ON THE RELATIONSHIP BETWEEN GROUNDWATER FLOW AND TECTONIC STRESSES: EVIDENCE FROM HYDROCHEMICAL SIGNATURES AND NUMERICAL MODELING

This research was undertaken to study the effects of compressional tectonics on a regional groundwater flow system. The site chosen for study was the Peshawar Basin, in the Himalayan foreland of northwest Pakistan, which is experiencing a tectonic compression of 90 MPa. The study area transects all major thrust faults of the Himalayas. A total of 71 springs (northern study area) and water wells (southern area) were surveyed and sampled for analysis of major and trace elements. We address three aspects of the ground water from the study area: field characteristics, hydrochemical signatures, and model hydrodynamic behavior.

Field measurements of spring- and well-water temperatures show an overall surplus over the mean annual air temperature. Chemical compositions and in-situ measured water temperatures, and calculated reservoir temperatures from several samples 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 (Main Mantle Thrust [MMT] and Main Boundary Thrust [MBT]), exhibit clear imprints of oil-brine admixture. Hydrochemical signatures of strontium (Sr), silica (SiO2), boron (B) - and other cation hydrochemical signatures - all indicate a deep circulation of the ground water. Spatial clustering of thermally and hydrochemically anomalous waters along major mapped faults suggest that these waters ascended from greater depths along the faults.

The basin has been divided into several hydrostratigraphic units in order to perform numerical simulations using the FEMWATER module of Groundwater Modeling System (v. 5.1). Pressure head data generated by the numerical simulations have been compared with the field measurements of hydraulic heads. Results of transient simulations indicate that topography alone is not sufficient to induce the pressure heads observed in the field. Instead, these simulations indicate the presence of uniformly positive residuals over the topography-driven flow, which indicates the additional effect of tectonic compression on subsurface water flow. These initial data support the previously-published, “tectonic-squeegee” model for groundwater flow.