TRANSIENT READJUSTMENT OF REGIONAL-SCALE FLOW SYSTEMS DUE TO CLIMATE CHANGE

Historically, the main reason for a transient analysis of groundwater flow systems has been to account for adjustments due to groundwater withdrawals from wells. Temporal variability in infiltration and recharge rates is not often considered in basin-scale flow models and instead constant, long-term average rates are used. There is growing awareness of just how variable climate can be over a variety of different time scales, and the concomitant influence of these changes on recharge. Our focus here is to examine the impacts caused by the variability in rainfall since the last interglacial (LIG) period on groundwater flow in a major segment of the Death Valley Flow System that passes through Yucca Mountain. Unique aspects of this flow system include the long time scale over which infiltration to the system has changed (tens-of-thousands of years), the thickness of the unsaturated zone and the large physical extent of the flow system (many tens-of-kilometers).

The numerical analysis involves a dual-porosity, dual-permeability model of variably-saturated flow developed for a two-dimensional cross-section that is about 40 km long. The hydrostratigraphic framework captures the key aquifers and aquitards in the upper 2000. Estimated paleo-infiltration rates are derived from a fully-coupled global climate model that extends from the last interglacial period (approximately 125 ka BP) to present. To understand the evolution of the flow system, the analysis includes simulating the migration of an ideal tracer and the calculation of mean groundwater ages. The flow model results suggest a relatively large time constant, because the system requires a very long time period to re-equilibrate following an adjustment to the rate of recharge. This result has important implications for the conceptualization and simulation of regional-scale groundwater flow systems.