Water Resources Professional Project Reports

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Approximately 9,600 to 10,600 gallons of aviation gasoline (avgas) were released into a shallow aquifer from a leaking underground storage tank located at a municipal airport in Central Wisconsin. A municipal wellfield that serves three neighboring communities is located approximately 3,500 to 4,000 feet downgradient of the release site. Emergency response remedial actions included the removal of contaminated soil from the source area. A sump was installed within the excavation backfill to recover free product and contaminated groundwater. Interim remediation efforts included equipping the sump with a pump and a vacuum well point system was installed. A site investigation was conducted by Earth Tech, Inc. (Earth Tech) to determine the degree, nature, and extent of the avgas release at the municipal airport. The investigation was conducted between November 10, 2003, and August 13, 2004, and included advancing soil borings, installing monitoring wells and piezometers, collecting and laboratory analyzing soil and groundwater samples, and conducting a geophysical survey. An exposure assessment was also completed prior to the preparation of a remedial action plan to evaluate the potential human health risks the avgas release imposes on the municipal wellfield. The following findings were generated as a result of these efforts. • Residual avgas contamination detected in soil exceeding Wisconsin Administrative Code NR 720 Residual Contaminant Levels (RCL) is limited to the immediate vicinity of the avgas releases and just side or downgradient of the releases. • Groundwater beneath the site has been impacted by the avgas releases at levels exceeding Wisconsin Administrative Code NR 140 Enforcement Standards (ESs). The downgradient extent of the impacted groundwater plume is estimated at approximately 300 feet southeast of the release locations. • An undissolved contaminant plume, containing toluene, benzene, lead, and other toxic products that are known carcinogens, has been encountered in 10 wells at the site. • The geophysical survey indicated a bedrock ridge separates two shallow basin structures in the investigation area. The ridge causes the southeasterly movement of groundwater and the plume to divide into southerly and easterly flow components that flow toward the shallow basins. In the southeastern portion of the site, the water table descends into less permeable seams, and the rising bedrock surface causes a thinning of the saturated zone. These natural conditions have helped to minimize the downgradient migration of the contaminant plume. The final remedial action plan needs to address the following relevant transport mechanisms, exposure pathways, and potential receptors affected by the avgas releases: • Leaching and groundwater transport are considered relevant transport mechanisms that final remedial actions will need to address. • Mobile free-liquid migration is considered a relevant transport mechanism at the site; yet, it is not considered a transpOlt mechanism that will continue to release substantial quantities of contaminant mass into the environment. • Inhalation of contaminant vapors in a below grade confined space is considered a relevant exposure pathway at the site. Exposure prevention measures will be required if utility or construction work is required within the impacted area. • Ingestion of contaminated media in a below grade confined space is considered a relevant exposure pathway at the site. Exposure prevention measures will be required if utility or construction work is required within the impacted area. • Construction/utility workers working in below grade conditions within the impacted area at the site would be considered relevant receptors to direct contact with contaminated soil or groundwater or inhalation of contaminant vapors. • The municipal wellfield is considered a relevant receptor at the site; but it is not considered to be a threatened receptor under current site conditions.

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Geology, Hydrogeology, Soil Sampling, Groundwater Sampling, Compound, Aquifer Testing, Volatilization, Leaching


A Professional Project Report Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Water Resources Policy/Management Concentration, Water Resources Program, The University of New Mexico, Albuquerque, New Mexico,December 2004