HANFORD TANK FARMS VADOSE ZONE MONITORING PROJECT

The Hanford Tank Farms Vadose Zone Monitoring Project (VZMP) was established in fiscal year (FY) 2001 for routine monitoring of 760 existing boreholes in Hanford single-shell tank farms. A baseline record of existing contamination associated with gamma-emitting radionuclides in the vadose zone was established between 1995 and 2000 using high-purity germanium (HPGe) detectors. Evaluation of the baseline data allows specific borehole intervals and frequencies to be identified for monitoring purposes. Because radionuclide identity and concentration are known from the baseline data, it is only necessary to detect changes between successive log runs. Long-term stability of a contaminant plume can be demonstrated by showing that changes between successive log runs are consistent with the radioactive decay process.

The logging system used for monitoring utilizes thallium-activated sodium iodide (NaI[Tl]) detectors to measure gamma activity over a wide range. Although less precise, it is simpler, faster, and more cost-effective. Monitoring measurements collected are compared to the baseline data to assess the long-term stability of the radionuclide contaminant profile in the vadose zone.

Approximately 430 of the 760 boreholes have been monitored as of July 2003. Only 30 of these boreholes have shown evidence of contaminant movement, most of which cannot be confirmed in the short term. The monitoring system and moisture measurements are currently being utilized to evaluate potential impacts to the vadose zone during the tank waste retrieval process. These data will also prove valuable during the tank/tank farm closure process to help determine the long-term mobility of contaminants remaining in the vadose zone.

Results of the Hanford Tank Farms Vadose Zone Monitoring Project are posted on the Internet at: http://www.gjo.doe.gov/programs/hanf/HTFVZ.html.