Groundwater quality and its suitability for drinking and agricultural use ...

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Environ Geol (2005) 47: 1099–1110 DOI 10.1007/s00254-005-1243-0

T. Subramani L. Elango S. R. Damodarasamy

Received: 29 July 2004 Accepted: 4 January 2005 Published online: 9 April 2005  Springer-Verlag 2005

T. Subramani Æ S. R. Damodarasamy Department of Civil Engineering, Government College of Engineering, Tirunelveli, 627 007, Tamil Nadu, India L. Elango (&) Department of Geology, Anna University, Chennai, 600 025, Tamil Nadu, India E-mail: [email protected] Tel.: +91-44-22203311 Fax: +91-44-22352870 URL: www.geocities.com/elango

ORIGINAL ARTICLE

Groundwater quality and its suitability for drinking and agricultural use in Chithar River Basin, Tamil Nadu, India

Abstract Hydrochemistry of groundwater in Chithar Basin, Tamil Nadu, India was used to assess the quality of groundwater for determining its suitability for drinking and agricultural purposes. Physical and chemical parameters of groundwater such as electrical conductivity, pH, total dissolved solids (TDS), Na+, K+, Ca2+, Mg2+, Cl), HCO3), CO32), SO42), NO)3, F), B) and SiO2 were determined. Concentrations of the chemical constituents in groundwater vary spatially and temporarily. Interpretation of analytical data shows that mixed Ca–Mg–Cl, Ca–Cl and Na–Cl are the dominant hydrochemical facies in the study area. Alkali earths (Ca2+, Mg2+) and strong acids (Cl), SO42)) are slightly dominating over alkalis (Na+, K+) and weak acids

Introduction Quality of groundwater is equally important to its quantity owing to the suitability of water for various purposes. Water quality analysis is an important issue in groundwater studies. Variation of groundwater quality in an area is a function of physical and chemical parameters that are greatly influenced by geological formations and anthropogenic activities. Sujatha and Rajeswara Reddy (2003) have studied groundwater and its suitability for irrigation in the southeastern part of the Ranga Reddy district, Andrapradesh, India. Ahmed and others (2002) have compared the analytical results of groundwater in Rajshahi city of Bangladesh with the recommended limits

(HCO3), CO32)). The abundance of the major ions is as follows: Na+ ‡ Ca2+ ‡ Mg2+ > K+ = Cl) > HCO3)> SO42) > NO3) > CO32) . Groundwater in the area is generally hard, fresh to brackish, high to very high saline and low alkaline in nature. High total hardness and TDS in a few places identify the unsuitability of groundwater for drinking and irrigation. Such areas require special care to provide adequate drainage and introduce alternative salt tolerance cropping. Fluoride and boron are within the permissible limits for human consumption and crops as per the international standards. Keywords Groundwater Æ Drinking and irrigation water quality Æ Chithar basin Æ India

suggested by World Health Organisation (WHO 1971). They have classified groundwater into various types. Anbazhagan and Nair (2004) have used the geographical information system (GIS) to represent and understand the spatial variation of various geochemical elements in Panvel Basin, Maharashtra, India. Saleh and others (1999) have prepared correlation matrixes for the relationship between physical and chemical parameters of groundwater. They focused on boron concentration in groundwater to classify the irrigation water for various types of crops in the Damman and Kuwait group of aquifers, Kuwait. Knowledge on hydrochemistry is more important to assess the quality of groundwater for understanding its suitability for various needs.

1100

Previous investigations in the Chithar Basin, Tamil Nadu, India include groundwater estimation by Balasubramanian and Sastri (1994) and water level fluctuation, pump test analysis and major ion chemistry of groundwater by the Indian Public Works Department (2002). However, spatial variation of concentrations of common trace elements in groundwater such as fluoride and boron and the suitability of groundwater for drinking and irrigational needs were not included. As groundwater in the Chithar basin is intensively exploited for water supply and irrigation and the Chithar River is non-perennial, it is essential to assess the suitability of groundwater for drinking and agricultural uses. There are no major industries in this basin. Hence, the present work had the objective of understanding the spatial distribution of hydrogeochemical constituents of groundwater related to its suitability for agriculture and domestic use.

Methodology

Dry climatic conditions prevail and the plain lands of this basin fall under semi-arid climatic type. However, the areas adjacent to Western Ghats are of dry to moist sub-humid climatic types (Rammohan 1984). The average maximum temperature recorded during April and May is about 39C. The average minimum temperature is about 24C recorded usually during the months of November and December. The average annual rainfall of the basin is about 918 mm. The northeast monsoon from October to December contributes almost 70% of the total rainfall. A considerable amount of rainfall is also received during the southwest monsoon from June to September. A well-developed dentritic type of drainage system indicates the occurrence of rocks of uniform resistance (Thornbury 1969). Fine textured patterns noticed in the hill slopes indicating high surface run off and the coarser patterns in the plains are indicative of high rainfall infiltration. The regional slope is towards the east. Rain is the primary source of water in this basin. Banana, paddy and sugarcane are the common crops.

Description of the study area

Geology and hydrogeology

The Chithar basin is approximately between latitudes 8 48¢ to 9 14¢ North and longitudes 77 11¢ to 77 46¢ East. It covers an area of 1,722 km2 (Fig. 1). Chithar River is an important tributary of Tamirabarani River. The trunk system of Chithar River originates near Coutrallam with five beautiful falls in Poigaimalai and confluences with Tamirabarani near the village of Rajavallipuram. Azhuthakanni Ar, Intharuvi, Harihara Nathi, Hanuman Nathi, Gundar and Karuppa Nathi are the major tributaries to Chithar in the hill ranges of Western Ghats (Fig. 1). This river is considered to be a non-perennial, as it carries water only during the months of November and December.

The basement of the study area consists of quartzite, calc-granulite, crystalline limestone, chanockite and biotite gneiss with or without garnet of Archean age (Balasubramanian and Sastri 1994). Quartz veins, pegmatite and granitic intrusions can be observed in some places. Kankar (lime rich top soil) observed in a few locations are of recent to sub-recent in age (Fig. 2). The study of structural and tectonic history indicates several episodes of deformation, which caused repeated folds, faults, joints and fracture systems (Narayanaswami and Poorna 1967). The basement rocks are overlain by black, red and red sandy soils of thickness

Fig. 1 Chithar River Basin and location of monitoring wells

Fig. 2 Geological map of Chithar River Basin

Understanding the quality of groundwater is as important as its quality because it is the main factor

0.36 2213 1268 191 19 232 115 619 127 9.4 158 15.7 15.5 0.29 0.28 987 2.3 19 17.7 22.7 0.57 804 434 69 3 44 51 248 66 22 41 18 21 0.27 0.2 297 1.4 6 13 16.6 7.7 – – 106 13 66 85 128 317 0 58 0 63 0.26 0.65 – – – – – 8.5 1180 – 92 4 48 129 156 275 0 72 5 89 0.29 0.4 – – – – – 7.7 1665 956 166 13 88 62 293 366 0 113 7.5 66 0.25 0.74 481 2.5 )6 39.4 52.7 8.4 1145 543 89 5.5 51 51 176 137 12 45 17 80 0.24 0.41 344 2 )4 32 49.7 7.8 2529 1448 182 18.8 197 98 567 335 4 155 13.4 65 0.33 0.78 900 3 )12 37.7 50.5 8.2 1320 655 97 9.7 63 63 259 153 16 49 21 75 0.29 0.45 417 2.1 )5.3 35 50.7 8.6 9040 5051 897 86 880 485 2482 531 30 576 56 89.9 1.03 1.3 4216 10.7 0.22 59.9 86.9 9.2 3740 2045 304 34 176 243 1163 275 90 173 52 112 1.45 0.94 1444 5.69 1.6 67 86.3 7.4 240 135 23 4 16 5 43 49 0 14 0 29.2 0.02 0.19 60.8 0.34 )77 5.2 11.6 6.9 80 48 4 2 10 2 7 18 0 0 1 20.9 0 0.17 32.8 0.3 )26.6 14.1 25.8

July-2002 July-2001 July-2002 July-2001 July-2001 July-2002 July-2001

July-2002

July-2001

July-2002

July-2001

July-2002

Mode Median Average Maximum Minimum

– lS/cm mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l – meq/l % %

Groundwater chemistry

pH EC TDS Na+ K+ Ca2+ Mg2+ Cl) HCO3) CO32) SO42) NO3) SiO2 B F) TH SAR RSC %Na PI

Results and discussion

Units

Groundwater samples were collected from 24 representative open wells during July 2001. Eighteen samples were also collected during July 2002. Electrical conductivity (EC) and pH were measured using digital meters immediately after sampling. Water samples collected in the field were analysed for chemical constituents, such as sodium, potassium, calcium, magnesium, chloride, bicarbonate, carbonate, sulphate, nitrate, fluoride, boron, silica and total dissolved solids (TDS) , in the laboratory using the standard methods as suggested by the American Public Health Association (APHA 1989, 1995). Ca2+, Mg2+, HCO3), CO32), Cl) and TDS were analysed by volumetric titrations. Concentrations of Ca2+ and Mg2+ were estimated titrimetrically using 0.05N EDTA and 0.01N. H2SO4was used to determine the concentrations of HCO3) and CO32) . AgNO3 was used to estimate Cl). Flame photometer was used to measure Na+ and K+ ions. SO42), NO3), F) and SiO2 were determined by spectrophotometric techniques. Boron concentration was determined with the help of Atomic Absorption Spectrophotometer. The accuracy of the chemical analysis was verified by calculating ion-balance errors where the errors were generally around 10%.

Parameters

Field and laboratory methods

Table 1 Minimum and Maximum values of physical and chemical parameters of groundwater with statistical parameters

ranging from 1 m to 1.5 m in most places. Thin layer of alluvial flood plain deposits, mainly sand are found along the banks of Chithar River. Borehole lithology records reveal that the thickness of alluvial deposits is more in bajada and valley fills (about 10–15 m). Weathered and fractured zones are areas for groundwater occurrence. Intensity of weathering is not uniform in space and depth. It is considerably higher in gneissic rocks than in charnockite. Weathered zone thickness of the study area generally ranges from 8 m to 34 m below ground level. The groundwater of the area occurs under unconfined conditions. Rainfall infiltration and seepage of water from surface water bodies are responsible for groundwater fluctuation. Most of the wells used for irrigation are shallow and partially penetrating because once a considerable depth of water column is reached, farmers stop further deepening of wells. Hydrographs indicate that the groundwater table tends to rises during November and December to reach peak in January and declines from February onwards to reach a low in September or October.

Standard deviation

1101

1102

determining its suitability for drinking, domestic, agricultural and industrial purposes. The pH values of groundwater range from 6.9 to 9.2 with an average value of 8.2 during July 2001. However, during July 2002, it ranges from 7.4 to 8.6 with an average value of 7.8. This shows that the groundwater of the study area is mainly alkaline in nature. The EC values range from 80 lS/cm to 3,740 lS/cm during July 2001 and 240 lS/cm to 9,040 lS/cm during July 2002. Higher values are generally noticed in the northeastern area. TDS values range from 48 mg/l to 2,045 mg/l during July 2001 and 135 mg/l to 5,051 mg/l during July 2002. Physical and chemical parameters including statistical measures, such as minimum, maximum, average, median, mode and standard deviation, are reported in Table 1 for July 2001 and July 2002. The abundance of the major ions in groundwater is in the following order: Na+ ‡ Ca2+ ‡ Mg2+ > K+ = Cl) > HCO3)> SO42) > NO3) > CO32) . Silica in groundwater varies from 20.9 mg/l to 112 mg/l during July 2001 and 29.2 mg/l to 89.9 mg/l during July 2002. The average values are 75 mg/l and 65 mg/l, respectively, during July 2001 and July 2002. The average value of boron concentration in groundwater is 0.29 mg/l in July 2001 and 0.33 mg/l in July 2002. Correlation of physicochemical parameters of groundwater Correlation coefficient is a commonly used measure to establish the relationship between two variables. It is

simply a measure to exhibit how well one variable predicts the other (Kurumbein and Graybill 1965). The correlation matrices for 16 variables were prepared for July 2001 (Table 2) and July 2002 (Table 3) and illustrate that EC and TDS show good positive correlation with Mg2+ and Cl) . TDS and Mg2+ also exhibit high positive correlation with SO42) and Cl) ions, respectively, during July 2001 and July 2002. EC–SO4, TDS– Ca, TDS–B, Ca–Mg, Ca–Cl, Mg–B, Cl–SO4 and Cl–B are also the more significant correlation pairs during July 2001. However, Na–SO4 and Mg–B are the good positive correlation pairs during July 2002. Furthermore, Na and SO4 ions also show more significant correlation pairing with NO3 during July 2002. pH and F exhibit negative correlation with most of the variables and SiO2 exhibit no significant correlation with any one of the variables in the matrixes. Hydrochemical facies The geochemical evolution of groundwater can be understood by plotting the concentrations of major cations and anions in the Piper (1944) trilinear diagram. The plot shows that most of the groundwater samples analysed during July 2001 and July 2002 fall in the field of mixed Ca–Mg–Cl type of water (Fig. 3). Some samples are also representing Ca–Cl and Na–Cl types. From the plot, alkaline earths (Ca2+ and Mg2+) significantly exceed the alkalis (Na+ and K+) and strong acids (Cl) and SO42)) exceed the weak acids (HCO3) and CO32)).

Table 2 Correlation of physiochemical parameters of groundwater of July 2001 EC EC pH TDS Na K Ca Mg Cl HCO3 CO3 SO4 NO3 B F SiO2 Cl HCO3 CO3 SO4 NO3 B F SiO2

1 0.01201 0.97363 0.7495 0.40479 0.83578 0.91595 0.94552 0.23406 )0.13113 0.87855 0.42081 0.82314 )0.01272 )0.01848 Cl 1 0.12126 )0.24052 0.86406 0.25807 0.89259 0.07802 0.06347

pH

TDS

Na

K

Ca

Mg

1 )0.01474 0.02457 0.34659 )0.17381 0.02997 )0.01133 )0.20482 0.64411 )0.23519 0.11115 )0.12717 0.07752 )0.00727 HCO3

1 0.74737 0.43077 0.87073 0.93949 0.97821 0.27024 )0.15894 0.91128 0.33308 0.86593 0.0902 0.01634 CO3

1 0.07076 0.41457 0.50446 0.62414 0.4836 0.1577 0.71432 0.41775 0.45005 0.12432 )0.17368 SO4

1 0.48413 0.47877 0.44408 0.00258 0.12641 0.35612 0.05611 0.38278 0.03515 0.21025 NO3

1 0.88789 0.89696 0.1328 )0.46067 0.78869 0.30366 0.84473 )0.03258 0.22378 B

1 0.9697 0.09023 )0.22128 0.81904 0.28103 0.89518 0.08795 0.08213 F

1 0.12506 0.30275 )0.07823 0.22906 0.22048 )0.56484

1 )0.23088 )0.13911 )0.25333 0.04679 )0.1229

1 0.28226 0.80021 0.03565 0.08083

1 0.06741 )0.10744 0.24678

1 )0.01917 )0.03267

1 )0.31144

1103

Table 3 Correlation of physiochemical parameters of groundwater of July 2002 EC EC pH TDS Na K Ca Mg Cl HCO3 CO3 SO4 NO3 B F SiO2 Cl HCO3 CO3 SO4 NO3 B F SiO2

1 0.0159 0.99784 0.8188 0.55234 0.47155 0.94 0.98024 0.67337 0.5343 0.83003 0.6493 0.80792 )0.15079 0.23483 Cl 1 0.62154 0.44016 0.72326 0.54946 0.80515 )0.05235 0.25205

pH

TDS

Na

K

Ca

Mg

1 0.03167 0.19289 0.42859 )0.30616 0.06762 )0.07301 0.09022 0.67288 0.25869 0.04418 )0.01729 )0.48142 )0.17577 HCO3

1 0.84722 0.5625 0.43184 0.93611 0.97047 0.65151 0.55187 0.85794 0.69407 0.7955 )0.18588 0.25928 CO3

1 0.47772 )0.09059 0.81194 0.72201 0.45315 0.63859 0.93191 0.87049 0.62665 )0.45546 0.24899 SO4

1 0.14427 0.58225 0.51115 0.54734 0.56834 0.48055 0.43835 0.47496 )0.29368 0.36335 NO3

1 0.28488 0.57676 0.44492 )0.10204 0.03822 )0.16585 0.28371 0.30499 0.11569 B

1 0.92009 0.60811 0.58016 0.79258 0.63236 0.92533 )0.06525 0.12356 F

1 0.32128 0.43332 0.19712 0.5182 )0.09208 )0.0026

1 0.68105 0.52061 0.45019 )0.44959 )0.09739

1 0.85891 0.64322 )0.39896 0.19319

1 0.50138 )0.33056 0.4328

1 0.16495 )0.05497

1 )0.00807

Aquachem software was used for plotting the Piper diagram. Drinking water quality The analytical results of physical and chemical parameters of groundwater were compared with the standard guideline values as recommended by the World Health

Organisation (WHO 1971, 1983) for drinking and public health purposes (Table 4). The table shows the most desirable limits and maximum allowable limits of various parameters. The concentrations of cations, such as Na+, Ca2+, and Mg2+, are within the maximum allowable limits for drinking except a few samples. Total dissolved solids To ascertain the suitability of groundwater for any purposes, it is essential to classify the groundwater depending upon their hydrochemical properties based on their TDS values (Catroll 1962; Freeze and Cherry 1979), which are presented in Table 5. The groundwater of the area is fresh water except a few samples representing brackish water. Most of the groundwater samples are within the maximum permissible limit for drinking as per the WHO international standard, except six samples in July 2002 and 1 sample in July 2001. The TDS zonation map for July 2002 (Fig. 4) was prepared by setting the most desirable (500 mg/l) and maximum allowable (1,500 mg/l) limits. The map shows that 2/3 of the basin is below 500 mg/l of TDS, indicating low content of soluble salts in groundwater which can be used for drinking without any risk. Total hardness

Fig. 3 Chemical facies of groundwater in Piper diagram

The classification of groundwater (Table 6) based on total hardness (TH) shows that a majority of the

1104

Table 4 Groundwater samples of the study area exceeding the permissible limits prescribed by WHO for drinking purposes and the resulting undesirable effect on human system Parameters

WHO international standard (1971, 1983)

Wells exceeding permissible limits

Most desirable limits

Maximum allowable limits

July-2001

July-2002

pH TDS (mg/l)

7–8.5 500

9.2 1500

Nil 10

Nil 6,8–10,19,24

TH (mg/l) Na (mg/l) Ca (mg/l) Mg (mg/l) Cl (mg/l) SO4 (mg/l) NO3 (mg/l) F (mg/l)

100 – 75 50 200 200 45 –

500 200 200 150 600 400 – 1.5

4,9–11,19,22 11 Nil 10 10 Nil 6,9,19,22 Nil

6–11,13,19,24 11,16,17,19,24 6,8,9,19 9,10,24 6,8–10,19,24 9,24 24 Nil

groundwater samples fall in the hard water category

Undesirable effect

Taste Gastrointestinal irritation Scale formation – }Scale } formation Salty taste Laxative effect Blue baby Fluorosis

allowable limit of TH for drinking is 500 mg/l and the

Table 5 Nature of groundwater based on TDS values TDS (mg/l)

100000

Nature of water

Fresh water Brackish water Saline water Brine water

July-2001

July-2002

Representing wells

Total no. of wells

Representing wells

Total no. of wells

1–8,12–18,20,21,23,24 9,10,11,19,22 Nil Nil

19 5 Nil Nil

1,2,5,13,15–18,22,23 6–11,19,24 Nil Nil

10 8 Nil Nil

most desirable limit is 100 mg/l as per the WHO international standard. Six samples out of 24 collected during July 2001 and nine samples out of 18 collected during July 2002 exceed the maximum allowable limits (Table 1). The study area is delineated into three zones based on the desirable and maximum permissible limits of TH for July 2002 (Fig. 5). Chloride

Fig. 4 Drinking water quality based on TDS values of July 2002

(Sawyer and McCartly 1967). TH of the groundwater was calculated using the formula given below (Hem 1985; Ragunath 1987) TH (as CaCO3) mg/l ¼ ðCa2þ þ Mg2þ Þmeq/l  50:

ð1Þ

The hardness values range from 32.8 mg/l to 1,443.9 mg/l with an average value of 417 mg/l during July 2001 and 60.8 mg/l to 4,216 mg/l with the average value of 900 mg/l during July 2002. The maximum

The chloride ion concentration varies between 7 mg/l and 1,168 mg/l in July 2001 samples and 43 mg/l to 2,482 mg/ l in July 2002 samples. However, the average values calculated for July 2001 and 2002 are within the prescribed limits. The spatial distribution of chloride concentration in groundwater of the study area for July 2002 is illustrated in the Fig. 6. This figure shows that only one sample exceeds the maximum allowable limit of 600 mg/l. However, six samples of July 2002 exceed this limit. Nitrate The nitrate concentration in July 2001 groundwater samples range from 1 mg/l to 52 mg/l with an average value of 21 mg/l. In July 2002 samples, it ranges from 0 mg/l to 56 mg/l with the average of 13.4 mg/l. The concentration of nitrogen in groundwater is derived

1105

Table 6 Classification of groundwater based on hardness Total hardness as CaCO3 (mg/l)

Water class

300

Soft Moderately hard Hard Very hard

July-2001

July-2002

Representing wells

Total no. of wells

Representing wells

Total no. of wells

23 1,15 All other wells Nil

1 2 21 Nil

1 Nil All other wells 9

1 Nil 16 1

Fig. 5 Drinking water quality based on hardness values of July 2002

Fig. 7 Spatial distribution of nitrate during July 2001

from the biosphere (Saleh et al. 1999). Nitrogen is originally fixed from the atmosphere and then mineralized by soil bacteria into ammonium. Under aerobic conditions nitrogen is finally converted into nitrate by nitrifying bacteria (Tindall et al. 1995). Four samples of July 2001 and one sample of July 2002 exceed the

desirable limit of 45 mg/l as per WHO norms. The high concentration of nitrate in drinking water is toxic and causes blue baby disease/methaemoglobinaemia in children and gastric carcinomas (Comly 1945; Gilly et al. 1984). As most of the study area is intensively irrigated, the fertilizers used for agriculture may be the source for

Fig. 6 Spatial distribution of chloride during July 2002

Fig. 8 Spatial distribution of Sulphate during July 2002

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Fig. 9 Spatial distribution of fluoride during July 2001

the elevated concentration of nitrate in a few locations. The spatial variation of nitrate during July 2001 in groundwater of the basin is illustrated in the Fig. 7. Sulphate Sulphate is unstable if it exceeds the maximum allowable limit of 400 mg/l and causes a laxative effect on human system with the excess magnesium in groundwater. Only two samples of July 2002 exceed the prescribed value. However, three samples of July 2002 and one sample of July 2001 exceed the maximum allowable limit of magnesium (150 mg/l) suggested for drinking. This may result in gastrointestinal irritation to the human system.

Fig. 10 Salinity and alkalinity hazard of irrigation water in US salinity diagram

Table 7 Quality of irrigation water based on electrical conductivity EC (lS/cm)

Water class

< 250 250–750 750–2000 2000–3000 >3000

Excellent Good Permissible Doubtful Unsuitable

July-2001

July-2002

Representing wells

Total no. of wells

Representing wells

Total no. of wells

23 1,13,15 2–8,12,14,16–18,20,21,24 9,11,19,22 10

1 3 15 4 1

1 Nil 2,5,13,15,16–18,22,23 7,11 6,8,9,10,19,24

1 Nil 9 2 6

Table 8 Alkalinity hazard classes of groundwater SAR

< 10 10–18 18–26 >26

Alkalinity hazard

S1 S2 S3 S4

Water class

Excellent Good Doubtful Unsuitable

July-2001

July-2002

Representing wells

Total no. of wells

Representing wells

Total no. of wells

All wells Nil Nil Nil

24 Nil Nil Nil

All wells except 24 24 Nil Nil

17 1 Nil Nil

1107

Table 9 Suitability of groundwater for irrigation based on percent sodium % Na

80

Water class

Excellent Good Permissible Doubtful Unsuitable

July-2001

July-2002

Representing wells

Total no. of wells

Representing wells

Total no. of wells

3,5 1,2,4,8,9,10,12,13,14, 16,19,20,22,23 6,11,15,17,18,21,24 7 Nil

2 14 7 1 Nil

6,9,13 7,8,10,11,19,23 1,2,5,15–18,22,24 Nil Nil

3 6 9 Nil Nil

contour map of sulphate concentration in groundwater during July 2002 is presented in the Fig. 8. Fluoride Fluoride is one of the main trace elements in groundwater, which generally occurs as a natural constituent. Bedrock containing fluoride minerals is generally responsible for high concentration of this ion in groundwater (Handa 1975; Wenzel and Blum 1992; Bardsen and others 1996). The concentration of fluoride in groundwater of the basin varies between 0.07 mg/l and 0.94 mg/l during July 2001 with an average value of 0.45 mg/l and median of 0.41. The concentration is slightly higher during July 2002, ranging between 0.19 mg/l and 1.3 mg/l with an average of 0.78 mg/l. However, all samples examined exhibit suitability for drinking. The spatial distribution of fluoride concentration in groundwater during July 2001 is illustrated in the Fig. 9.

Irrigation water quality Salinity and alkalinity hazard

Fig. 11 Suitability of groundwater for irrigation in Wilcox diagram

Significant positive correlation (+0.8) is also noticed between magnesium and sulphate (Tables 2, 3). The

Electrical conductivity is a good measure of salinity hazard to crops as it reflects the TDS in groundwater. Five out of 24 samples of July 2001 and eight out of 18 samples of July 2002 exceed (Table 7) the permissible limit for irrigation (Ragunath 1987). Excess salinity reduces the osmotic activity of plants and thus interferes with the absorption of water and nutrients from the soil

Table 10 Quality of groundwater based on residual sodium carbonate RSC (meq/l)

2.5

Remark on quality

Good Doubtful Unsuitable

July-2001

July-2002

Representing wells

Total no. of wells

Representing wells

Total no. of wells

All wells except No.7 7 Nil

23 1 Nil

All wells Nil Nil

18 Nil Nil

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Table 11 Permissible limits of boron in irrigation water for several types of crops Boron class

Excellent Good Permissible Doubtful Unsuitable

Semi-sensitive crops

Semi-tolerant crops

Tolerant crops

Range (mg/l)

Range (mg/l)

Range (mg/l)

< 0.33 0.33–0.67 0.67–1 1–1.25 >1.25

Total no. of wells July-2001

July-2002

21 2 Nil Nil 1(Well.10)

13 2 2 1(Well.19) Nil

< 0.67 0.67–1.33 1.33 – 2 2–2.5 > 2.5

Total no. of wells July-2001

July-2002

23 Nil 1 Nil Nil

15 3 Nil Nil Nil

3.75

Total no. of wells July-2001

July-2002

23 1 Nil Nil Nil

17 1 Nil Nil Nil

of exchangeable sodium (Fig. 10). Few samples fall in the field of C4S1, indicating very high salinity and low alkalinity hazard. This can be suitable for plants having good salt tolerance and also restricts their suitability for irrigation, especially in soils with restricted drainage (Karanth 1989; Mohan et al. 2000). The sodium percentage (Na %) is calculated using the formula given below: Na% ¼

Fig. 12 Spatial distribution of boron during July 2002

(Saleh et al. 1999). Sodium adsorption ratio (SAR) is an important parameter for determining the suitability of groundwater for irrigation because it is a measure of alkali/sodium hazard to crops. SAR is defined by (Karanth 1987) SAR ¼

Naþ ðCa2þ þ Mg2þ Þ1=2 =2

;

ð2Þ

where the concentrations are reported in meq/l. The SAR values range from 0.3 to 5.69 with an average value of 2.1 during July 2001 and 0.34 to 10.67 during July 2002 with an average value of 3. Groundwater samples of the study area fall in the low sodium class (S1) except one sample (Well No. 24) of July 2002 having the SAR value of 10.67 (Table 8). This implies that no alkali hazard is anticipated to the crops. If the SAR value is greater than 6 to 9, the irrigation water will cause permeability problems on shrinking and swelling types of clayey soils (Saleh et al. 1999). The analytical data plotted on the US salinity diagram (Richards 1954) illustrates that most of the groundwater samples fall in the field of C3S1, indicating high salinity and low sodium water, which can be used for irrigation on almost all types of soil with little danger

ðNaþ þ Kþ Þ  100 ; ðCa þ Mg2þ þ Naþ þ Kþ Þ 2þ

ð3Þ

where all the concentrations are expressed in meq/l. The Na % indicates that the groundwater is excellent to permissible for irrigation (Table 9) except one sample of July 2001(Ragunath 1987). The Wilcox (1955) diagram relating sodium percentage and total concentration (Fig. 11) shows that most of the groundwater samples fall in the field of good to permissible except a few samples falling in the fields of doubtful and unsuitable for irrigation. When the concentration of sodium is high in irrigation water, sodium ions tend to be absorbed by clay particles, displacing Mg2+ and Ca2+ ions. This exchange process of Na+ in water for Ca2+ and Mg2+ in soil reduces the permeability and eventually results in soil with poor internal drainage. Hence, air and water circulation is restricted during wet conditions and such soils are usually hard when dry (Collins and Jenkins 1996; Saleh et al. 1999). Residual sodium carbonate In addition to the SAR and % Na, the excess sum of carbonate and bicarbonate in groundwater over the sum of calcium and magnesium also influences the unsuitability of groundwater for irrigation. This is denoted as residual sodium carbonate (RSC), which is calculated as follows (Ragunath 1987): 2þ 2 RSC ¼ (HCO þ Mg2þ Þ; 3 þ CO3 Þ  (Ca

where the concentrations are reported in meq/l.

ð4Þ

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The classification of irrigation water according to the RSC values is presented in Table 10, where the category of groundwater is good except one sample of July 2001.

distribution of boron concentration during July 2002 is given in the Fig. 12.

Conclusion Permeability index The permeability index (PI) values also indicate that the groundwater is suitable for irrigation. It is defined as follows (Ragunath 1987): p ðNaþ þ HCO 3 Þ  100 ; ð5Þ PI ¼ 2þ 2þ ðCa þ Mg þ Naþ Þ where the concentrations are reported in meq/l. The PI ranges from 25.8% to 86.3% during July 2001 and 11.6% to 86.9% during July 2002. The average value is about 51% during July 2001 and July 2002, which comes under class-1 of Doneen’s chart (Domenico and Schwartz 1990). Boron Boron concentration in the groundwater of the area during July 2001 ranges between 0 mg/l to 1.45 mg/l with an average value of 0.29 mg/l. In July 2002, it ranges from 0.02 mg/l to 1.03 mg/l with an average value of 0.33 mg/l. Boron is also toxic to crops at high concentration. The proposed limits of boron concentration in irrigation water and the total number of groundwater samples of the study area representing the boron classes are presented in Table 11 (McCarthy and Ellery 1994). Out of 24 samples analysed during July 2001, only one sample (Well No. 10) is unsuitable for semi-sensitive crops, but it can also be used for semitolerant and tolerant types of crops. All of the eighteengroundwater samples analysed during July 2002 are within the permissible limits for tolerant and semi-tolerant crops. However, one sample (Well No. 19) falls in the doubtful class for semi-sensitive crops. The spatial

Interpretation of hydrochemical analysis reveals that the groundwater in Chithar Basin is hard, fresh to brackish and alkaline in nature. The sequence of the abundance of the major ions is in the following order: Na+ ‡ Ca2+ ‡ Mg2+ > K+ = Cl) > HCO3)> SO42) > NO3) > CO32) . Alkali earths (Ca2+ and Mg2+) slightly exceed alkalis (Na+ and K+) and strong acids (Cl) and SO42)) exceed weak acids (HCO3) and CO32)). This leads to a mixed Ca–Mg–Cl type of groundwater. However, some groundwater samples of the study area represent Ca–Cl and Na–Cl types. Magnesium, chloride and sulphate ions show positive correlation with EC and TDS. Magnesium also exhibits good positive correlation with chloride and boron. Similarly, sodium, sulphate and nitrate ions have good relationship. TH is generally high in the groundwater thereby, causing the groundwater in one fourth of the study area to be unsuitable for drinking as per July 2001 and nearly 50% unsuitable as per July 2002. Groundwater in one third of the study area exceeded the recommended limits of TDS during July 2002 as per the international drinking water standard. The concentrations of major ions in groundwater are within the permissible limits for drinking except in some places. Concentration of fluoride is within the permissible limit for drinking. Due to high to very high salinity hazard, the groundwater in nearly 40% of the study area is beyond the maximum allowable limit for irrigation even though it has low alkalinity hazard. This shows that groundwater in a few places can be used for plants having good salt tolerance and also restricts its suitability for irrigation, especially in soils with restricted drainage. Hence, such areas need adequate drainage and salt tolerance cropping patterns to overcome suitability problems for irrigational purpose. Boron concentration in groundwater is, however, safe for all types of crops.

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