Hydrochemistry and Turnover of the Kerala Tertiary Aquifers, India

Abstract

Coastal aquifers are subject to considerable stress globally. As coastal regions are densely populated areas, the water requirements are large which includes a demand for water within agriculture. Tertiary sections of the coastal sediments in southern Kerala have been investigated since the 1980s, and results are presented here. Four sections have been identified in the Tertiary sequence in southern Kerala, the Warkali, the Quilon, the Vaikom and the Alleppey beds. The Warkali and Vaikom beds are productive aquifers. The Warkali beds are extensively used for water extraction, and the groundwater level has sunk below sea level while there are boreholes into the Vaikom beds that are still artesian having a pressure head of up to 6 m above the present sea level. This indicates that the offshore cover of fine sediments is tight and little discharge occurs onto the seabed. The investigation has shown that the groundwater was recharged 22–34 Ka BP. when the sea level was 80–90 m below the present sea level. The recharge may have been interrupted by a drier period. Some of the samples show elevated chloride levels, but isotopic investigations (δ¹⁸O) show that this is not due to seawater intrusion but rather due to diffusion from pore water in intercalated clay layers. In spite of considerable pumping from the aquifers, there has been no remarkable change in chloride levels over the past 35 years. This is probably due to the freshwater/saline interface which is offshore, a common phenomenon caused by the low sea level during the last glacial maximum. Some of the samples show a reduced water with elevated iron concentrations. Unlike in the Bengal Delta, there is no connection with elevated arsenic concentration. The arsenic concentration has been found to be below 10 µg/l.