Considering Monitoring Well Bias in the Delineation of Benzene Plume Lengths

Underground storage tanks (USTs) primarily store and dispense fuel products (petroleum hydrocarbons) such as gasoline and diesel as well as some hazardous substances. When a release occurs from an UST, it must be cleaned up and remediated as necessary. As part of the site characterization process, monitoring wells are installed to measure petroleum hydrocarbon concentrations, such as benzene, to help site managers make decisions on remediation strategies and effectiveness, determine site status, and whether sites pose a risk to local receptors-with implications to human health and the environment. Previous studies have used various approaches to estimate benzene plume lengths using these monitoring well data. Examining one state's large database of monitoring well data from leaking underground storage tank (LUST) cleanup sites, and through statistical and spatial analysis, we find that studies that used existing monitoring well data may have underestimated plume lengths. Our study identified elevated benzene concentrations at the boundaries of monitoring well extents, including at the leading edge of plumes. For example, in 3,495 LUST sites, 83% exceeded a selected threshold concentration of 5 ug/L in a monitoring well at the site boundary, and 76% of sites exceeded 5 ug/L at the monitoring well furthest from a defined source, the point used to delineate length. Of the total 17,816 monitoring wells that delineated UST site boundaries, 47% had concentrations exceeding 5 ug/L. These data analyses and results are done over the lifetime of a monitored UST site, not targeted over time periods when decisions are being rendered. However, results suggest that studies to date using existing monitoring well data may have underestimated plume lengths and extents. Furthermore, results suggest monitoring wells in combination with other site information, such as modeling, should be used to guide site managers in their decisions regarding leaked UST products, the area and extent of contamination, risks to receptors, and remedial approaches.