The Treated Effluent Disposal Facility (TEDF) is a site where treated non-hazardous and non-radioactive liquid wastes are collected and disposed of in two state-permitted disposal basins near the 200 East Area at the Hanford Site. In 2016, a renewal application was submitted for TEDF that requested mass loading effluent limits instead of concentration-based effluent limits. The Washington State Department of Ecology denied this request due to a lack of groundwater monitoring wells that can be used to identify and characterize the impact of TEDF discharges on the underlying groundwater. Current TEDF monitoring wells are screened in the confined aquifer below the Ringold Lower Mud (RLM) unit and are not representative of TEDF discharges that can potentially mound above the RLM unit.
There is a need to identify an appropriate location to install a well that can be used to sample intermittent perched water from TEDF discharges above the RLM unit. The perched water samples will be used to determine if water quality is in compliance prior to reaching the water table. To help identify an optimal location for a monitoring well, geophysical methods will be used to image subsurface transport of TEDF discharges, and to identify important stratigraphic features influencing migration pathways, such as where the RLM unit pinches off to the north. Both electrical resistivity tomography (ERT) and electromagnetic methods will be evaluated for these purposes.
The overall objective of this task is to support the renewal of the TEDF discharge permit by conducting geophysical surveys to identify geologic contacts and the location of the RLM interface that creates perched water conditions. Data from this survey will also be used to aid monitoring well placement by identifying the flow path for subsurface discharge. Before geophysical surveys were performed, numerical simulation was used to estimate the anticipated performance of ERT for identifying the RLM interface and monitoring transport through the subsurface. Since the simulations demonstrated a strong potential for ERT to image TEDF releases, field surveys were approved and are scheduled for FY22. The geophysical surveys will include baseline ERT and towed Time Domain ElectroMagnetic (tTEM) surveys conducted prior to a discharge event, both of which image the bulk electrical conductivity of the subsurface After discharge, time-lapse ERT imaging will be used to survey effluent migration through the subsurface. This document summarizes the simulations and data collected to date.