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 po st-Pleistocene hydrogeology of a layered sequence of elastic and evaporite sediments. This work is part of an effort to evaluate the suitability of th e Waste Isolation Pilot Plant (WIPP), New Mexico, USA, as a repository for transuranic waste. The numerical model is three-dimensional, extends latera lly to topographic features that form the actual boundaries of a regional g roundwater system, and uses a free surface with seepage face as an upper bo undary condition to simulate the effect of change in recharge rate on the p osition of the water table. Simulation results suggest that the modern-day flow field is still adjusting to the drying of the climate that has occurre d since the end of the Pleistocene Epoch. A wetter climate at the end of th e Pleistocene resulted in a shallow water table, and patterns of groundwate r flow were controlled by the intermediate features of the land-surface top ography. As the climate became drier and the water table declined. groundwa ter flow began to increasingly reflect the land-surface topography at the s cale of the entire groundwater basin. The modern-day flow pattern has not e quilibrated with either the present recharge rate or the position of the wa ter table.