Evaluation of hydrogeochemical characteristics of phreatic alluvial aquifers in southeastern coastal belt of Prakasam district, South India

The assessment of hydrogeochemical processes that govern the water quality of inland freshwater aquifers in coastal environment, especially in Indian sub-continent, is occasionally attempted. To bridge the gap, a detail hydrochemical evaluation of groundwater occurring in coastal alluvium is attempted. Single set of high-density water sampling is done from a limited area to gain an in-depth knowledge of the processes that govern the water chemistry of the sandy aquifers. The water is of weak alkaline nature and less mineralized, EC being < 1,000 μS/cm in many samples. Major ion composition indicates that water is contaminated with excess concentration of nitrates. Ionic abundance is in the order of Cl^? > Na⁺  > Ca²⁺ > HCO₃ ^? > SO₄ ^2? > Mg²⁺  > NO₃ ^?. Na⁺ and Cl^? are almost in similar proportions implying the influence of coastal climate on water quality. The water shows modest variation in their ionic assemblage among different sample points as evident from Schoeller scheme. Groundwater can be classified into three distinct facies viz. Cl^?–Ca²⁺–Mg²⁺, Na⁺–Cl^? and Ca²⁺–Mg²⁺–HCO₃ ^? types. The ionic assemblages, their indices, ratios and cross-plots substantiate that multiple processes were involved in the evolution of the water chemistry. Among them, silicate weathering, halite dissolution, ion exchange and base exchange played prominent role in the ion enrichment of groundwater. The aquatic chemistry is further influenced and modified by marine environment, evapotranspiration and anthropogenic inputs which is authenticated by good correlation (r ² = 1) among the Na⁺–Cl^?, EC–Mg²⁺, Na⁺ and Cl^?. Gibbs plots established that evaporation is more responsible for contribution of minerals to the groundwater than aquifer material. Nitrate contamination can be attributed for poor sewerage disposal mechanism which is aggravated by fertilizer inputs, irrigation practices and agriculture activity. A contrasting correlation (r ² ≥90 to <0.40) among select pairs of ions reassures dissimilar source of those ions, involvement of multiple processes and limited interaction of formation water with aquifer material.