A SOURCE-RESPONSIVE MODELING APPROACH FOR EXAMINING ANTHROPOGENICALLY AUGMENTED RECHARGE AND ACCELERATED CONTAMINANT TRANSPORT AT THE IDAHO NATIONAL LABORATORY (INL)

Groundwater contamination associated with past operations at the Idaho National Laboratory (INL) poses a threat to the underlying Snake River Plain aquifer. Recharge to the aquifer is controlled largely by the alternating stratigraphy of fractured volcanic rocks and sedimentary interbeds within the overlying vadose zone, as well as by the availability of water through percolation. Beneath the chemical processing facilities, zones of perched saturation have developed on the low-permeability interbeds due to a combination of ongoing anthropogenic sources of water (e.g., leaky pipes and drainage ditches) and episodic natural sources of recharge (i.e., annual snowmelt and transmission losses from intermittent streamflow). These perched saturated zones are of particular concern as they may accelerate transport of known contaminants through the vadose zone by facilitating lateral flow, which bypasses underlying low-permeability materials to reach well-connected fractures that allow rapid downward flow. Simple approaches for quantitative characterization of this complex, variably-saturated flow system are needed for assessing best- and worst-case contaminant transport scenarios for proposed corrective actions.

Observations from perched zone piezometers and tensiometers at various depths, in combination with temperature, precipitation, and stream discharge records, indicate possible sources and timing of recharge to the perched water. We applied the recently developed source-responsive model, which considers water availability at the entrances of macropore networks, to estimate preferential fluxes through the unsaturated zone originating from the recharge sources identified from data analyses. The model was parameterized, adjusting the macropore facial area density, M (m^-1), to simulate water-level dynamics in the shallow perched zones. Results correspond well to the observed piezometer / tensiometer data, indicating that source-responsive flow through a limited number of unsaturated fractures contributes significantly to vertical fluxes.