Quantifying freshwater resource in coastal barriers: the joint use of transient electromagnetic and magnetic resonance soundings

Abstract

Abstract. Many human communities living in coastal areas in Africa and Asia rely on thin fresh water lenses for their domestic supply. Population growth together with change in rainfall patterns and sea level will probably impact these vulnerable groundwater resources. A spatial knowledge of the aquifer properties and the use of groundwater model are required for the sustainable management of the resource. This paper presents a ready-to-use methodology for estimating the key aquifer properties based on the joint use of two non-invasive geophysical tools together with common hydrological measurements. We applied the proposed methodology on a coastal sandy barrier in South-Western India. We found that the joint use of magnetic resonance and transient electromagnetic soundings allows to map the fresh water lens and to estimate the specific yield, the hydraulic conductivity, the water salinity and the water table recharge. From the geophysical results, we estimate the fresh water reserve to range between 400 and 700 l m−2 of surface area according to the location and to the season. Using time lapse geophysical measurements and common groundwater monitoring, we also estimate the recharge of a rainy event to be about 100% of the rain, and the net recharge at the end of the monsoon to be less than 10% of the rain. Thus, we conclude that a change in rainfall patterns will probably not impact the groundwater resource since most of the rain water recharging the aquifer is flowing towards the sea and the river. However, a change in sea level will impact both the groundwater reserve and net recharge.