Seasonal Variations in Groundwater Chemistry of a Phreatic Coastal and Crystalline Terrain of Central Kerala, India

This article investigates the seasonal variation in geochemical characteristics of groundwater in phreatic aquifers of the coastal plain and crystalline terrain of central Kerala, India. Within the coastal plain, the western barrier island area shows values of electrical conductivity (EC, > 3 times), Na+, K+, Cl− (four times), Mg2+ (10 times), Ca2+ (two times), and HCO3 −+ CO3 2− (approximately two times) higher than those of the eastern coastal plain during both seasons. In contrast, pH and SO4 2− concentrations showed little variation. The alkaline nature and high EC of groundwater in the western barrier island–coastal plain is found to result from the intrusion of saltwater from the adjacent Vembanad estuary. In contrast, groundwater of the crystalline terrain is found to be slightly acidic with low EC. Within the crystalline terrain, the area east of Muvattupuzha River showed high EC values during both seasons while other parameters showed marginal variation. The trilinear diagram reveals that most of the groundwater samples from the crystalline terrain, which is of type IV (Ca2+–Mg2+–Cl−–SO4 2−) during the premonsoon period, changed character to type II (Na+–K+–Ca2+–HCO3 −) during the postmonsoon period. This shift is possibly caused by the addition of Na+ and HCO3 through the leaching of the country rock. Results for the western barrier island reveals that most of the samples are affected by the saline water incursion. Fecal contamination is found to be comparatively high in the groundwater of the coastal plain versus that of the crystalline terrain. This study indicates that the groundwater from the crystalline terrain is of higher quality than that of the coastal plain. The study further reveals the need for seasonal or multi-seasonal sampling when a geochemical characterization is performed and the recognition of physical events, such as heavy precipitation or droughts, when interpreting the geochemical characteristics of an aquifer.