A groundwater-basin approach to conceptualize and simulate post-Pleistocene subsurface flow in a semi-arid region, southeastern New Mexico and western Texas, USA

Numerical simulation was used to enhance conceptual understanding of the post-Pleistocene hydrogeology of a layered sequence of elastic and evaporite sediments. This work is part of an effort to evaluate the suitability of the Waste Isolation Pilot Plant (WIPP), New Mexico, USA, as a repository for transuranic waste. The numerical model is three-dimensional, extends laterally to topographic features that form the actual boundaries of a regional groundwater system, and uses a free surface with seepage face as an upper boundary condition to simulate the effect of change in recharge rate on the position of the water table. Simulation results suggest that the modern-day flow field is still adjusting to the drying of the climate that has occurred since the end of the Pleistocene Epoch. A wetter climate at the end of the Pleistocene resulted in a shallow water table, and patterns of groundwater flow were controlled by the intermediate features of the land-surface topography. As the climate became drier and the water table declined. groundwater flow began to increasingly reflect the land-surface topography at the scale of the entire groundwater basin. The modern-day flow pattern has not equilibrated with either the present recharge rate or the position of the water table.