Hydrogeological Behavior of Fractured Shallow Aquifers: a Case Study in the Vieira do Minho Granite Area (NW Portugal) Luís Macedo, Dr., Atrevo – Laboratório Ambiental, Lda.; Alberto Lima, Ph.D., Universidade do Minho Abstract Despite the poor “ingestion” capability classically admitted in hard rock environments, the renewable ground water resources quantified in this study area – about 50% of the rainfall – are capable of supplying the needs of 945 people/year consuming an average of 200 l/person/day. The convergence of shallow fast flow pathways and large recharge rates in the studied fractured granite aquifer determines a discreet ground water chemical signature evolution that reflects the occurrence of silicate weathering processes as well as the influence of concentration, by evaporation, of sea salts introduced by precipitation. 1. Introduction Ground water resources have an important role in suppressing public consumption needs and complementing classical distribution systems based on surface water sources, with quality and economic advantages. The present work comprises a hydrogeological study aiming the estimation of the renewable ground water resources in a selected region of NW Portugal, following the interest revealed by the local municipal entity – Câmara Municipal de Vieira do Minho – on its potential to supply isolated urban areas where the accentuated relief makes it very difficult to implement conventional distribution systems. Several methodological approaches were selected: spring hydrograph analysis, hydrometeorologic techniques (daily water budget and chloride tracer mass balance) and a geochemical study based on ground water and rainwater samples. 2. Framework 2.1. Geology and Geomorphology The support area of this study is located in Louredo (Fig. 1), Vieira do Minho region (NW Portugal). In this region, the geological setting is characterized by the occurrence of porphyiritic biotite-rich, coarse to medium coarse-grained granites. Also represented are the metamorphic rocks of Vila Nune unit (Ribeiro et al., 2000) in the NE sector, as well as the Holocenic fluvial deposits along the edge of many rivers such as the Ribª de Cantelães (Fig. 1). The study area corresponds to the western surface of a relief marked by two major faults with an NNE-SSW direction, cut by a third one, striking 34º East of North dipping 70º East. Several pegmatitic rock bodies of varying thickness with abundant quartz and tourmaline were registered. The present granite, known as the Vieira do Minho granite, is a biotite-rich coarse-grained granite with large dimensions feldspars. The plagioclase minerals present are zoned and have oligoclase-andesine compositions. The K-feldspar is microcline, microcline-perthite and perthite. Biotite occurs in large slabs (Noronha et al. 1983). 188 to 202
Transcript
Hydrogeological Behavior of Fractured Shallow Aquifers: a Case Study in the
Vieira do Minho Granite Area (NW Portugal)
Luís Macedo, Dr., Atrevo – Laboratório Ambiental, Lda.; Alberto Lima, Ph.D., Universidade do Minho
Abstract Despite the poor “ingestion” capability classically admitted in hard rock
environments, the renewable ground water resources quantified in this study area – about 50% of the rainfall – are capable of supplying the needs of 945 people/year consuming an average of 200 l/person/day. The convergence of shallow fast flow pathways and large recharge rates in the studied fractured granite aquifer determines a discreet ground water chemical signature evolution that reflects the occurrence of silicate weathering processes as well as the influence of concentration, by evaporation, of sea salts introduced by precipitation.
1. Introduction Ground water resources have an important role in suppressing public consumption
needs and complementing classical distribution systems based on surface water sources, with quality and economic advantages. The present work comprises a hydrogeological study aiming the estimation of the renewable ground water resources in a selected region of NW Portugal, following the interest revealed by the local municipal entity – Câmara Municipal de Vieira do Minho – on its potential to supply isolated urban areas where the accentuated relief makes it very difficult to implement conventional distribution systems. Several methodological approaches were selected: spring hydrograph analysis, hydrometeorologic techniques (daily water budget and chloride tracer mass balance) and a geochemical study based on ground water and rainwater samples.
2. Framework
2.1. Geology and Geomorphology
The support area of this study is located in Louredo (Fig. 1), Vieira do Minho
region (NW Portugal). In this region, the geological setting is characterized by the occurrence of porphyiritic biotite-rich, coarse to medium coarse-grained granites. Also represented are the metamorphic rocks of Vila Nune unit (Ribeiro et al., 2000) in the NE sector, as well as the Holocenic fluvial deposits along the edge of many rivers such as the Ribª de Cantelães (Fig. 1). The study area corresponds to the western surface of a relief marked by two major faults with an NNE-SSW direction, cut by a third one, striking 34º East of North dipping 70º East. Several pegmatitic rock bodies of varying thickness with abundant quartz and tourmaline were registered. The present granite, known as the Vieira do Minho granite, is a biotite-rich coarse-grained granite with large dimensions feldspars. The plagioclase minerals present are zoned and have oligoclase-andesine compositions. The K-feldspar is microcline, microcline-perthite and perthite. Biotite occurs in large slabs (Noronha et al. 1983).
188 to 202
Fig. 1. Geological aspects of the Vieira do Minho region: γπg – Vieira do Minho granite; a – recent fluvial deposits; δ – basic rock bodies; - Louredo study area; -
LRDE2 spring (adapted from sheets 5-B, 6-A, 5-D e 6-C of the Carta Geológica de Portugal at scale 1: 50 000).
2.2. Climate
The study area is inserted in the province of Minho in NW Portugal which is one of
the most pluvious areas of the Portuguese continent (Lima 2001). According to this author, the rainfall spatial distribution is strongly influenced by orography, having found a pluviometric gradient of 103 mm/100 m. At a regional scale, the rainfall concentrates in the October to March semester, although April and May are, generally speaking, quite pluvious constituting, in some cases, secondary spikes. Summer rainfall has a small contribute on the annual values and the wet period contrasts with a short but accentuated dry period. The study area falls into the 2300 mm/year of average annual rainfall value. As for the temperature, January is the coldest month and July and August register the highest temperatures. Lima (2001) refers the existence of a thermal gradient of -0,5ºC/100 m with an average annual temperature value of about 15ºC in the areas close to sea level and an average value of 10ºC in the more mountainous inner areas, although the temperature is shown to behave independent of altitude up to 70-80 m. The actual evapotranspiration values accompany the potential evapotranspiration, depicting an increase tendency from NE to SW, ranging from 720 mm/year to 750 mm/year.
3. Hydrodynamic characterization The discharge of the LRDE2 spring was monitored from July 23, 2003 to June 9,
2004 on a total of 323 days. The hygrograph obtained is represented in Fig. 2. The maximum value recorded was 182,083 m3/day and the minimum 6,839 m3/day, with an average value of 80,479 m3/day, attesting its good yield but also its seasonal character.
Fig. 2. LRDE2 spring hydrograph and daily rainfall from the Salamonde pluviometric
station.
From the beginning of the monitoring period to the end of October 2003 the spring discharge is characterized by a constant regular decrease. At this time, a sudden increase occurs in which the maximum value is registered and beyond this point the discharge displays a more irregular behavior expressing some vigorous variations until achieving again, towards the end of the monitoring period, a stable decrease tendency. The analysis of rainfall vs. discharge reveals a relatively ready response to the variations of hydraulic head associated to direct recharge from rainfall, especially under the influence of concentrated rainy events.
In the 47 weeks of monitoring the spring discharge, four recession periods are
identifiable (Fig. 3). The recession coefficients take on values of n.10-2 order which, according to Lima (1994, 2001), are compatible with aquifers associated to shallow underground flow pathways in more or less permeable systems connected to a weathered layer of variable thickness. The recession values evolution, over the time, depicts variations, even thought discrete. According to Wanielista (1990), a spring recession coefficient is rarely constant over time, a behavior to which the variation of the saturated geological materials properties contributes. In fact, the spring drainage seems to manifest two distinct behaviors: an initially slower drainage that, parallel to the increasing intensity and persistency of rainy events as well as the increasing discharge, gives place to faster drainage when the highest recession value is registered (-0,0253/day). The last recession period manifests a value close to the initial, although reflecting an even slower drainage. The described spring behavior should be interpreted as a result of vertical permeability variation of the saturated geological materials, more specifically, a reduction of the aperture and density of the fracture network. As such, faster drainage periods should be the result of a circulation materialized by wider and more permeable fractures, then
giving place to slower circulations which would occur in a less denser and less permeable fracture network with the contribute of the granite weathered layer.
Fig. 3. Recession periods for the LRDE2 spring and their respective coefficient values.
This incipient hydrogeological conceptual model seems to be supported by field
observations, namely the ground water electrical conductivity values measured in the samples collected which vary from 15,1 µS/cm to 18,3 µS/cm, thus reflecting an incipient mineral content.
4. Recharge
4.1. Daily water budget
The implementation of the daily water budget model to the Louredo study area
involved the use of a spreadsheet developed by Canas et al. (2003). Its utilization determined the input of several data: number of days in the months of the hydrologic year in appreciation, daily rainfall data; monthly evapotranspiration values, curve number, soil maximum storage capacity and soil moisture content in the beginning of the calculations.
The daily rainfall data was obtained directly from the Sistema Nacional de
Informação de Recursos Hídricos website regarding the Salamonde pluviometric station which was selected attending the proximity criteria as well as the altitude similarity. Evapotranspiration is a parameter dependent on numerous and highly variable factors very difficult to estimate. Given the fact that the detailed approach of this phenomena doesn’t express the main goal of this work, the values used in the daily water budget model were obtained from Lima (2001) using the Thornthwaite (1944) method in works in the near Braga region (Table 1).
Table 1. Monthly potential evapotranspiration values (ETP - mm) calculated for the
region of Braga by Lima (2001) through the Thornthwaite (1944) method. OCT NOV DEC JAN FEB MAR APR MAI JUN JUL AUG SEP YEAR
The maximum soil storage capacity of 100 mm used was estimated by Lima (2001) in similar studies conducted in the Braga region. Finally, the curve number can be consulted in the appropriated literature. The values were obtained from Lencastre and Franco (1992). The hydrologic soil type of the study area was classified under the B type on the Continental Portugal Soil Classification Chart and, based on the characteristics typified by the Soil Conservation Service, its was included in two types regarding its utilization, giving rise to two curve numbers. The average value of 69 was used, corresponding to medium soil moisture conditions, adjusted to 50 in extreme low moisture conditions and to 84 in very wet conditions.
Having applied this daily water budget model to the 323 days monitoring period,
the accumulated rainfall has a value of 1169 mm, of which 152 mm drained on the surface and 574 mm reached the aquifer – recharge. As such, the aquifer recharge expresses a value of 49% of the total rainfall and the runoff a value of 13%. The remaining 411 mm (35%) were returned to the atmosphere by evapotranspiration and 3% correspond to soil moisture change. The distribution of the recharge, rainfall and discharge values is represented in Fig. 4. There are two distinct periods of recharge: (i) the first period from late October 2003 to middle January 2004 that concentrates about 70% of the total recharge which occurs regularly; (ii) the second period involves scattered recharge events in March, April and May. The recharge events correspond, generally speaking, to the precipitation spikes, with some exceptions that reflect water stress periods after which the rainfall input is used in replenishing the soil moisture storage capacity and in the evapotranspiration process.
Fig. 4. Recharge, rainfall and LRDE2 discharge values (07/23/2003 to 06/09/2004).
The analysis of the LRDE2 discharge vs. recharge reveals that the increases in
discharge correlate well with the recharge events. In an attempt to calibrate the daily water budget model, namely the soil attributed parameters (curve number and maximum storage capacity), three recharge periods were chosen as well as the closest discharge increases that the analysis of Fig. 4 shows related. According to Custodio and Llamas (1983), aquifer drainage is described by an exponential law:
Qt = Q0e-αt (1)
where, Qt – spring discharge in time t (m3/day); Q0 – spring discharge in the beginning of the recession period (m3/day); α – recession coefficient (day-1); and t – time interval
considered (days). With the spring recession coefficient it is possible to estimate the volume of water stored above the drainage level at any time (Vt) based on the spring discharge:
Vt = Qt/α (2) Using both these expressions, the recharge based on the spring discharge was
estimated for the three recharge periods estimated by the daily water budget (Table 2) considering a recharge area of approximately 60 000 m2, determined by analysis of topographic maps. The recharge values estimated present encouraging similarities, especially in the 03/12-24/2004 period. For the other periods the differences are higher. However, in respect to the last period, it should be noted that the curve number used in the daily water budget for October 2003 reflects the poor moisture conditions calculated for September 2003. But, considering that in October 28 the soil achieves its maximum moisture storage capacity, an adjustment for optimum moisture conditions lowers the recharge value estimate to 306,67 mm, thus yielding a smaller difference to the value estimate based on discharge – 46,19 mm – from a previous percentage difference of 32% to 15%. The results obtained allow assuming that the soils conceptual model is fairly representative of the study area conditions.
Table 2. Recharge estimated for three periods by daily water budget and spring discharge
Recharge period considered
Recharge estimated by daily water budget
Recharge estimate based on spring discharge Ar - R
Due to its chemical and biological stability, chlorine can be considered as an almost
ideal tracer. According to Custodio and Llamas (1983), considering nil the chlorine quantity washed from the soil, introduced artificially and that there is equilibrium between formation and decay of organic matter, recharge can be estimated by the following expression:
Inf. =i
ps
C
C
P
E
−1 (3)
where, Inf. – Infiltration (%); Es – Surface runoff (mm); P – Rainfall (mm); Cp – Chlorine rainfall concentration (mg/l); and Ci – Chlorine ground water concentration (mg/l).
Table 3 expresses the statistics of the chlorine concentrations in the ground water and the rainfall samples collected. The variability of the ion concentrations observed in the rainfall contrast with the stability observed in the ground water as a result of the aquifer “assimilation” (Fig. 5). The variations in chlorine concentrations in precipitation could be related to several factors besides distance to the shore, such as: wind direction and velocity, intensity and duration of rainfall, rainfall type and industrial discharges. Using eq. 3, a recharge rate of 49% was estimated using the median Cl values and Es/P based on total rainfall and runoff estimated by daily water budget. Subtracting the fraction correspondent to the surface runoff, the actual evapotranspiration is estimated to be about 38% of the total rainfall.
Table 3. Statistics of Cl concentrations in rainfall (Cp) and ground water (Ci) samples collected between 12/10/2003 and 05/26/2004.
N = 16 Cp (mg/l) N = 16 Ci (mg/l) Average 1,78 Average 2,48 Median 1,41 Median 2,50 Standard deviation 1,19 Standard deviation 0,07
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
0 2 4 6 8 10 12 14 16 18 20 22 24
Week
Cl- (mg/l)Ci
Cp
Fig. 5. Chlorine concentration in rainfall (Cp) and ground water (Ci) samples collected
between 12/10/2003 and 05/26/2004.
5. Hydrogeochemistry
5.1. Rainwater chemistry
5.1.1. Major and minor constituents
Table 4 holds the statistical analysis of the 16 rainwater samples collected during
the monitoring period. The rainwater reveals a slightly acidic character and a strong dispersion of its chemical constituents. Na is the dominant cation, followed by K and Mg. Ca values are lower than the detection limit of the analytical method (700 µg/l) except in one sample. On the anions, the median composition reveals that Cl is the most abundant, followed by HCO3 and SO4. The temporal variability of the cations concentrations is paralleled by the anionic ones, except for the last sample in which SO4 and NO3 display a contrary trend that could be related to anthropic activities.
Table 4. Statistic on the chemistry of the rainwater samples collected (chemical
parameters in mg/l; EC – µS/cm; SD – Standard deviation; CV – Coefficient of variation)
N = 16 pH EC Na Mg K Ca NO3 SO4 Cl HCO3 Average 5,25 13,3 1,24 0,15 0,18 0,69 0,89 1,78 1,99 Median 5,12 13,4 0,93 0,12 0,16 0,53 0,63 1,41 1,33
Taking chlorine as a tracer element of marine origin, the contribution of the marine component of a given element in the rain [Xi(rain)] can be estimated by the expression:
Xi(rain) = (Xi/Cl)sea * Cli (rain) (4) The non marine contribution of a given element is then calculated by subtracting the
marine input (Eq. 4) to the median concentration in the rainwater. The results are expressed in Table 5.
Table 5. Rainwater median chemical composition according to the origin of the main
HCO3 0,008 1,330 0,011 0,8% 1,319 99,2% Na, Mg and Cl witness the influence of sea salts in the rainwater composition. The
good correlation between Na, Cl and Mg seems to support a common source for these elements (Fig. 6).
Fig. 6. Relationship between Na, Mg, Cl, SO4 and NO3 values in rainwater. On the other hand, SO4, K and Mg witness the continental effect. The correlation
between NO3 and SO4 (Fig. 6) suggests a common source that could be related to anthropogenic activities, such as the burning of fossil fuels and/or the agricultural activity.
5.2. Ground water
5.2.1. Main constituents
Na = 0,7473Cl - 0,0897
R2 = 0,9393
Mg = 0,0772Cl + 0,0098
R2 = 0,7875
0,00
0,50
1,00
1,50
2,00
2,50
3,00
3,50
4,00
0,00 1,00 2,00 3,00 4,00 5,00Cl (mg/l)
Na, Mg
(mg/l)
SO4 = 1,7229NO3 - 0,3078
R2 = 0,8348
0,00
0,50
1,00
1,50
2,00
2,50
3,00
3,50
0,00 0,50 1,00 1,50 2,00
NO3 (mg/l)
SO4 (mg/l)
The statistic of the ground water samples (Table 6) shows a good stability when compared to rainwater. Such as in the rainwater, Ca in ground water samples is very low, usually lower than 700 µg/l, except for the LRD-N12 sample (Ca = 0,72 mg/l).
Table 6. Statistic of ground water samples main composition (chemical parameters in mg/l; electrical conductivity in µS/cm; SD – Standard deviation; CV – Coefficient of
variation). pH EC SiO2 Na Mg K Ca Sr HCO3 NO3 SO4 Cl TDS
Given the geology of the area and the common silicates weathering processes, the
pH values suggest a reduced water-rock contact time not favoring hydrolysis reactions. Indeed, the median value of ground waters (5,8) is only slightly higher than that of rainwater (5,12), thus suggesting a little buffering action of silicate hydrolysis. The EC values reflect the waters poor mineral content along with an average TDS value of 18,6 mg/l. The ground waters EC maximum value (18,3 µS/cm) is lower than that of rainwater (35,7 µS/cm), but their median values are close. Na is the main cation present followed by K and Mg. HCO3 and Cl are two of the main constituents and present a good stability. According to Hounslow (1995), in opposition to open systems such as superficial waters in which the HCO3 content tends to diminish, in ground water, it tends to remain constant. As for the Cl, its origin should be related to concentration by evaporation of rainwater given its conservative statute that allows its wide use as a natural tracer. SiO2 is the main constituent and has a median participation of about 49% in the TDS content.
As for the chronological evolution of the main constituents, Na, Mg and K (Fig. 7)
manifest an increase trend paralleled by SO4 and NO3 (Fig. 8) in a less pronounced form. HCO3 depicts a very stable behavior (Fig. 8). The data points towards a discreet geochemical evolution of the ground water samples through the monitoring period.
Fig. 8. NO3, SO4 and HCO3 values in the ground water samples collected.
5.2.2. Hydrochemical evolution
The correlation between SiO2 and TDS as well as the correlation between SiO2 and
the sum of cations points towards the participation of silicates weathering in the ground water chemical evolution (Fig. 9). Nevertheless, the slight variation of the major constituents as well as the low pH values reflects a discreet weathering process. SiO2 values manifest a correlation with Na, Mg and K (Fig. 10). However, the correlation between SiO2 and K should be interpreted carefully, because of the low correlation coefficient value. As it can be seen in Fig. 10, the samples form two groups. The first one, a less concentrated group, is comprised by the initial 6 samples collected (from 12/10/2003 to 01/28/2004) and the latter, a more concentrated group, comprehends the remaining samples collected. The gap between the two groups is a consequence of absence of rainfall, totalizing 42 days without any sample collection. The analysis of the rainwater distribution points towards a seasonality effect that reflects the diminishing pluvious events that, together with slower flows and increasing evapotranspiration values, favour water-rock interaction and explain the cations increasing concentrations.
Fig. 9. Relationship between SiO2, Σcations and TDS in ground water.
Σcations = 0,197SiO2 + 1,4239
r = 0,96
2,5
2,7
2,9
3,1
3,3
3,5
3,7
7 7,5 8 8,5 9 9,5 10 10,5 11
SiO2 (mg/l)
Σcations
(mg/l)
TDS= 1,3377SiO2 + 6,2674
r = 0,995
14
15
16
17
18
19
20
21
7 8 9 10 11
SiO2 (mg/l)
TDS(mg/l)
SiO2:Na: r = 0,9542; Na = 1,128 + 0,177*SiO2
SiO2:Mg: r = 0,9328; Mg = 0,0973 + 0,0121*SiO2
SiO2:K: r = 0,8206; K = 0,1355 + 0,0149*SiO2
7,0 7,5 8,0 8,5 9,0 9,5 10,0 10,5 11,0
SiO2(mg/l)
2,3
2,4
2,5
2,6
2,7
2,8
2,9
3,0
3,1N
a (m
g/l)
0,16
0,18
0,20
0,22
0,24
0,26
0,28
0,30
0,32
Mg a
nd K
(m
g/l)
Na(L)
Mg(R)
K(R)
Fig. 10. SiO2 versus Na, Mg and K values in ground water samples.
The data seems to support the occurrence of silicates weathering which should be a
consequence of hydrolysis reactions. The formation of carbonic acid in the root zone, where the CO2 partial pressure is higher, should favor that chemical process. This is consistent with the granitic nature of the geologic medium. As such, the abundance of Na should be related to the Na-plagioclases weathering (Eq. 5). K and Mg should proceed from biotite weathering (Eq. 6). Furthermore, the correlation between these constituents supports their common source (Fig. 11).
The correlations between SO4 and Mg (r = 0,86) between SO4 and NO3 (r = 0,82)
suggest the possibility of a common origin probably related to the contribute of the decay of organic matter (grazing animals droppings) or even the weathering of N and S bearing minerals such as, according to Reimann and Caritat (1998), the K-feldspars or biotite. Still regarding SO4, it is noted that the ground water values are inferior to those present in rainfall (median value of 0,63 mg/l). S is a plant macronutrient and SO4 is the main source (Raven, 1999). As such, its absorption at a root level of the plants could explain this reduction in the ground water.
NaAlSi3O8 + H+ + 2
9H2O =
2
1 Al2Si2O5 (OH)4 + Na+ + 2Si(OH)4 (5)
(Albite) (Kaolinite)
KMg3AlSi3O10(OH)2 + 7H+ + 2
1 H2O =
2
1 Al2Si2O5 (OH)4 + K+ + 3Mg2+ + Si(OH)4 (6)
(Biotite) (Kaolinite)
K = 1,17Mg + 0,028
r = 0,84
0,2
0,22
0,24
0,26
0,28
0,3
0,32
0,15 0,17 0,19 0,21 0,23 0,25
Mg (mg/l)
K (mg/l)
Fig. 11. Relationship between Mg and K in ground water.
Considering Cl as the conservative element in the infiltration process, the
expression (4), used to determine the marine contribution in the rainwater chemical composition, was applied, making the necessary adjustments, to estimate the water-rock interaction contribution in the ground water chemical signature. Table 7 holds the results obtained. Within the major cations, Na manifests the highest water-rock contribute which should be materialized by the Na-plagioclases weathering, whereas Mg depicts a low water-rock interaction contribution. As such, the weathering of biotites and other magnesium iron silicates should produce a discreet contribute. As for K, the negative value registered must be analyzed carefully taking into account the preferential participation of this ion in the almost irreversible exchange processes with clay minerals that, along with the calculation difficulties related to the strong dispersion of the rainwater chemical composition, difficult clearer interpretations as to the water-rock interaction contribution. The origin of Sr is almost equally distributed by rainfall introduction and by water-rock interaction. In fact, this minor constituent can occur in granite primary minerals such as feldspars and micas and has high mobility under acid and oxidizing conditions (Reimann and Caritat, 1998) like those prevailing in the study environment. As for NO3, its origin should be mainly related to introduction trough rainfall although a small fraction (15%) could be associated to some contamination resulting from organic matter decay, namely the excrements of the animals that usually graze in the vicinities. As said previously, the NO3 contribute of the weathering of N bearing silicates is not excluded, yet this constituent presents strongly reduced values. SiO2 should be exclusively originated from the silicate weathering as previously discussed. Also noted regarding the origin of the ground water constituents, is the importance of concentration, by evaporation, of seal salts such as Na, Mg and, naturally, Cl.
Table 7. Ground water average chemical composition according to the origin of
Further investigation to the role of Na, Mg and also K in the ground water mineral content involves the analysis of their concentrations together with the discharge of the spring where the ground water samples were collected (Fig. 12). Na presents a good negative correlation trend (r = -0,89) with the spring discharge which becomes even more clear beyond the end of January 2004 as a consequence of the stabilization of the spring decreasing discharge rate. Slower drainage corresponds to lower discharge values. In these circumstances, the longer period of the water-rock interaction favors the occurrence of weathering reactions with the most unstable minerals and, as a consequence, the water mineral content is enriched. In this case, the plagioclase hydrolysis reactions are favored in these conditions. Mg also manifests the same behavior towards the spring discharge with a high correlation coefficient (r = -0,87). As well as in the Na case, the slower drainage favors water-rock interaction and the biotite hydrolysis reactions. As a result, the ground water mineral content is enriched with this constituent. As for K, the negative correlation with the spring discharge (r = -0,63) seems to indicate the contribute of the weathering of biotites or even K-feldspars, although its role in the ground water chemical evolution is more difficult to interpret in light of the considerations previously made.
Na (mg/l)
Mg (mg/l)
Discharge (m3/day)
12/05/03
12/25/03
01/14/04
02/03/04
02/23/04
03/14/04
04/03/04
04/23/04
05/13/04
06/02/04
60
80
100
120
140
160
180
200
Dis
charg
e (m
3/d
ay)
2,20
2,40
2,60
2,80
3,00
3,20
3,40
3,60
Na (m
g/l)
0,185
0,190
0,195
0,200
0,205
0,210
0,215
0,220
0,225
0,230
0,235
Mg (m
g/l)
Fig. 12. Na and Mg in ground water samples versus spring discharge.
5.2.3. Water-rock equilibrium
The study of the water-rock equilibrium demonstrates that the ground water is
undersaturared to the entire mineral phases considered (Table 8) except for kaolinite, gibbsite and quartz. The values attest the ground water poor mineral content. The K-feldspar and albite saturation index depict a correlation trend (r = 0,83 and r = 0,79, respectively) with the SiO2 contents, thus supporting the claim made relative to silicates weathering in the ground water mineral content evolution. Furthermore, Na and K manifest a correlation trend with the kaolinite saturation index suggesting the importance of this clay mineral as a result of silicate weathering. In fact, given the apparent good drainage conditions of the study area, the fact that kaolinite is the preferred clay mineral in the granites weathering process is easily admitted.
Table 8. Statistic on the ground water samples saturation indexes relative to some mineral phases; (a) – amorphous; SD – Standard deviation; CV – Coefficient of variation.
The recharge estimates produced by the different methods express interesting
similarities – about 50% of the rainfall. This rate seems to oppose the classical poor “ingestion” capability attributed to granite aquifers. Despite the accentuated relief in which the drained aquifer is located, the existence of permeable structures (fractures) facilitates the infiltrating process. The vertical variation of the permeability of the geological materials is translated by the spring different recession periods and, parallel to the ground water electrical conductivity, seems to support this hydrogeological conceptual model of a fast, shallow flow pathway circuit. Considering an average rainfall value for the spring recharge area of 2300 mm/year (Lima 2001) and the aquifer recharge area (60 000 m2), the renewable ground water resources of the study area are capable of supplying the needs of 945 people/year consuming an average of 200 l/person/day.
The convergence of fast flow pathways and accentuated recharge values determines
an incipient mineral content. Despite the extraordinary chemical stability, the ground water evolution reflects, nonetheless, an enrichment that seems to be related to two main processes: (i) concentration, by evaporation, of the rainwater and (ii) water-rock interaction. The contribution of the last process in the water salinity is witnessed by the variation of SiO2, Na, and Mg. It is admitted that the hydrolysis of the granite primary minerals such as plagioclases, biotites and eventually other iron magnesium silicates, materialize the process by which the geologic medium participates in the ground water chemical evolution that, in Na case, is estimated to achieve a value around 41%. Na, Mg and Cl attest the importance of sea salts introduced by precipitation in the ground water chemical composition. Given the fractured nature of the aquifer, is admitted that the aeration zone should play an important role in the ground water geochemical evolution.
References
Canas A, Lima A S, Fonseca J, 2003, Quantificação dos Recursos Hídricos Subterrâneos da Região de Montalegre (Norte de Portugal). Ciências da Terra, Volume Especial
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Author’s biographical sketches and addresses
Luís Macedo is a graduate in Biology and Geology by the University of Minho – Portugal and has developed an investigation on fractured shallow granite aquifers for is MSc degree. Mail address: Atrevo - Laboratório Ambiental de Estudos, Intervenções e Consultadoria, Lda., Rua do Eiteiro, n.º76, 4705-593 Ruílhe E-mail: [email protected] Alberto S. Lima is a graduate in Biology and Geology Teaching from University of Minho, Braga, 1986, Portugal, where he returned later to take his MSc (1994) and Ph.D. (2001) in Hydrogeology of Hard Rocks. In addition to the research developed at University of Minho, especially devoted to thermal and mineral groundwater, he’s teaching also many courses in graduate and postgraduate programs. Mail address: Departamento de Ciências da Terra, Universidade do Minho, 4710-057 Braga, Portugal E-mail: [email protected]
