Jayasuriya and Jayasena /Int.J.Econ.Environ.Geol.Vol. 10(4) 07-13, 2019 7 c Refined GIS Mapping to Reinvestigate Groundwater Mining Potential Surrounding the Manmade Reservoirs and Tributaries in the Deduru Oya Basin, Sri Lanka C. Jayasuriya, 1 H.A.H. Jayasena 2 1 Geological Survey and Mines Bureau, 569, Epitamulla Road, Pitakotte, 10100, Sri Lanka 2 Department of Geology, University of Peradeniya, Peradeniya, 20400, Sri Lanka *E mail: cjayasena@pdn.ac.lk Received: 06 April, 2019 Accepted: 31 January, 2020 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. Keywords: Deduru Oya, fractures, reservoirs, recharge, GIS mapping, groundwater mining. Introduction With the increasing demand for the available surface water, the emphasis is now being laid on the extraction of shallow and deep-seated groundwater in crystalline rocks (Jayasena et al., 1986; Jayasena, 1989; Jayasena, 1993; Jayasena, 1995; Jayasena and Dissanayake, 1995). During the driest periods of the year, shallow groundwater extracted from the regolith aquifer (average up to 10 m) through dug wells may dry up. Therefore, government and private agencies pay special attention to extract deep groundwater (fractured aquifer average up to 30 m and deep lineaments) (Jayasena et al., 2018). The availability of groundwater in crystalline rocks is generally less because of the extremely low primary porosity and low interconnectivity (Bell, 1980; Jayasena et al., 1986). However, fractures, joints, deep lineaments, faults and shear zones developed due to local and regional tectonic events could generate a dense fracture network facilitating the movement and occurrence of a significant amount of groundwater. The overall objective is to investigate the hydrogeologic behavior of a fractured crystalline rock terrain, with special reference to identifying areas with high potential for groundwater. It specifically aims at understanding the influence of fracture distribution, manmade reservoirs and diversion canals on groundwater yield in the Deduru Oya Basin (DOB) with the support of ArcGIS (Fig. 1). Hydrogeologic and geomorphic background of the study area The DOB originates from the headwater around 850 m above mean sea level (MSL) in the central highlands and lies between the latitudes 7° 19’ N and 7° 51’ N and the longitudes 79° 47’ E and 80° 34’ E. The elevation drops towards the northwest, and the ridge and valley topography give way to an area of isolated ridges and rock knobs near the center of the basin (Jayasena, 1989; Jayasena, 1998). The mainstream length is about 115 km and the basin covers approximately 2,600 km 2 (Somaratne et al., 2003) which mainly underlain by Precambrian metamorphic rocks. However, recent deposits formed during the Quaternary period cover as a mantle consist of alluviums about 3 km wide floodplain at the western end of the DOB, colluviums flanked by isolated ridges and rock knobs near the center of the basin (Jayasena, 1989; Jayasena, 1998) and regolith as thick as 10 m (Jayasena et al., 2018). The regolith plays a significant role in retaining a substantial amount of groundwater except in the north-central part of the basin. In terms of groundwater exploration, smaller geologic units such as Pegmatites, Dolerites and the sequence of Jurassic sedimentary rocks occur in the northwestern faulted basins significantly control the groundwater Open Access ISSN: 2223-957X Int. J. Econ. Environ. Geol. Vol. 10 (4) 07-13, 2019 Journal home page: www.econ-environ-geol.org Copyright © SEGMITE