A quick and easy way to detect ground water in semi-arid hard rock areas that also happens to be economical could improve the siting of boreholes to improve clean water supply in the developing world. Details of the approach are outlined in the International Journal of Hydrology Science and Technology in June.

P.D. Sreedevi, Dewashish Kumar and Shakeel Ahmed National Geophysical Research Institute in Hyderabad, India, explain how electrical conductivity (EC) logs of hard rock terrain recorded before and after the monsoon season can reveal differences that show where water accumulates most in subterranean rock fissures. By comparing the data with other geological measurements and drilling experiments, the team is available to correlate the EC data with regions of underground water without additional test drilling.

Understanding hard rock aquifers relies on hydrology of fractured rock and knowing details of the subterranean environment. Data is commonly obtained through drilling test boreholes or investigating underground openings. Such work is hazardous and time-consuming and does not necessarily reveal the most appropriate site to sink a water well. However, anomalies in electrical conductivity measurements of which many have been made in various regions might be useful in finding the most abundant sources of ground water.

The researchers demonstrated how effective the approach might be in correlating information from 25 boreholes in the Maheshwaram watershed situated in the Ranga Reddy district of Andhra Pradesh, India, more than 18 miles south of Hyderabad, covering an area of about 23 square miles. The area is semi-arid with average annual rainfall of 29.5 inches. The bedrock is mostly granite. The team points out that, based on the detailed geological and hydrogeological studies, the aquifer is classified as a two-tier coupled system with weathered and fractured layers that exist over almost the entire area. However, due to over-exploitation, the ground water levels have affected the weathered layers, and ground water flow currently is in the fractured rock aquifer. There are no rivers feeding the aquifers, so the system relies on the monsoon to for replenishment.  

"Our approach is fast and cost effective and could be very useful as a screening tool prior to conducting hydraulic testing and water sampling," the team concludes.