Groundwater geochemistry and identification of hydrogeochemical processes in a hard rock region, Southern India PDF

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Environ Monit Assess (2010) 162:123–137 DOI 10.1007/s10661-009-0781-4 Groundwater geochemistry and identification of hydrogeochemical processes in a hard rock region, Southern India T. Subramani · N. Rajmohan · L. Elango Received: 12 May 2008 / Accepted: 27 January 2009 / Published online: 28 Februa...

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Environ Monit Assess (2010) 162:123–137 DOI 10.1007/s10661-009-0781-4

Groundwater geochemistry and identification of hydrogeochemical processes in a hard rock region, Southern India T. Subramani · N. Rajmohan · L. Elango

Received: 12 May 2008 / Accepted: 27 January 2009 / Published online: 28 February 2009 © Springer Science + Business Media B.V. 2009

Abstract Hydrogeochemical investigations were carried out in Chithar River basin, Tamil Nadu, India to identify the major geochemical processes that regulate groundwater chemistry. For this study, long-term (1991–1997) and recent water quality data (2001–2002) for 30 groundwater wells spread over the study area were used to understand the groundwater geochemistry and hydrogeochemical process regulating groundwater quality. Groundwater quality data obtained from more than 400 water samples were employed. Results of electrical conductivity and chloride express large variation between minimum and max-

T. Subramani Department of Civil Engineering, Government College of Engineering, Salem, 636 011, Tamil Nadu, India e-mail: [email protected] N. Rajmohan Department of Waste Treatment and Conditioning Research, Nuclear Energy Division, Commissariat a l’energie atomique (CEA), Centre De Valrho/Marcoule, BP 1717F 30207 Bagnols-Sur-Ceze Cedex, France e-mail: [email protected] L. Elango (B) Department of Geology, Anna University, Chennai, 600 025, Tamil Nadu, India e-mail: [email protected] URL: www.geocities.com/elango

imum values and high standard deviation, which suggests that the water chemistry in the study region is not homogeneous and influenced by complex contamination sources and geochemical process. Nitrate and depth to water table expose the influences of surface contamination sources, whereas dissolved silica, fluoride and alkalinity strongly suggest the effect of rock–water interaction. In the study region, weathering of carbonate and silicate minerals and ion exchange reactions predominantly regulate major ion chemistry. Besides, the concentrations of sulphate, chloride and nitrate firmly suggest the impact of agricultural activities such as irrigation return flow, fertiliser application, etc on water chemistry in the study region. Keywords Groundwater · Geochemistry · Hydrogeochemical processes · Water–rock interaction · Chithar River basin · Tamil Nadu · India

Introduction Chemistry of groundwater is an important factor determining its use for domestic, irrigation and industrial purposes. Interaction of groundwater with aquifer minerals through which it flows greatly controls the groundwater chemistry. Hydrogeochemical processes that are responsible for

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altering the chemical composition of groundwater vary with respect to space and time. In any area, groundwater has unique chemistry due to several processes like soil/rock–water interaction during recharge and groundwater flow, prolonged storage in the aquifer, dissolution of mineral species, etc. (Hem 1985). Earlier studies reported the importance of hydrogeochemical studies of groundwater in a particular region (Sikdar et al. 2001; Apodaca et al. 2002; Elango et al. 2003; Tesoriero et al. 2004; Devadas et al. 2007; Möller et al. 2007), and hydrogeochemical studies will help to create suitable management plans to protect aquifer as well as remedial measures for contaminated groundwater by natural and manmade activities. Moreover, detailed knowledge about geochemical process regulates groundwater chemistry is necessary to overcome the groundwater-related issues. Some of the previous studies carried out in Chithar River basin focused on groundwater potential assessment (Balasubramanian and Sastri 1994), groundwater level fluctuation and quality monitoring (Public Works Department (PWD), Tamil Nadu, India; PWD 2002), groundwater quality and its suitability for drinking and agricultural purposes (Subramani et al. 2005a) and study on the occurrence of various rock types and their mineral composition (including petrographic studies; Subramani et al. 2005b). Hence, it is apparent that the hydrogeochemical processes that control the groundwater chemistry of the basin have not been attempted in the previous studies. As groundwater is being continuously exploited in this basin to meet the demand for water supply and irrigation due to insufficiency of surface water storage in reservoirs and non-perennial River, it is essential to know the hydrogeochemical processes that take place in the aquifer system. Thus, the present work was carried out with the main objective of identifying the major hydrogeochemical processes that are responsible for groundwater chemistry in the study region.

Study area Chithar River basin is located in the extreme south of Tamil Nadu state, India, between lati-

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tudes 8◦ 48′ to 9◦ 14′ N and longitudes 77◦ 11′ to 77◦ 46′ E (Fig. 1). It occupies an area of 1,722 km2 , and the regional slope of the basin is towards east. The plain lands of this basin fall under semi-arid climatic type, and the areas adjacent to Western Ghats are of dry to moist sub-humid climatic types (Rammohan 1984). Hence, the study area experiences mostly dry climatic condition with average maximum temperature of 39◦ C during the months of April and May and average minimum temperature of 24◦ C during the months of November and December. The average annual rainfall recorded at eight rain gauge stations spread over the basin is 917.88 mm. Among which, northeast monsoon from October to December contributes almost 70% of the total rainfall and the rest by southwest monsoon from June to September. Considerable amount of rain showers are also received during the transitional period.

Geology and hydrogeology The Proterozoic (Post-Archaean) basement of the area consists of quartzite, calc-granulite, crystalline limestone, charnockite and biotite gneiss with or without garnet (Yoshida 1992; GSI 1995; Jayananda et al. 1995). Kankar (lime-rich top soil) observed in few places are of recent to sub-recent age. Quartzite shows NW–SE trend in most of the places with numerous NS minor joints. In some places, it is associated with calc-granulite and crystalline limestone. Calc-granulite and crystalline limestone also show the NW–SE trend. Calciticand dolomitic-limestones are being quarried in a few places for cement industries. Charnockite outcrops as massive hills in the western part of the basin. Garnetiferous charnockites are also noticed in a few well cuttings in the eastern part. Most of the basin is composed of biotite gneiss with or without garnet. The different lithological units encountered in the Chithar River basin are illustrated in the Fig. 2. A thin layer of topsoil, varying between 1 to 1.5 m in most of the places, overlies the basement rocks. Three major soil types were identified in the study area: black cotton, deep red and red sandy soils. Lithological cross sections plotted along X–X′ , Y–Y′ and X–Z (see Fig. 2) are illustrated in the Figs. 3, 4 and 5.

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Fig. 1 Chithar River basin and location of monitoring wells

Groundwater recharge, transmission and discharge of a basin are controlled by the basin geomorphology, geology and structural patterns of a region (Salama et al. 1994). It is inferred from the cross sections that the occurrence of weathered zone is not uniform in both space and depth. Weathered zone thickness ranges from 8 to 34 m below ground level. Groundwater in this basin occurs under water table conditions in weathered crystalline formations. In the study area, depth to water table varies between 2.1 and 16 m during pre-monsoon while between 0.91 and 16 m during post-monsoon. Overall view of groundwater level fluctuation and the hydrograph results suggest that the water table tends to rise during November and December to reach the peak in January and

starts declining from February onwards to reach the valley in September or October (Subramani 2005). Aquifer parameters of various geological formations established by the PWD (PWD 2002) from 30 bore wells spread over the basin reveals that hydraulic conductivity varies from 0.1 to 24 m/day in weathered rock formations.

Materials and methods Groundwater sampling and analysis Based on detailed well inventory survey in Chithar River basin, 30 representative groundwater wells (Created by PWD, Government of

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Fig. 2 Geology of Chithar River basin

Fig. 3 Lithological cross-section along X–X′

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Tamil Nadu, India for regular monitoring of water level (monthly) and water quality (Bi-annual)) were selected for groundwater sampling program (Fig. 1). Most of these wells are shallow irrigation wells, and groundwater sampling campaigns were carried out during July 2001 and 2002. Twentyfour samples during July 2001 and 18 samples during July 2002 were collected from the 30 representative wells. In overall, 42 samples were collected for chemical analysis. Field parameters such as electrical conductivity (EC), pH and temperature were measured in the field using portable meters. Groundwater samples collected from these wells were transported to the laboratory in the same day and filtered using 0.45 µm Millipore filter paper. The cations and silica samples were acidified to pH < 2 with several drops of

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Rock sampling and analysis Detailed geological field survey was also carried out in the basin to relate the groundwater chemistry with lithology. Thirty-nine rock samples were collected from outcrops, quarries and well cuttings in the field. Similar rock types were grouped together in the laboratory after megascopic studies. Among them, 25 samples were chosen for thin section preparation (microscopic studies) to understand the petrographic nature of various rocks. Data collection and analysis Long-term groundwater quality data (from 1991 to 1997, pre-monsoon (July) and post-monsoon (January) periods, 361 samples) for the same PWD wells (30 wells used for 2001 and 2002 groundwater sampling) were collected from PWD to understand the long-term variation in groundwater chemistry and hydrogeochemical process (PWD 2004). This long-term water quality data are considered as secondary data (SD).

Fig. 4 Lithological cross-section along Y–Y′

ultra-pure HCl in the laboratory and then refrigerated at 4◦ C until analysis. Samples were analysed for major ions, nutrients, silica and minor ions in the laboratory using the standard methods (APHA 1995). Samples were analysed for sulphate, nitrate, phosphate, fluoride and silica using UV/visible spectrophotometer; sodium, potassium using flame photometer and calcium, magnesium, chloride and alkalinity (HCO3 + CO3 ) by the titration technique. Overall, measurement reproducibility and precision for each analysis were less than 2%. The analytical precision for the total measurements of ions was checked again by calculating the ionic balance errors and was generally within ±5%.

Fig. 5 Lithological cross-section along X–Z

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Results and discussion Groundwater geochemistry Groundwater samples of this basin are alkaline, with a pH ranging between 6.9 and 9.2, showing an average value of 8.3 (Table 1). Electrical conductivity ranges from 79 to 9,300 µS/cm with an average of 1,595 µS/cm. The statistical summary of water quality parameters is given in Table 1. Table 1 illustrates that EC and chloride show large variation between minimum and maximum value and also express high standard deviation. This inference not only suggests that the water chemistry in the study region is not homogeneous but also reveal the influences of complex contamination sources and geochemical process. Moreover, nitrate and depth to water table expose the influences of surface contamination sources, whereas dissolved silica, fluoride and alkalinity strongly suggest the effect of rock–water interaction (Table 1). Table 1 also shows a variation in the mean values of chemical parameters between primary and secondary data. The mean and median values of EC and major ions are higher in primary data compared to the secondary data, which apparently suggests the enhancement of solute load in the aquifer.

Hydrogeochemical processes Reactions between groundwater and aquifer minerals have a significant role on water quality, which are also useful to understand the genesis of groundwater (Cederstorm 1946). As mentioned earlier, groundwater chemistry in the study region is regulated by diverse processes and mechanisms. Since the study region experiences dry and semiarid climatic condition, evaporation may also contribute in water chemistry in the study region. Hence, Gibbs plot is employed in this study to understand and differentiate the influences of rock– water interaction, evaporation and precipitation on water chemistry (Gibbs 1970). Figure 6 illustrates that most of the groundwater samples of the Chithar River basin fall in the water–rock interaction field and few samples plotted on evaporation zone, which suggests that the weathering

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of rocks primarily controls the major ion chemistry of groundwater in this region. Therefore, the concentrations of major ions in groundwater and the mineralogy of different rocks have been used to determine the source of these major ions to the groundwater and their relation to regional geology and weathering processes.

Carbonate weathering and dissolution Calcium and magnesium are the dominant cations present in groundwater next to sodium in this region. Similarly, bicarbonate is also present in considerable amounts. Carbonate-rich rocks such as crystalline limestone, dolomitic limestone, calcgranulite and kankar (lime-rich weathered mantle overlies carbonate rocks) are the major sources for carbonate weathering. The available carbonates in these rocks might have been dissolved and added to the groundwater system during irrigation, rainfall infiltration and groundwater movement. In Ca2+ + Mg2+ vs alkalinity + SO4 2− scatter diagram (Fig. 7a), the points falling along the equiline (Ca2+ + Mg2+ = alkalinity + SO4 2− ) suggest that these ions have resulted from weathering of carbonates and sulphate minerals (gypsum or anhydrite; Datta and Tyagi 1996). Moreover, if the Ca2+ and Mg2+ solely originated from carbonate and silicate weathering, these should be balanced by the alkalinity alone. However, most of the points are placed in the Ca2+ + Mg2+ side, which indicates excess calcium and magnesium derived from other process such as reverse ion exchange reactions. In silicate terrain, if the calcium and bicarbonate in groundwater are solely originated from calcite, the equivalent ratio of dissolved Ca2+ and HCO3 − in the groundwater is 1:2, whereas from dolomite weathering, it is 1:4 (Garrels and Mackenzie 1971; Holland 1978). Similarly, if the calcium and sulphate in groundwater derived from dissolution of gypsum or anhydrite, then the Ca2+ /SO4 2− ratio is almost 1:1 (Das and Kaur 2001). In Ca2+ vs Alkalinity scatter diagram (Fig. 7b), some groundwater samples follow the 1:2 and 1:4 lines and indicate the contribution of both calcite and dolomite weathering on groundwater chemistry in this basin. Moreover, in Ca2+

Parameter Primary (2001 and 2002)

Secondary (1991–97)

Total

Minimum Maximum Mean Med Standard deviation n Minimum Maximum Mean Med Standard deviation n Minimum Maximum Mean Med Standard deviation n

EC (µS/cm)

pH

Wl (mbgl)

Na (mg/l)

K (mg/l)

Ca (mg/l)

Mg (mg/l)

Cl (mg/l)

Alk (meq/l)

SO4 (mg/l)

NO3 (mg/l)

SiO2 (mg/l)

F (mg/l)

PO4 (mg/l)

80.0 9,040 1,838 1,225 1,661 42 79.0 9,300 1,566 1,150 1,354 361 79.0 9,300 1,595 1,160 1,389 403

6.9 9.2 8.0 8.0 0.5 42 7.0 9.2 8.4 8.4 0.3 258 6.9 9.2 8.3 8.4 0.4 300

2.60 17.2 8.28 7.14 4.10 34 0.91 16.4 7.10 6.65 4.09 160 0.91 17.2 7.30 6.80 4.10 194

4.00 897 133 106 140 42 4.00 840 146 92.0 134 361 4.00 897 145 92.0 134 403

2.00 86.0 13.6 10.5 14.7 42 BDL 297 21.4 9.00 35.7 361 BDL 297 20.6 9.00 34.2 403

10.0 880 120 66.0 167 42 4.00 620 78.2 46.0 93.7 361 4.00 880 82.6 48.0 104 403

2.00 485 78.1 54.0 85.1 42 2.00 462 67.6 49.0 60.4 361 2.00 485 68.7 50.0 63.4 403

7.00 2,482 391 213 466 42 6.50 2,552 336 209 361 361 6.50 2,552 342 209 373 403

0.30 8.70 4.15 3.39 2.22 42 0.10 14.8 3.56 3.11 2.06 361 0.10 14.8 3.62 3.20 2.08 403

BDL 576 94.5 60.0 119 42 BDL 624 75.4 46.0 83.9 361 BDL 624 77.4 48.0 88.2 403

BDL 56.0 17.9 10.0 17.2 42 NA NA NA NA NA NA BDL 56.0 17.9 10.0 17.2 42

20.9 112 70.8 71.2 19.3 42 NA NA NA NA NA NA 20.9 112 70.8 71.2 19.3 42

0.17 1.30 0.59 0.60 0.28 42 NA NA NA NA NA NA 0.17 1.30 0.59 0.60 0.28 42

BDL 0.89 0.06 0.00 0.18 42 NA NA NA NA NA NA BDL 0.89 0.06 0.00 0.18 42

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Table 1 Summary of water quality parameters

n number of samples analysed, Wl groundwater level, Med median, NA not analysed, BDL below detection limit

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130 Fig. 6 Gibbs plots explain groundwater chemistry and geochemical process in the study region

Environ Monit Assess (2010) 162:123–137 Primary data

100000

Secondary data

100000

10000

tio

n

ra

o ap

o ap

Ev

TDS (mg/l)

TDS (mg/l)

1000

Rock water interactio n 100

Rock water interactio n 100

Pre c

ipi

10

Pre c

tat

ion

ipi

10

1

tat

ion

1 0.0

0.2

0.4 0.6 (Na+K)/(Na+K+Ca)

0.8

1.0

0.0

Primary data

25

0.2

0.4 0.6 Cl/(Cl+Alk)

0.8

1.0

Secondary data

20 18

20

10

5

ne

14

1:2 li

1:1 li

Alkalinity (meq/l)

ne

16

15

b)

1:4 line

a) Alkalinity+SO4 (meq/l)

n

Ev

1000

Fig. 7 Relation between Ca, Mg, SO4 and alkalinity in the groundwater

tio

ra

10000

12 10 8 6 4 2

0

0 0

20

40

60

80

100

Ca+Mg (meq/l)

c) 1:1

lin e

1:2 li ne

16

SO4 (meq/l)

14 12 10 8 6 4 2 0 0

10

20

30

Ca (meq/l)

10

20

30

Ca (meq/l)

20 18

0

40

50

40

50

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vs SO4 2− scatter diagram (Fig. 7c), few samples   fall along the 1:1 equiline Ca2+ = SO4 2− , and most of them show excess calcium over sulphate. Groundwater samples following the 1:1 equiline seem to be derived from gypsum or anhydrite dissolution, whereas excess calcium highlights additional geochemical process. Similarly, excess sulphate over calcium in few samples expresses the removal of calcium from the system likely by calcite precipitation. Mayo and Loucks (1995) explained that if Ca2+ / Mg2+ molar ratio is equal to one, dissolution of...