Evaluation of hydrochemical variations in coastal alluvial aquifers of Prakasam district, A.P, India

Abstract

The unconfined coastal alluvial aquifers, although occurring in limited extent along ocean tracts, cater to the growing water requirements of most of the seaside habitation. By virtue of the physical location, the chemistry of groundwater in marine environment undergoes rapid modifications and is also susceptible to contamination. Through this paper, an attempt is made to chronicle the temporal changes in water chemistry and extent of contamination in the sandy aquifers utilizing the closely monitored data of 13 sampling sessions spread over 4 years. The water is of weak alkaline nature and less mineralized, electrical conductivity (EC) being <1000 μS/cm in many samples. Major ion composition indicates that 60 to 80% of the samples are polluted with nitrates. Apart from nitrate, the water is also contaminated with respect to pH, EC, Ca2+, Mg2+, Cl−, and SO4 2− as about 5 to 20% of samples in different seasons have some of these parameters in more than the maximum permissible limits of Bureau of Indian Standards drinking water standards. Ionic abundance is in the order of Cl− > Na+ > Ca2+ > HCO3 − > Mg2+ > SO4 2− > NO3 −. Na+ and Cl− are almost in similar proportions implying affect of coastal climate on water quality. Even though spatial variations among constituent species are limited, the water can be classified into two distinct facies viz. Ca-Mg-Cl and Na-Cl-SO4 types. The stoichiometry analysis of long-term water chemistry data enunciate that multiple processes were involved in evolution of aquatic chemistry. Water-rock interaction with the aquifer matrix and carbonate dissolution was the most significant. It was further influenced by evaporation process and anthropogenic inputs which are authenticated by mean ionic ratios of Na+/Cl−, SO4 2−/Cl, Mg2+/Ca2+, and HCO3 −/Cl−. Nitrate contamination, which is more rapid in habitat areas, can be attributed to poor sewerage and domestic waste disposal mechanism. Fertilizer inputs and continuous irrigation and agriculture activity can be accounted for the nitrate enrichment in agriculture areas. With the contaminant transport dynamics being uniform in the aquifer, difference in the intensity of contamination can be attributed to land use variations. Point source pollution from domestic and animal waste landfills is causing enormous damage to the groundwater. The study substantiate that shallow phreatic coastal aquifers suffers more from human interference than coastal climate effect.