Results of Tritium Tracking and Groundwater Monitoring at the Hanford Site 200 Area State-Approved Land Disposal Site Fiscal Year 2000

Abstract

The Hanford Site 200 Area Effluent Treatment Facility (ETF) processes contaminated liquids derived from Hanford Site facilities. The clean water generated by these processes is occasionally enriched in tritium, and is discharged to the 200 Area State Approved Land Disposal Site (SALDS). Groundwater monitoring for tritium and other constituents is required by the state-issued permit at 22 wells surrounding the facility. Water level measurements in nearby wells indicate that a small hydraulic mound exists around the SALDS facility as a result of discharges. This feature is directing groundwater flow radially outward a short distance before the regional northeasterly flow predominates. Evaluation of this condition indicates that the networks currently adequate for tracking potential effects of the SALDS on the groundwater. During FY 2000, average tritium activities inmost wells declined from average activities in 1999. The exceptions are deep well 699-48-77C, where tritium results reached a maximum value of 710,000 pCi/L as a result of the delayed penetration of effluent deeper into the aquifer, and in well 299-W7-3, along the northern boundary of the 200 West Area, which has apparently been affected for the first time by the SALDS tritium plume, with a tritium activity of 1,400 pCi/L measured in August 2000. Of the 12 constituents with permit enforcement limits, which are monitored in SALDS proximal wells, all were within groundwater limitations during FY 2000. The arrival of tritium at well 299-W7-3 demonstrates excellent agreement with the current numerical groundwater model, by virtue of both arrival time and predicted tritium concentration. Analyses for conductivity, total dissolved solids, sulfate, chloride, sulfate, dissolved calcium, and dissolved sodium indicate that well 699-48-77A and, to a lesser extent, well 699-48-77D show the effects of dilute effluent entering groundwater, resulting in a depression of concentrations of these constituents below natural background levels. Recommendations for future monitoring include temporarily increasing the frequency of tritium sampling at wells 299-W7-3, 299-W7-5, 299-W7-6, and 299-W7-7 to quarterly. This measure will assist in a more accurate determination of the southern bounds of the SALDS-generated tritium plume, provide estimates of travel time for model comparisons, and help preserve the distinction between this plume and the older 200 West tritium plume further east. Because of the accurate numerical model predictions thus far, reapplication of the model will occur only after a recognizable departure from model predictions is observed through the increased frequency in well monitoring.

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