Near-field impact of 216-U-10 (U-Pond) decommissioning on the unconfined aquifer

The Hanford Site has been an active liquid waste disposal facility since 1944. Infiltration of liquid effluents from surface ponds, trenches, and high-volume cribs has been directly responsible for the formation of a conical ground-water mound in the unconfined aquifer below 200 West Area. This mound covers more than 143 km2 (56 mi2) of the unconfined aquifer. As of 1980, the crest of the mound was approximately 139 meters (490 feet) above mean sea level, indicating a water-table rise of 24 meters (80 feet) beneath 200 West Area since 1944. The proposed decommissioning of the 216-U-10 surface infiltration pond (U-pond) has raised the question as to the future impact of decommissioning on the unconfined aquifer below the site, and the unconfined ground-water monitoring network. In order to determine the impact of decommissioning, a finite-difference computer model has been calibrated and validated against historical water-level measurements; the model has accurately represented the past and present response of the unconfined aquifer to inflow from 11 liquid waste disposal sites. This computer model was used to predict the water-level decline in the unconfined aquifer in response to U-pond decommissioning. This analysis assumed all inflow into U-pond will be shut off in 1982. The model predicts declines of about 10 meters (33 feet) within the unconfined aquifer 7 years after U-pond shutdown. The model predictions were used to assess the impact of water-table declines associated with U-pond shutdown on Rockwell Hanford Operations and Pacific Northwest Laboratory ground-water monitoring networks; 19 wells will require redrilling, reperforating, or cleaning before 1989 if U-pond is decommissioned in 1982