Volatile Organic Compound Investigation Results, 300 Area, Hanford Site, Washington
Unexpectedly high concentrations of volatile organic compounds (VOC) were discovered while drilling in the unconfined aquifer beneath the Hanford Site’s 300 Area during 2006. The discovery involved an interval of relatively finer-grained sediment within the unconfined aquifer, an interval that is not sampled by routine groundwater monitoring. Although VOC contamination in the unconfined aquifer has been identified and monitored, the concentrations of newly discovered contamination are much higher than encountered previously, with some new results significantly higher than the drinking water standards. The primary contaminant is trichloroethene, with lesser amounts of tetrachloroethene. Both chemicals were used extensively as degreasing agents during the fuels fabrication process. A biological degradation product of these chemicals, 1,2-dichloroethene, was also detected. To further define the nature and extent of this contamination, additional characterization drilling was undertaken during 2007. Four locations were drilled to supplement the information obtained at four locations drilled during the earlier investigation in 2006. The results of the combined drilling indicate that the newly discovered contamination is limited to a relatively finer-grained interval of Ringold Formation sediment within the unconfined aquifer. The extent of this contamination appears to be the area immediately east and south of the former South Process Pond. Samples collected from the finer-grained sediment at locations along the shoreline confirm the presence of the contamination near the groundwater/river interface. Contamination was not detected in river water that flows over the area where the river channel potentially incises the finer-grained interval of aquifer sediment. The source for this contamination is not readily apparent. A search of historical documents and the Hanford Waste Information Data System did not provide definitive clues as to waste disposal operations and/or spills that might have resulted in groundwater contamination in this sediment, although several relatively small accidental releases of VOCs have occurred in the past in the northern portion of the 300 Area. It is likely that large quantities of degreasing solutions were disposed to the North and South Process Ponds during the 1950s and 1960s, and that evidence for them in the upper portion of the unconfined aquifer has been removed because of groundwater movement through the much more transmissive sediment. Also, investigations to date have revealed no evidence to suggest that a dense, non-aqueous phase liquid remains undetected in the subsurface. Potential pathways for contamination to migrate from this finer-grained sediment include groundwater movement through the interval to offshore locations in the Columbia River channel, dispersion out of the finer-grained interval into the overlying transmissive sediment (again, with transport to the riverbed), and potential future withdrawal via water supply wells.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 936597
- Report Number(s):
- PNNL-17666; 830403000; TRN: US200818%%1224
- Country of Publication:
- United States
- Language:
- English
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