Evaluation of groundwater monitoring results at the Hanford Site 200 Area Treated Effluent Disposal Facility

The Hanford Site 200 Area Treated Effluent Disposal Facility (TEDF) has operated since June 1995. Groundwater monitoring has been conducted quarterly in the three wells surrounding the facility since 1992, with contributing data from nearby B Pond System wells. Cumulative hydrologic and geochemical information from the TEDF well network and other surrounding wells indicate no discernable effects of TEDF operations on the uppermost aquifer in the vicinity of the TEDF. The lateral consistency and impermeable nature of the Ringold Formation lower mud unit, and the contrasts in hydraulic conductivity between this unit and the vadose zone sediments of the Hanford formation suggest that TEDF effluent is spreading laterally with negligible mounding or downward movement into the uppermost aquifer. Hydrographs of TEDF wells show that TEDF operations have had no detectable effects on hydraulic heads in the uppermost aquifer, but show a continuing decay of the hydraulic mound generated by past operations at the B Pond System. Comparison of groundwater geochemistry from TEDF wells and other, nearby RCRA wells suggests that groundwater beneath TEDF is unique; different from both effluent entering TEDF and groundwater in the B Pond area. Tritium concentrations, major ionic proportions, and lower-than-background concentrations of other species suggest that groundwater in the uppermost aquifer beneath the TEDF bears characteristics of water in the upper basalt confined aquifer system. This report recommends retaining the current groundwater well network at the TEDF, but with a reduction of sampling/analysis frequency and some modifications to the list of constituents sought