Quantifying aquifer properties and freshwater resource in coastal barriers: a hydrogeophysical approach applied at Sasihithlu (Karnataka state, India)

J.-M Vouillamoz,A Legchenko,L Nandagiri,D Caron,M Grammare,J Hoareau

doi:10.5194/hess-16-4387-2012

Abstract

Abstract. Many human communities living in coastal areas in Africa and Asia rely on thin freshwater lenses for their domestic supply. Population growth together with change in rainfall patterns and sea level will probably impact these vulnerable groundwater resources. Spatial knowledge of the aquifer properties and creation of a groundwater model are required for achieving a sustainable management of the resource. This paper presents a ready-to-use methodology for estimating the key aquifer properties and the freshwater resource based on the joint use of two non-invasive geophysical tools together with common hydrological measurements. We applied the proposed methodology in an unconfined aquifer of a coastal sandy barrier in South-Western India. We jointly used magnetic resonance and transient electromagnetic soundings and we monitored rainfall, groundwater level and groundwater electrical conductivity. The combined interpretation of geophysical and hydrological results allowed estimating the aquifer properties and mapping the freshwater lens. Depending on the location and season, we estimate the freshwater reserve to range between 400 and 700 L m−2 of surface area (± 50%). We also estimate the recharge using time lapse geophysical measurements with hydrological monitoring. After a rainy event close to 100% of the rain is reaching the water table, but the net recharge at the end of the monsoon is 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.

Highlights

In Africa and Asia, climate change along with rapid population growth will probably impact all water resources
The resistivity of the first layer is not known because (1) the TEM measurement has poor resolution from ground surface to about 1.5 m deep with the used configuration and equipment and (2) resistive targets are not well resolved with TEM measurements (Albouy et al, 2001). Crossing both magnetic resonance sounding (MRS) and TEM results obtained with the sequential inversion, one can propose an obvious hydrogeological interpretation (Fig. 5c): from ground surface down, we interpret a dry sand layer, a sandy layer saturated with probably brackish water, a sandy layer saturated with sea water
We propose a methodology which enables to estimate the key parameters required for quantifying the groundwater reserve and for managing the resource

Summary

Introduction

In Africa and Asia, climate change along with rapid population growth will probably impact all water resources. The management of groundwater in coastal areas is already critical since the highest concentrations of human settlements occur along the coasts where the rate of water withdrawals has increased three times faster than the rate of population growth since the 1900s (Unesco, 2006). Small islands and coastal barriers in Africa and Asia are usually populated with fishermen communities. Do these communities rely on groundwater for their domestic supply and for providing water to the small fishingrelated industries. In such areas, the infiltration of rain supplies a freshwater lens lying on the top of saline groundwater.

Methods

Results

Discussion

Conclusion