NUMERICAL SIMULATION OF GROUNDWATER FLOW IN A BASIN UNDER TECTONIC COMPRESSION

The Peshawar Basin and its surroundings in the Himalayan foreland fold-and-thrust belt of northwestern Pakistan are experiencing a total stress of 90 MPa resulting from the Cenozoic India-Eurasia collision. Our study program was undertaken to investigate the relationship between fluid flow and tectonic stresses. The study area extends from Main Karakoram Thrust (MKT) and Main Mantle Thrust (MMT) in the north, though Main Central Thrust (MCT) and Main Boundary Thrust (MBT) in the center to Salt Range Thrust (SRT) in the south. Abundant springs with normal and anomalously high temperatures are present in the northern part of the study area. The southern part is divided into isolated basins with a number of drilled wells and dug-wells. A total of 71 springs and water wells were surveyed and sampled to analyze major and trace elements.

At several of the sampling sites, analyzed water compositions, measured water temperatures, and calculated reservoir temperatures, all point to waters that are anomalous in both chemistry and temperature. Water samples from one shallow well and three deeper wells, all located in the immediate vicinity of major thrust zones (MMT and MBT), demonstrate clear imprints of admixture of oil-brines. Hydrochemical signatures of strontium (Sr), silica (SiO2), boron (B) - and the geothermometric signatures - all indicate a deep circulation of the emerging groundwater. Remarkable clustering of all the thermal and hydrochemical anomalies along the major mapped faults suggests that the anomalous waters ascended along these faults from greater depths.

The basin has been divided into several hydrostratigraphic units to perform numerical simulations using FEMWATER module of GMS (v 5.1). The numerical model is being used to simulate both, deformation and induced fluid flow. Input parameters for the model include recharge, discharge, bulk compressibility of the media and ramping structures to incorporate tectonic compression. The ultimate objective of the numerical simulations is an attempt to distinguish between topography-driven and tectonically induced flow. The results from both simulations will be compared with the field measurements of hydraulic heads to see the extent - if any – of positive residuals, signifying pressure excess over the topography-driven hydraulic heads.