Refined GIS Mapping to Reinvestigate Groundwater Mining Potential Surrounding the Manmade Reservoirs and Tributaries in the Deduru Oya Basin, Sri Lanka

Abstract

A hydrogeologic study was carried out to understand the influence of Man-Made Reservoirs (MMR), tributaries and fracture intensity on well yields within the Deduru Oya Basin (DOB), Sri Lanka. A number of cascaded MMRs interconnected by tributaries are distributed throughout the basin. Fracture traces, lineaments and reservoir boundaries were initially demarcated using aerial photographs, however, subsequently re-plotted them on to a Google Earth map with corrections to rectify the distortion. The GPS based well locations were regenerated and plotted to obtain accurate dimensions. ArcGIS was used to redraw the buffer zones from 0-200, 200-400 and 400-600 m away from the MMRs and tributaries. After eliminating dry wells, box plots were prepared where lower and upper quartiles indicate yield variations from 18-470; 15.8-165 and 12.8–55 liters/minute respectively. It clearly exhibits decreasing yields with respect to distance away from the MMR. However, wells drilled within the alluvial plains of tributaries after filtering those controlled by the MMRs and eliminating dry wells indicate different yield variations, viz: 7-36.8; 12.8-67.5 and 6.5-142.5 liters/minute. The results assigned higher yields to the wells located away from the tributaries with steep hydraulic gradients whereas lower yields to the wells closer to the tributaries with gentle hydraulic gradients. Moreover, wells drilled at fracture interconnections indicate a potential for high yields compared with those drilled along with a single fracture. The study concludes that the potential for groundwater mining can be enhanced by identifying high recharging areas such as MMRs, zones of steeper hydraulic gradients and high fracture interconnectivity.