Indian Journal of Marine Sciences Vol. 37(2), June 2008, pp. 186-192 A statistical evaluation of ground water chemistry from the west coast of Tamil Nadu, India. Asa Rani L 1 & D.S.Suresh Babu Centre for Earth Science Studies Akkulam, Thiruvananthapuram-695031 Received 14 December 2007, revised 19 May 2008 Ground water chemistry in the coastal area between Kollamkode and Kanyakumari has been studied. Quality assessment was made through the estimation of ph, EC, salinity, DO, Na +, K +, Ca 2+, Mg 2+, Cl -, SO 4 2-, PO 4 3-, NO 3 -, NO 2 -, HCO 3 -, and total hardness. The wells in general showed high values of conductivity, salinity, chloride, hardness and calcium. The chemical relationship exhibited by Piper s diagram suggests that the groundwater mainly belongs to the hydrochemical facies Na-Ca and Cl-SO 4 -HCO 3. Samples from the coastal fringe zone seldom represent fresh groundwater. Seasonal landward shift of freshwater-saltwater interface was observed throughout the coastal zone. It was found that Na + and Cl - are the dominant cation and anion in the area. Correlation analysis showed perfect correlation between EC and TDS, reveals that EC is a measure of dissolved solids in ground water. The clusters defined by Q-mode analysis reflect the spatial distribution of samples and the R-mode cluster conveys that salinity, TDS, EC and Na + form one group. The interface zone that extends up to 200-250m from the shoreline has to be carefully exploited, after identifying the discharge and the probable zones. Keywords: Coastal aquifer, Groundwater chemistry, frequency distribution, Correlation analysis, Cluster analysis, Hydrochemical facies. Introduction Groundwater is an important water-supply source worldwide. It is the major source of drinking water in both urban and rural India. A number of studies on groundwater quality with respect to drinking and irrigation purposes have been carried out in different parts of the country 1-3. Groundwater in the coastal area is relatively vulnerable to the contamination by seawater intrusion, which makes it unsuitable for drinking or irrigation. In this paper an attempt has been made to evaluate the chemistry of groundwater in the west coast of Tamil Nadu. The disposition of study area is given as Fig. 1. The area experiences a tropical climate with an average annual rainfall of 1465mm. It is drained by four main rivers, Neyyar, Tamaraparani, Valliyar and Pazhayar. Geologically, rocks belonging to the Khondalite suite, Charnockite series, Granite gneiss, pegmatites and dykes of Precambrian age underlie the area. The tropical climate favoured intense chemical weathering resulting in the development of deep laterite profiles, 1 Corresponding Author: asasuresh@rediffmail.com Asa Rani L., Centre for Earth Sciences Studies, Akkulam, Thiruvananthapuram-695031, Kerala, India. Tel: 0471-2511621 often exceeding 20-30m in thickness, which is overlained by Precambrian rocks. The basement rocks are also overlain conformably by mixed chemical and clastic sediments of Neogene age, at selected pockets of the coastal zone. Considerable portion of the study area is covered by red loam on the surface. Geomorphologically, the area belongs to moderately rolling type and possesses two crystalline promontories/sea cliffs in the beach, at Kolachal and at Muttom. Material and Methods Water table positions were recorded monthly from March 05 to November 2006. A total of 53 wells were monitored up to 1km from the shoreline and of these 35 wells were selected for hydrochemical analysis. Four sets of samples were collected during March 05, May 05, Feb 06 and Nov 06, which represents all the seasons of the year and the parameters like ph, EC, salinity were measured insitu using a portable electronic water quality analyser. Concentration of major cations, anions, total hardness and total dissolved solids were determined, following the standard procedures of water analyses. 4 The hydrochemical facies were determined using the Piper s trilinear diagram using the software,
ASA RANI et al.: GROUND WATER CHEMISTRY FROM THE WEST COAST 187 Fig. 1 Location map of the study area Groundwater chart (ver. 1.5). Statistical analysis of the data, including frequency distribution of major cations and anions, correlation analysis and hierarchical cluster analysis were carried out using the software, SPSS (ver. 10.0). Results and Discussion Hydrogeology The unconfined coastal aquifer zone falling between Kollamkode and Kanyakumari, in the west coast of Tamil Nadu has been evaluated.the depth to basement varies from 35 to 40 m and the depth to water table (bgl) ranges between 0.07 m (well no. 43) during Nov 06 and 24.22 m (well no. 24) during April 05. The area is drained by four rivers namely Neyyar River, Tamaraparani River, Valliyar River and Pazhayar River. The elevation of water table above mean sea level also increases eastward. The area is receiving an annual rainfall of 1465 mm and it is a major parameter in recharging the aquifer. Ground water chemistry The ph of ground water varies from 3.05 to 8.48. However, the acceptable concentration of ph in drinking water given by WHO (1984) 5 is 7-8.5. During the study period, none of the samples exceeded the maximum limit of 8.5. But fourteen samples showed significantly lower values of ph, which might be attributed to the ionic interaction. 6 The water from these wells is not used for potable purposes. Maximum electrical conductivity of 9380 µs/cm and a minimum of 61µs/cm were recorded in the study area and a total of 14 wells are found to be falling in the freshwater category. Concentration of salinity ranges from 53.80 to 5300 mg/l. Most of the wells located at the fringe of coastal area showed very high values. Maximum TDS value of 6097 mg/l and a minimum of 39.65mg/l were recorded and the monthly average values of most of the wells in the study area fall in the brackish water category. Hardness ranges between 12 mg/l and 1664.93 mg/l and the monthly average values of
188 INDIAN J. MAR. SCI., VOL. 37, NO. 2, JUNE 2008 hardness noticed during the study period showed most of the wells fall under the Very hard category. Wells located very near to the coastal region showed high values of hardness, which is due to saline water mixing. The concentration of calcium in the study area ranges between 14mg/l and 974.59 mg/l, while the maximum desirable limit of calcium in drinking water is 75 mg/l and maximum permissible limit is 200 mg/l 7. It is noticed that most of the samples exceed the permissible limit except 17 wells which are out of the interface zone. It was observed that the calcium content increases towards the shoreline wells. Concentration of magnesium in the study area ranges between 1.46 mg/l and 286.67 mg/l, which are found to be within the desirable limit. The WHO guideline limit for sodium in drinking water is 200mg/l. Most of the samples are showing concentration of sodium above the permissible limit. Only 9 wells have the concentration within the limit. Samples which are located within the interface are found to have higher concentration of sodium. Concentration of potassium in natural waters given by WHO is 10 mg/l. Concentration of potassium in the area ranges between 6.5-201 mg/l. In most of the samples in the study area concentration of potassium is above the limit. Among the anions (SO 4 2-, NO 3 -, NO 2 -, HCO 3 -, PO 4 3- ), only the concentration of phosphate exceeds the desirable limit. In this area phosphorous contamination is observed to be high in wells located very near to the coast and so phosphate concentration may be due to saline water mixing rather than domestic sewage and detergents. Statistical analysis The major ions are plotted in frequency versus percentage milli-equivalents of the total cations and anions are provided in Fig. 2 & 3. Sen and Al- Dakheel (1986) 8 defined the relative content of a cation or anion as the percentage of the milliequivalent per liters (meq/l) of the total cations or the total anions, respectively. Pie diagram of median Fig. 2 Percentage frequency distribution of major cation concentrations
ASA RANI et al.: GROUND WATER CHEMISTRY FROM THE WEST COAST 189 Fig. 3 Percentage frequency distribution of major anion concentrations Fig. 4 Pie diagrams of median values of major cations (a) and anions (b) concentration of major cations and anions in milliequivalent are plotted, since median values are much more robust descriptors of non- normal distributors than mean values (Fig. 4a & 4b). This figure shows that Na + and Cl are the dominant cation and anion. These are the only ions that exceeded 50% of the total cations or anions in the area. It also shows that the order of relative abundance of major cations in the groundwater of the area is Na + >Ca 2+ >Mg 2+ >K + while that of anions is Cl - >HCO 3 SO 4 2->NO 3 - > PO 4.
190 INDIAN J. MAR. SCI., VOL. 37, NO. 2, JUNE 2008 Statistical analysis of the hydrochemical parameters indicates that the area is generally occupied by brackish to saline water (Table 1). Correlations between major cations and anions were carried out using Pearson s correlation (Table 2). A correlation analysis is a bivariate method applied to describe the degree of relation between two hydro chemical parameters. A high correlation coefficient (near 1 or 1) means a good relationship between two variables and its value around zero Parameter & Permissible limits Table 1 Statistical summary of the Hydrochemical parameters of the study area Minimum Maximum Median Mean Standard Deviation Remarks PH (7-8.5) 4.51 7.34 6.48 6.47 0.65 Below limit-14 and Within the limit-21 EC 188 6213.25 1281.75 1711.12 1539.95 Fresh water-14 and Saline water-21 Salinity 117.93 3450 575 879.41 809.17 Related to EC Hardness (100) 89.51 1235.97 318.92 366.47 366.47 Very hard-12, Hard-8, Moderately hard-12 & Soft-3 TDS(500 mg/l) 122.2 4038.61 833.14 1112.22 1112.22 Fresh water-14, Saline water-21 Na + (50 mg/l) 62.5 6016.25 812.500 1322.37 1322.37 Wells within the permissible limit-9 K + (10mg/l) 6.25 201.25 30 34.99 34.12 Wells within the permissible limit-5 Ca 2+ (75 mg/l) 35.07 462.74 208.59 209.36 209.36 Within the limit-17 Mg 2+ (150 mg/l) 7.45 198.31 21.89 41.27 41.27 Beyond the limit-2 HCO3 (600mg/l) 12.99 408.44 126.13 158.93 96.65 All the samples within the limit Cl (200 mg/l) 65.28 1892.26 262.17 442.96 443.20 Related to EC & Salinity NO 3 - (45 mg/l) 0.25 0.86 0.77 0.711 0.15 All the samples within the limit NO 2 - (0.001mg/l) 0.003 0.334 0.02 0.06 0.08 All the samples within the limit SO4 (200 mg/l) PO4 (mg/l) 0.75 14.4 6.01 5.86 3.31 All the samples within the limit 0.02 1.62 0.15 0.29 0.34 All the samples within the limit and towards the coast concentration increases Table 2 Linear correlation of different hydrochemical parameters of the area Parameter ph EC Salinity Hardness TDS Na + K + Ca 2+ Mg 2+ HCO3 Cl NO 3 NO 2 SO4 PO4 ph 1 0.200 0.108 0.276 0.200 0.121 0.007 0.388 0.179 0.811 0.109-0.221 0.457 0.258 0.493 EC 1 0.945 0.891 1 0.973 0.384 0.78 0.811 0.429 0.966-0.173 0.302 0.802 0.320 Salinity 1 0.855 0.945 0.937 0.364 0.694 0.825 0.323 0.991-0.230 0.249 0.676 0.234 Hardness 1 0.891 0.881 0.267 0.850 0.936 0.533 0.877-0.177 0.254 0.739 0.329 TDS 1 0.973 0.384 0.798 0.811 0.429 0.966-0.173 0.302 0.802 0.320 Na + 1 0.267 0.710 0.848 0.346 0.955-0.157 0.217 0.719 0.200 K + 1 0.139 0.299 0.043 0.384-0.189 0.218 0.172 0.263 Ca 2+ 1 0.625 0.648 0.728-0.218 0.037 0.845 0.476 Mg 2+ 1 0.391 0.837-0.098 0.151 0.536 0.185 HCO3 1 0.331-0.145 0.371 0.451 0.531 Cl - 1-0.204 0.263 0.634 0.255 NO 3 1-0.693-0.181-0.474 NO 2 1 0.330 0.710 SO4 1 0.420 PO4 1 * Good correlation values are given in bold
ASA RANI et al.: GROUND WATER CHEMISTRY FROM THE WEST COAST 191 means no relationship between them at a significant level of < 0.05. More precisely it can be said that parameters showing r > 0.7 are considered to be strongly correlated where as r between 0.5 and 0.7 shows moderate correlation. Table 2 shows perfect correlation between EC and TDS, which indicates that EC is a measure of dissolved solids in the ground water. The matrix shows fairly high correlation of EC with salinity, Na +, and Cl - and salinity with TDS, Na + and Cl -. Also very high correlation of hardness with Ca 2+ and Mg 2+, TDS with Na + and Cl -, and Na + with Cl - was observed. Since the correlation coefficient of Na + and Cl - is fairly high, it can be deduced that for most of the groundwater samples Na + and Cl - originate from a common source. This means that the salinity content is due to modern seawater mixing and not due to formation salinity. Hierarchical cluster Analysis is a powerful tool for analyzing water chemistry data and this method groups samples into distinct populations that may be significant in the geologic/hydrologic context, as well as from a statistical point of view 9. The classification is based on the similarity of object attributes. The clusters are characterized by high similarity within the cluster and high dissimilarity between the clusters. Cluster method used was Ward s method and the output is given in the form of dendrograms. For examining the similarity among the samples of Q- mode cluster and for evaluating interaction among the variables, R-mode cluster was used. Figure 5a shows the Q-mode cluster with two major clusters, cluster1 and cluster2, which in turn have two subdivisions, Cluster1a & 1b and Cluster 2a & 2b. It is observed that each group represents similar hydro chemical environment. Samples belonging to cluster1 have high EC, salinity, hardness, Ca 2+, Mg 2+, Na +, K +, Cl - 2-, SO 4 and TDS and the corresponding wells are located in the near shore environment. R-mode cluster shows two major clusters, cluster1 and cluster2. Cluster1 has two subdivisions, 1a & 1b (Fig. 5b). Cluster2 also has two sub divisions, 2a & 2b. It shows salinity, TDS, EC and Na + form one group and the rest of the parameters form a separate cluster. Hydrochemical Evaluation The geochemical evolution of groundwater can be understood by plotting the concentrations of major cations and anions in Piper (1944) 10 trilinear diagram (Figure 6). It reveals that most of the samples fall in the field, alkalies exceed alkaline earths and all the Fig. 5 Dendrogram showing Q-mode (a) and R-mode (b) cluster of the study area Fig. 6 Piper trilinear diagram of the ground water of the study area samples except one strong acids exceed weak acids. The data plot suggests that the groundwater in the area is Na-Ca and Cl-SO 4 -HCO 3 facies dominant. Following are the major hydrochemical facies identified in the area: Na-Ca-Cl-SO 4 -HCO 3 (48.57), Ca-Na-Cl- SO 4 - HCO 3 (20 %), Na-Ca-Cl-SO 4 (20 %), Ca-Na- HCO 3 -Cl-SO 4 (5.71%), Na-K-Cl-SO 4 -HCO 3 (2.86 %). The trilinear diagram revealed that majority of the samples is enriched with Na + and Cl - ions.
192 INDIAN J. MAR. SCI., VOL. 37, NO. 2, JUNE 2008 Coastal aquifers with high percentage of sodium and chloride indicates saline water intrusion 11. Conclusion The Hydrochemical investigation of the study area reveals that the groundwater of the area up to a distance of 250 m was found to be brackish to saline in nature. Concentration of anions like nitrate, nitrite and sulphate are within the limit prescribed by WHO (1984) 5 and the phosphate concentration was above the limit, which may be due to saline water mixing. The order of abundance of major cations in the ground water of the area is Na + > >Ca 2+ >Mg 2+ >K + while that of anion is Cl - >HCO - 3 >SO 2-4 >NO - 3 >PO - 4. Correlation analysis of the hydrochemical parameters investigated showed a perfect correlation between EC and TDS, which indicates that EC is a measure of dissolved solids in the ground water. Since the correlation coefficient of Na + and Cl - is fairly high, it can be deduced that for most of the groundwater samples Na + and Cl - originate from a common source. From the Q-mode cluster analysis it was found that samples with high EC, salinity, hardness, Ca 2+, Mg 2+, Na +, K +, Cl - 2-, SO 4 and TDS form one cluster, corresponding to the near shore environment. R-mode cluster indicates that the hydro chemical parameters salinity, TDS, EC and Na + form one group. For most of the samples, alkalies exceed alkaline earths and in all the samples, except one, strong acids exceed weak acids. The dominant hydro chemical facies in the area is Na-Ca-Cl-SO 4 -HCO 3 indicating seawater mixing. Acknowledgements The authors thank Dr. M. Baba, Director, Centre for Earth Sciences Studies, Thiruvananthapuram for giving the necessary facilities to carry out this work. The first author is thankful to Department of Science and technology (DST), Govt. of India for financial support. References 1 Sreedevi, P.D., Groundwater quality of Pageru river basin, Cuddapah district, Andhra Pradesh. J. Geol. Soc. India. 64 (5), (2004) 619-636. 2 Subba Rao, N., & John Devadas, D., Quality criteria for groundwater use for development of an area. J. Appl. Geochem. 7(1), (2005) 9-23. 3 Srinivas Gowd, S, Assessment of groundwater quality for drinking and irrigation purposes: a case study of Peddavanka watershed, Anantapur district, Andra Pradesh, India. Environ, Geol. 48, (2005) 702-712. 4 APHA., Standard methods for the examination of water and wastewater, 19th ed., American Public Health Association, Washington D.C. (1995). 5 Reddy, U.V.B., Reddy K.P,.Sudarsan.V., & Reddy, B.R., Hydrochemistry of Musy River and groundwater, Hyderabad city, Ind.Jour. Environ. Prot., 15(6) (1995) 440-446. 6 Indian Standards Institution, ISI, Indian Standard specification for drinking water, IS-10500 (1983). 7 Sen, Z., Al-Dakheel, A., Hydrochemical facies evaluation in Umm Er Radhuma limestone, Eastern Saudi Arabia Groundwater 21, (1986) 626-636 8 Guler, C, Thyne G, McCray J.E., Turner A.K., Evaluation of graphical and multivariate statistical methods for classification of water chemistry data. Hydrogeol. J. 10: (2002) 455-474. 9 Piper, A.M., A graphic procedure in the geochemical interpretation of water analysis, Union Trans.,Jour. Amer.Geophysics., 25, (1944) 914-928. 10 Laluraj, C.M., Gopinath, G. & Dinesh Kumar, P.K., Groundwater chemistry of shallow aquifers in the coastal zones of Cochin, India. Applied Geology and Environmental Research 3(1): (2005) 133-139. 11 World Health Organisation, WHO, Guidelines for drinking water quality, 1, Recommendations, Geneva. (1984).