: Wetlands

Radius of Influence: South Pasco Wellfield

Camp

Thomas

Lake Levels vs. Pumping

“Thirsty Tampa Bay ponders huge desalination plant” April 20, 2000

• Want to build the largest desalination plant in

the Western Hemisphere

• Projected cost of $100 million

• Could supply about 25 MGD

(about 1/10 of the region's needs)

• Critics are concerned that the high salinity wastewater pumped back into the bay will hurt the environment.

Impact of Pumping on Heads

Eldridge Wilde Mitchell Effect of Pumping on Observed Heads

0

5

10

15

20

25

1100 1600 2100 2600 3100 3600

Time (Days)

Head (Ft)

0

8

16

24

32

40

Q (Ft3/Day)

ObservedHeads

PumpingRates

Rainfall/Recharge

53 rainfall observation pointsMonthly Readings from

January 1989-January 2000

Average Rainfall Observations

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Jan-89 Jan-90 Jan-91 Jan-92 Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98

Time

Inches/Month

1

Rainfall to Recharge

Assigned rain gages to basins & used recharge equation from previous studies:

Rech (node) = Radj * ((Rechss/P(b,ss))*P(b,m)

Radj = Runoff adjustment per basin per month Rechss = Recharge of node in May 1989 Steady State Model P(b,ss) = May 1989 Rainfall per Basin P(b,m) = Rainfall per basin per month

Uses of ModelingA model is designed to represent reality in such a way that

the modeler can do one of several things:

Quickly estimate certain aspects of a system (screening models, analytical solutions, “back of the envelope” calculations)

Determine the causes of an observed condition (contamination, subsidence, flooding)

Predict the effects of changes to the system (remediation, development, waste disposal)

Types of Ground Water ModelsAnalytical Models

1-D solution, Ogata and Banks (1961)

2-D solution, Wilson and Miller (1978)

3-D solutions, Domenico & Schwartz (1990)

Numerical ModelsFlow-only models (MODFLOW)

Transport-only models (MT3D, RT3D, MODPATH, etc.)

Require a coordinated flow model, such as MODFLOW

Combined flow and transport models (BIOPLUME, FEMWATER, FLOTRAN)

NTB Model & MODFLOW Three dimensional finite difference groundwater flow model

(McDonald & Harbaugh, 1988)

Simulates horizontal flow based on following inputs: Aquifer properties - Pumpage

Recharge - Evapotranspiration

River/spring flow - General Head Boundaries

Allows for vertical interchange between layers

Surficial Aquifer

Upper Floridan (1)

Upper Floridan (2)

NTB MODFLOW Model Description

GMS

Encompasses all/or part of five counties

1500 mile2 area

Variable grid spacing (0.25 - 1.0 miles2)

62 Rows & 69 Columns

Three layers

MODFLOW Cell-centered, 3D, finite difference groundwater flow

model

Iterative solver

Initial values of heads are provided

Heads are gradually changed through “time steps” until governing equation is satisfied

Divided into a series of packages

Each package forms a specific task

Each package stored in a separate input file

MODFLOWMODFLOW based on the following partial differential equation for

three-dimensional movement of groundwater of constant density through porous earth material :

Kxx, Kyy, and Kzz = hydraulic conductivity (x, y, and z axis)

h = potentiometric head

W = volumetric flux per unit volume pumped

Ss = specific storage of the porous material

t = time

t

hSW

z

hK

zy

hK

yx

hK

x szzyyxx ∂∂

=−⎟⎠

⎞⎜⎝

⎛∂∂

∂∂

+⎟⎟⎠

⎞⎜⎜⎝

⎛

∂∂

∂∂

+⎟⎠

⎞⎜⎝

⎛∂∂

∂∂

Recharge EquationAssigned rain gages to basins & used recharge

equation from previous studies:

Rech (node) = Radj * ((Rechss/P(b,ss))*P(b,m)

Radj = Runoff adjustment per basin per month

Rechss = Recharge parameter in May 1989 Steady State Model

P(b,ss) = May 1989 Rainfall per Basin

P(b,m) = Rainfall per basin per month

Runoff Parameter

Watersheds - Runoff AdjustmentAnclote RiverBrooker CreekCoastal RiversCypress CreekDouble Branch CreekHillsborough RiverPithlachascotee RiverRocky CreekSweetwater CreekWithlacoochee River

5 0 5 10 Miles

N

Runoff AdjustmentsWatersheds

1, 3, 7, 8, 9, 10

2, 4, 5, 6

Jun 0.8 0.7

Jul 0.5 0.4

Aug 0.4 0.3

Sep 0.4 0.3

Oct 0.4 0.3

Nov 0.5 0.4

Dec 0.5 0.5

Jan 0.6 0.5

Feb 0.7 0.6

Mar 0.8 0.7

Apr 1.0 0.9

May 1.0 1.0

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

Calibration Quotient

Thiessen Polygons for May 1989 precipitation

Recharge Parameter for Calibrated Model

5 0 5 MilesN

Recharge Rate Coefficient (ft/day)Low- 0.001Moderate - 0.004High - 0.009

MODFLOW InputsRecharge/Rainfall

Variable parameter, dependent on type of rainfall used in recharge calculation

Pumping Well DataOver 1500 wells used for pumping information

Starting HeadsStarting heads interpolated from May 2000 data, Inverse Distance

Weighted Method

Observation Coverage–544 Observation PointsCreated per layer in the grid for each month; data were obtained from

District monitoring wells

MODFLOW Inputs

Qualitative Analysis

Ending Heads for NxrdRaw run for Layer 2

Qualitative Analysis – NxrdRaw Layer 1

Quantitative Analysis

∑=

−=n

iico hh

nME

1

)(1

i

n

ico hh

nMAE ∑

=

−=1

1

2

2

11

22

11

2

1112

⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜

⎝

⎛

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛−

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛−

⎟⎠

⎞⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛−⎟

⎠

⎞⎜⎝

⎛

=

∑∑∑∑

∑∑∑

====

===

n

io

n

io

n

ic

n

ic

n

io

n

ic

n

ioc

hhnhhn

hhhhnr

where hc = computed head, ho = observed head

and n = number of observations

Quantitative AnalysisRainGage Run, May 01

r2 = 0.9131

-200

20406080

100120140

-20 0 20 40 60 80 100 120 140

observed head, ft

computed head, ft

NxrdRaw Run, May 01

r2 = 0.9144

-200

20406080

100120140

-20 0 20 40 60 80 100 120 140

observed head, ft

computed head, ft

NxrdAvg Run, May 01

r2 = 0.9128

-200

20406080

100120140

-20 0 20 40 60 80 100 120 140

observed head, ft

computed head, ft

NxrdGeo Run, May 01

r2 = 0.9129

-200

20406080

100120140

-20 0 20 40 60 80 100 120 140

observed head, ft

computed head, ft

NxrdTemp Run, May 01

r2 = 0.9121

-20

0

20

40

60

80

100

120

140

-20 0 20 40 60 80 100 120 140

observed head, ft

computed head, ft

The Major Aquifers of Texas

The Minor Aquifers of Texas

The Edwards Aquifer

The Edwards Aquifer

Pumpage to Date: 33,035.30 mg (million gallons)

Average Daily Pumpage: 144.26 mg

Minimum Edwards Level for 2000: 649.7’

Historic Minimum (8/17/56): 612.5’

Maximum Edwards Level for 2002: 690.5’

Historic Maximum (6/14/92): 703.3’

The Edwards Aquifer

When the limestone was exposed, it was extensively eroded creating cavities and conduits making it capable of holding and transmitting water

Then it was covered over with relatively impermeable sediments forming a confining unit

Geology of Edwards Aquifer

• Primary geologic unit is Edwards Limestone

• one of the most permeable and productive aquifers in the U.S.

• The aquifer occurs in 3 distinct segments:

-The drainage, recharge, and artesian zones

Drainage Zone of Edwards Aquifer

• located north and west of the aquifer in the region referred to as the Edwards Plateau or Texas Hill Country

• largest part of the aquifer spanning 4400 sq. miles

• water in this region travels to recharge zone

Recharge Zone of Edwards Aquifer

• Geologically known as the Balcones fault zone

• It consists of an abundance of Edwards Limestone that is exposed at the surface

-provides path for water to reach the artesian zone

Artesian Zone of Edwards Aquifer

• The artesian zone is a complex system of interconnected voids varying from microscopic pores to open caverns

• located between 2 relatively less permeable layers that confine and pressure the system

• underlies 2100 square miles of land

Artesian Wells• A well whose source of water is a confined aquifer

• The water level in artesian wells is at some height above the water table due to the pressure of the aquifer

•This level is the potentiometric surface and if it is above the land surface, it is considered a flowing artesian well

The Edwards Group

The Edwards Group

The Edwards limestone is between 300-700 ft. thick

Outcrops at the surface is tilted downward to the south and east and is overlain by younger limestone layers and thousands of feet of sediment The immense weight of this sediment layer

caused faulting in the region

Typical Dip Section

Regional Dip Section

Flowpaths of the Edwards Aquifer