Evaluation and Management of Groundwater in Coastal Regions

Transcription

1 Evaluation and Management of Groundwater in Coastal Regions K. Manikandan, P. Kannan, and M. Sankar Maintaining groundwater quality is the most critical issue in coastal region and it necessitates understanding of seawater intrusion and salinization process in this region for the wise management of groundwater Chemical composition of groundwater in coastal region differs broadly depending on diverse geo-hydrology, hydrometeorology, topography, drainage and other artificial conditions imposed Coastal region is receiving more attention than earlier because of need for more lands to meet the demands of increasing population all over world. This created more pressure on coastal resources, which necessitates rational utilization of land and water in coastal region. Groundwater is the major irrigation source for coastal agriculture besides its domestic utility. The availability of groundwater depends mainly on rainfall and recharge from freshwater sources viz., river and canal. The demand for fresh water is increasing drastically over the years and groundwater is being exploited beyond its renewable capability. This extensive exploitation lower downs the groundwater level resulting in seawater intrusion and associated freshwater crisis in coastal region (Bear et al., 1999). India has a very long coastline and 25% of the country s population lives in the coastal zone. Similarly, urban centers are located mostly along the coast. Factors affecting the coastal ground water The characteristics of ground water in coastal region vary both spatially and temporally. Further the ground water system is so complex as is influenced by many factors in a particular site. Rainfall, landform, soil, lithology, seawater intrusion and other anthropogenic activity are some of the factors determining the ground water quality in coastal region. Sea Water Intrusion When the groundwater is drawn from aquifers having connection with the sea, there may be flow of saline water from the sea. It is known as seawater intrusion. It is very normal to indicate occurrence of any saline or brackish water along the coastal formations as sea water intrusion. In India, sea water intrusion is observed along the coastal areas of Gujarat and Tamil Nadu. The salinization processes in coastal area is very complex which may be due to multitude of factors viz., seawater intrusion, prawn culturing and pollution phenomena (Morell et al., 1996). 1

2 Geogenic Salinity This is the most common quality problem observed in the coastal aquifers. Here the salinity is due to the leaching of the salts in the aquifer material. In some cases, the formation water gets freshened year after year due to the leaching effect. Anthropogenic Impact Rivers are the major contributors of pollution of the coast and coastal aquifers. Almost all the rivers in our country are polluted mostly due to sewerages and industrial effluents. Sea Level Rise The rise in the sea level may push the fresh water seawater interface more inland along coastal aquifers and will submerge low lying areas with sea water, thereby making the shallow aquifers saline. For wise management of ground water it is inevitable to obtain the basic ground water data with respect to distance from sea and in different season. The obtained ground water data needs to be analyzed further and classified. Accordingly management practices need to be suggested for viable land use options in coastal region. Piper classification, Wilcox classification and use of multivariate statistical analysis are some of the measures for determining ground water characteristics. Chemical composition of groundwater in coastal region differs broadly depending on diverse geo-hydrology, hydrometeorology, topography, drainage and other artificial conditions imposed (Kim et al., 2005). Groundwater quality in this intermediary tract is the balance among fresh water recharge, groundwater use and seawater intrusion. Maintaining groundwater quality is the most critical issue in coastal region and it necessitates understanding of seawater intrusion and salinization process in this region for the wise management of groundwater. This article illustrates the methods of studying ground water characteristics and its further evaluation. Groundwater sampling Determination of sampling point and number of sample is most crucial as it decides the preciseness of ground water information. The study area might be divided into different transect zone to represent the region properly. The transect may be of distance from the sea, landform type, soil type or any other criteria if found suitable for the particular region. The number of transects varies with variability in landform, soil, total area and cost of the project. Transect based on distance from sea is most useful approach and inside that landforms influence can be obtained by locating sampling location appropriately. Number of sampling points depends on the local variability in landform, geology, soil, geography and other management practices. Detailed micro level studies require one sample per every ha. In medium scale studies one sample per every ha is suffice. In case of large scale, one sample per each landform is most satisfactory. The depth of sampling is more useful to assess the seawater intrusion in coastal region. 2

3 Water sampling with specific time interval is helpful in assessing the seasonal changes in ground water quality. Three-month interval is most suitable for all occasion. But it may be reduced or lengthened depending on the local conditions. Collection and characterization of groundwater Several ground-water sampling procedures can be used for representative sample collection but no single method or procedure is universally applicable to all types of groundwater sampling. But sample should be of representative for the region. The samples should be drawn as per any of the standard procedures recommended for particular condition. In case of wells and tube wells, it is advisable to collect the samples after minutes of initial pumping. The samples are to be collected in specific bottles for water sampling. Water samples were characterized for physico-chemical constituents viz., cations (Ca 2, Mg 2, Na, K ), anions (HCO 3 -, Cl -, SO 4 2- ), ph, EC, nutrients (N,P,K) and micro nutrients in relevance to agricultural purposes. If need arises microbiological load, BOD, COD, heavy metals also can be analyzed towards pollution analysis. Assessment of groundwater quality The ground water quality is assessed with respect to the any of the following means for agricultural uses. It includes Residual Sodium Carbonate (RSC), Sodium Adsorption Ratio (SAR), Potential Salinity (PS), Soluble Sodium Percentage (SSP), Residual Sodium Bicarbonate (RSBC), Magnesium Hazard (MH) and Permeability Index (PI). These are computed using specific formulae as given below: Residual Sodium Carbonate value (Richards, 1954) RSC = (CO 3 2- HCO 3 - ) (Ca 2 Mg 2 ) Soluble Sodium Percentage value (Richards, 1954) Na SSP = x CA Mg Na Sodium Adsorption Ratio (USSL Staff, 1954) SAR = (CA Potential Salinity (Doneen, 1975) Na Mg 2 2 SO 2- PS = Cl - 4 Permeability Index (Doneen, 1975) x100 ) / 2 (Na HCO3 ) Permeability index = 2 2 Ca Mg Na x100 3

4 Residual Sodium Bicarbonate (Gupta, 1983) RSBC = HCO Ca 2 Magnesium Hazard (Richards, 1954) 2 Mg MH = 2 Ca The values obtained for each index needs to be correlated further for its further management. Graphical methods of ground water classification This ordering of ion is done based on the concentration of major cations (Ca 2, Mg 2, Na, K ) and anions (HCO 3 -, CO 3 2-, Cl -, SO 4 2- ), the total of which is 100 per cent. a) Piper diagram Piper (1944) proposed a trilinear diagram for the classification of waters into different types as shown in Fig.1. In this, total cations and anions are set equal to 100 per cent. In Piper diagram, major ions are plotted as cation and anion percentage in two base triangles. The data points in the two triangles are then projected onto the diamond grid. Every sample is represented by three data points; one in each triangle and one in the diamond grid. Fig. 1: Piper s trilinear diagram 4

5 b) Wilcox diagram Wilcox diagram is a simple scatter plot of sodium hazard (SAR) on the y-axis and salinity hazard (Electrical conductivity, µs/cm) on the x-axis. This diagram was used to determine the viability of water for irrigation purposes (USDA, 1954). Multivariate Statistical Analysis for ground water classification a) Cluster analysis Cluster analysis groups samples by linking inter-sample similarities and illustrates the overall similarity of variables in the data set (Massart and Kaufmann, 1983). Cluster analysis of groundwater forms finite number of clusters among total samples and each cluster represents a specific hydrogeochemical composition of groundwater (Frapporti et al., 1993). Wenning and Erickson (1994) reported that application of cluster analysis was one of the unbiased methods that can help to indicate the natural association between ground water quality and variables. The hierarchical clustering diagram is prepared by adopting ward s method whereby well samples were formed into clusters on the x-axis and the linkage distances appear on the y-axis. The closer linkage distance indicates more similarity between the samples and wide distance indicates less similarity between the samples. b) Factor Analysis Factor analysis is more useful to find out the factors influencing the water quality. It assess the associations between variables as it indicates the participation of individual chemicals amongst several factors of influence (Meglen, 1992). The main stages in the factorial analysis are; the preparation of the Pearson Correlation matrix between standardized variables, the extraction of the initial factors and the transformation of these factors through processes of mathematical rotation until a final solution was reached (Davis, 1986). The varimax rotation procedure is useful in order to make the factors easier to interpret by maximizing the difference between variables. The first factor accounts for as much variance as possible in the data set. The second factor accounts for as much residual variances as possible, and so forth. The weights of the original variables in each factor are called loadings. These are a measure of the extent to which each factor is associated with a particular variable. Conclusion Groundwater quality in coastal region is influenced by multitude of factors viz., rainfall, artificial recharge, soil, topography, domestic exploitation, seawater intrusion and other artificially imposed conditions. Among this seawater intrusion is the prime one which determines the quality in total in coastal lands. Saline water contamination occurs mostly during summer season during which hydrostatic groundwater pressure from inland is low. Improper use of ground water in coastal region create salinity problem which reduces agricultural production besides restricting many other domestic uses. Hence it is essential to assess the ground water quality both spatially and temporally. This can be done using any of the above mentioned approached. This helps apt management of groundwater management in a sustainable manner. 5

6 Further readings Bear, J., A.H.D. Cheng, S. Sorek, D. Quazar and I. Herrera Sea Water Intrusion in Coastal Aquifers Concepts, Methods and practices, Kluwer Academic publishers, Dordrecht. pp.625. Doneen, L.D Water quality for irrigated agriculture. In: Plants in Saline Environments (eds.) A. Poljakoff- Mayber and J. Gale, Springer-Verlag, New York. pp Frapporti, G., R. Knoben and R. Buskens Fuzzy c-means clustering: a multivariate technique for the evaluation of surface-water-quality monitoring Network data. In: Proc. MTM-III - Fuzzy C-means Clustering. pp Gupta, S.K Variations of water table in Yamuna drainage basin of Haryana-Implications and management strategies. In: Proc. Seminar on Strategies for Irrigation Water Management, Patna, IV. pp Kim, J.H., R.H. Kim, J. Lee, T.T. Cheong, B.W. Yum and N.W. Chang Multivariate statistical analysis to identify the major factors governing ground water quality in the coastal area of Kimje, South Korea. Hydrol. Processes, 19: Morell, I., E. Gimenez and M.V. Esteller Application of principal components analysis to the study of salinization on the castellon plain, Spain. The Science of the Total Environ., 177: Richards, L.A Diagnosis and improvement of Saline and Alkali soils. USDA Handbook, 60, USDA, Washington. D.C. USSL, Diagnosis and improvements of Saline and Alkali soils. (ed.) L.A. Richards. Hand book No. 60. USDA, Washington, DC. USDA, Diagnosis and improvements of Saline and Alkali soils. (ed.) L.A. Richards. Hand book No. 60. USDA, Washington, DC. Massart, D.L. and L. Kaufmann The interpretation of analytical data by use of Cluster Analysis, Wiley, New York. Wenning, R.J. and G.A. Erickson Interpretation and analysis of complex environmental data using chemometric methods. Trends in Analytical Chem., 13: Meglen, R.R Examining large database: a chemometric approach using principal component analysis. Marine Chem., 39: Davis, J.C Statistics and data analysis in geology. 2 nd edition, John Wiley and Sons, New York. Dr. K. Manikandan is Scientist Probationer, Directorate of Mushroom Research, Chambaghat, Himachal Pradesh, Mr. P. Kannan Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu. Mr. M. Sankar is Scientist, Centre for Soil and Water Conservation Training Institute, Dehradun, Uttarkhand drkmaniyan@live.com 6