Groundwater Reservoir Types in Fold and Thrust Belts: Quetta Valley, Pakistan
The Quetta Valley is a N-S trending tectonic depression within folded and faulted Carboniferous-Jurassic limestone (Shirinab and Chiltan fm), and Tertiary shale (Ghazij fm) and conglomerate (Marap and Urak fm). Four million Pakistanis rely solely on the groundwater of the Quetta valley and surroundings (Lat: 29045'-30020', Long 66050'-67015') of the Balochistan Province for drinking and irrigation. Increasing population, excessive use of groundwater, the arid climate (precipitation: ~ 100 mm/yr) have all contributed to progressive decline in groundwater levels (1989-present: 3m decline/yr). An integrated (remote sensing, geochemistry, field, geophysics, modeling) approach was adopted to identify reservoir types, and to select criteria for locating potential productive well locations. Groundwater samples from various aquifers (e.g., alluvial and carbonate aquifers) and settings (e.g., springs, surface and groundwater) are isotopically similar (dD: -48.1 to -28.9 ; dO18: -8.15 to -5.8) and are consistent with compositions (dD: -74 to -37; dO18: -11 to -6.3) of meteoric precipitation over surrounding areas (Ravi River) with similar elevation (1.65 km a.m.s.l.). These results together with the available head data suggest that recharge is largely supplied from surrounding mountains and point to the importance of locating potential wells in bedrock. Criteria for selecting potential well locations in bedrock were facilitated by integrating relevant spatial data sets (Landsat TM, SRTM, SIR-C, geologic maps, cross sections, drainage patterns, structural elements, well locations, and soil maps) in a web-based GIS (http://www.esrs.wmich.edu/groundwater__resources_in_quetta_pakistan.htm). Thirty potential well locations were identified along highly fractured and faulted zones within the Chiltan limestone, a formation commonly exhibiting solution cavities and karstic features. Twelve productive wells were found to suffice these criteria. We also suggest that wells drilled in tectonic depressions reaching the Chiltan limestone might also provide additional groundwater resources. Implications for applying these findings to fold and thrust belts elsewhere are clear.