EVALUATION OF AQUIFER SYSTEM IN QUETTA VALLEY THROUGH GEOPHYSICAL METHODS AND GROUNDWATER FLOW MODELING

Abstract

The population of Quetta city and surroundings is almost totally dependent upon groundwater, which is abstracted from the aquifer system of the valley through tube wells. Due to overdraft of groundwater the aquifer system is under tremendous stress. The water table is declining continuously at an increasing rate. The average decline in water table in 2007 was about one meter. The situation is quite alarming and points towards the danger of the complete depletion of the aquifer system. To study the problem in-depth, the aquifer system of Quetta valley was evaluated through integrated application of geophysical methods and groundwater flow modeling. Geometry of an aquifer is a prerequisite for groundwater flow modeling, but there was no data available about the depth to bedrock, therefore, electrical resistivity, seismic reflection and gravity methods were employed. GM SYS; a gravity software of Northwest Geophysical Associates, USA was used for modeling of gravity data and the depth to bedrock determined through interpretation of electrical resistivity and seismic reflection data was used as constraint in the modeling. From the results of gravity modeling a map of depth to bedrock was prepared. For groundwater flow modeling Visual MODFLOW of the Waterloo Hydrogeologic Inc., Canada was used. The aquifer system of the valley was simulated by a simplified model consisting only of two layers, the top layer for the unconsolidated and semi-consolidated rocks aquifers and bottom layer for the consolidated rocks aquifers. Data of the depth to bedrock were put into the groundwater flow model as the bottom of the top layer. The model was successfully calibrated against the water table observation data from 1955 to 1995. Calibration was performed under both the steady and transient conditions. Verification of the model was done with the observation data from 1996 to 2007. With the assumption that the natural conditions of recharge and discharge will not change in future, four future scenarios differing only in the abstraction of groundwater were prepared. These scenarios were simulated and predictions by the model were made for the period 2008 to 2030. Results of the study show that groundwater abstraction is too much high in comparison to recharge of the aquifer, therefore negative water balance will continue and that water table will decline necessarily despite of attempting efforts to arrest it. At the most the decline in water table can be slowed down by keeping the groundwater abstraction at the level of 2007.