Delineation of freshwater zones in the shallow coastal aquifers of Ernakulam-Chettuva region, Central Kerala, India using electrical resistivity methods

Abstract

Freshwater zones in the shallow aquifers extending from Ernakulam to Chettuva region, central Kerala coast has been studied using electrical resistivity methods. Seasonal salinity patterns and hydrochemistry in the shallow aquifers in the region also has also been studied. The coastal zone is made up of shore parallel ridges and runnels formed by alternate marine trangressive and regressive events. Vertical Electrical Sounding (VES) was carried out at 33 locations in the area extending from the beach to approximately up to 10 km from the shoreline. The subsurface lithology and freshwater layers in the aquifer system was delineated from the resistivity model. The saline zones are intervening with fresh water zones at different depths along the coast. The salinity in the aquifer system seems to be either due to brackish water ingress or paleo-salinity. 107 dug wells from the study area were also monitored during 2008–2010 period in pre-monsoon, monsoon and post-monsoon seasons. The water samples collected in the post-monsoon 2008 was analysed for the physico-chemical parameters viz., pH, temperature, EC, total alkalinity, salinity, turbidity, TDS, chloride (Cl−), total hardness, Ca2+, Mg2+, Na+, K+, CO32−, HCO3−, Cl−, SO42−, NO3−N and Fe2+. The pH of the water samples varied from 4.47–8.32 in the post-monsoon season and 3.5–9.21 in the pre-monsoon and 5.55–9.05 in the monsoon season. The EC values ranges from 58-4500 µ mhos/cm in the post-monsoon, 59-3753 µ mhos/cm in the pre-monsoon and 51-2637 µ mhos/cm in the monsoon season. TDS values ranges from 35-2700 mg/L in the post-monsoon, 38-21190 mg/L in the pre-monsoon and 32-1668 mg/L in the monsoon season. Hill-Piper diagram indicated Ca-HCO3 is the dominant water type followed by Na-Cl type and Na-HCO3 type. Other water types are Ca-Cl, Mg-HCO3, Mg-Cl, Ca-SO4 and MgSO4.