Geoelectrical and hydrochemical characteristics of a shallow lateritic aquifer in southwestern India

Abstract

• The resistivity values of 50-200 Ω·m in second layer denote the groundwater zone. • The chemical parameters and elements except pH are within BIS and WHO standards. • The groundwater samples belong to Ca-Mg-Cl and Na-Cl type hydrochemical facies. Laterites are one of the important aquifers in the southwestern Indian region, which exhibit declining water levels and acute water shortage during the summer season. In this context, a micro-level study was conducted through vertical electrical sounding (VES) and chemical analyses to assess the characteristics and hydrochemistry of shallow lateritic aquifers of northern Kerala. The VES data indicate the presence of three layers in most of the study area with a H-type curve. The three layers include (1) dry topsoil and/or hard, unsaturated laterite of high resistivity (687-1200000 Ω·m), (2) porous, soft lateritic layer saturated with water exhibiting low resistivity (55.5-1374 Ω·m), (3) hard rock with the highest resistivity (8186-1600000 Ω·m). The resistivity values of 50 to 200 Ω·m are assumed to indicate the presence of groundwater in the region. The groundwater is present in the second layer (6 to 60 m thick) of weathered laterite at a depth ranging from 4.4 to 57.7 m. The spatial distribution maps of various resistivity layers indicate that the northern and north-western parts of the study area are favourable for groundwater exploration through open wells. The interpretations based on resistivity data are corroborated by the depth to water table map prepared from water level data of open wells in the region. The geochemical analysis indicates that the groundwater in the lateritic aquifers is slightly acidic in nature. The cation and anion concentration in most of the samples follow the order Na + > Mg 2+ > K + and HCO 3− > Cl − > SO 4 2− respectively. However, all chemical parameters except pH and elemental concentrations are within the standards of the World Health Organization (WHO) and Bureau of Indian Standards (BIS) for drinking purposes. The groundwater samples in the area predominantly belong to Ca-Mg-Cl (Mixed type) and Na-Cl type hydrochemical facies. The US salinity diagram indicates that all the groundwater samples belong to the category of low salinity and low sodium hazard. The ions in the groundwater mostly originate from rock-water interaction and a few samples belong to the category of precipitation dominance. However, the type of parent rock does not exhibit a significant relationship with groundwater chemistry. The study also highlights the usefulness of electrical resistivity methods for the characterization of shallow lateritic aquifers and assessing the depth of groundwater occurrence in the region.