Vadose Zone VOC Mass Transfer Testing At The SRS Miscellaneous Chemical Basin

Riha, B

Savannah River Site (S.C.)

关键词: Removal;    Mass Transfer;    Diffusion;    Testing;    Remedial Action;   

DOI  :  10.2172/890182
RP-ID  :  WSRC-TR-2005-00266
RP-ID  :  DE-AC09-96SR18500
RP-ID  :  890182

美国|英语

来源: UNT Digital Library

【 摘 要 】

Active remedial activities have been ongoing since 1996 to address low levels of solvent contamination at the Miscellaneous Chemical Basin at SRS. Contaminant levels in the subsurface may be approaching levels where mass transfer limitations are impacting the efficiency of the remedial action. Rate limited mass transfer effects have been observed at other sites in the vadose zone at the SRS, however, detailed measurements and evaluation has not been undertaken. Anecdotal evidence suggests that the mass transfer rates are very slow from the fine grain sediments. This conclusion is based on the observation that measured soil gas concentrations tend to be low in permeable zones relative to the higher concentrations found in fine grain zones. Decreasing soil gas concentration with depth below the ''upland unit'' at several areas at SRS is also evidence of slow diffusion rates. In addition, due to the length of time since disposal ceased at the MCB, we hypothesize that mobile solvents have migrated downward, and the solvent remaining in the upper fine grain zone (''upland unit'') are trapped in fine grain material and are primarily released by gas diffusion (Riha and Rossabi 2004). Natural weathering and other chemical solutions disposed with the solvents can further enhance this effect by increasing the micro-porosity in the clays (kaolinite). This microporosity can result in increased entrapment of water and solvents by capillary forces (Powers, et. al., 2003). Also supporting this conclusion is the observation that active SVE has proven ineffective on VOC removal from the fine grain zones at the SRS. Adsorption and the very slow release phenomenon have been documented similarly in the literature especially for old solvent spills such as at the SRS (Pavlostathis and Mathavan 1992; Oostrom and Lenhard 2003). Mass transfer relationships need to be developed in order to optimize remediation activities and to determine actual loading rates to groundwater. These metrics will aid in answering the question ''How clean is protective of the environment''? For this study, a field test was developed to measure a mass transfer factor by injecting clean air into the subsurface through a sample port or well and measuring the rebound VOC concentration over time. Interpretation of the he results of these tests will provide a mass transfer rate that will be used to determine the appropriate type of SVE for the area (passive, enhanced or active SVE), a measured field parameter to estimate mass loading to the groundwater, and time frame for cleanup.