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Extensive Database from Over 500 Sites and Three Years Allows Examination and Interpretation of Groundwater MtBE Plumes in Southern California

A unique regulator’s tool in the form of a database has been developed which allows sorting analysis and pinpointing of particular situations (i.e., high MtBE without benzene, rapid declines from high levels of MtBE). The Santa Ana Regional Water Quality Control Board has created an extensive database containing mainly groundwater concentration data. The database includes over 44,000 groundwater samples containing data from over 5300 wells from approximately 500 sites with two-thirds containing groundwater elevations and depths to water. Some groundwater treatment data, significant vapor extraction data containing amount removed and duration of vapor extraction, with some soil concentrations, are also part of the database. The vapor extraction data is useful to estimate the typical amounts of contamination removed and gives a feel for the elusive “volume of release” by comparison to the in situ concentrations. The database has the ability to create maps which show the proximity of UST sites and municipal wells within cities.
The data was analyzed in terms of frequency distribution of total petroleum hydrocarbons, benzene, toluene, ethylbenzene, total xylenes with emphasis on MtBE, on a quarterly basis. Some of the findings of quarterly breakdown concentrations of MtBE in groundwater shows the median concentrations increase over a three year period (1996 through 1998). The system increase in MtBE concentrations are consistent over all concentration ranges (10 to 100,000 µg/L). A site-specific analysis reveals a direct relationship between MtBE concentrations with water table elevation over a three year period. A comprehensive comparative study of MtBE concentration determination by EPA Methods 8020/8021 and 8260/8260A reveals several problems with EPA Method 8020/8021.