Source-identification investigations of petroleum contaminated groundwater in the Missouri Ozarks

Abstract The growing population in the Missouri Ozarks rely on the abundant supply of potable groundwater found in the fractured bedrock underlying the region. Protection of this valuable resource from surface, or near-surface contamination is essential to the health and safety of the citizens of Missouri. Petroleum products stored in underground storage tanks pose a threat to groundwater if a release occurs. “Pools” of free product of petroleum can become trapped in voids commonly found on the karst bedrock of the Ozarks. These pools become underground sources of contamination by slowly dissolving into the groundwater. Although the free product rarely travels far from the surface source, dissolved-phase contamination can migrate thousands of meters through underground conduits. Dissolved phase petroleum degasses and partitions out of the groundwater in these conduits. Consequently, contaminated groundwater is not commonly found more than a kilometer or two (few thousand feet) from the source. The Missouri Department of Natural Resources, Division of Geology and Land Survey's Leaking Underground Storage Tank Unit (DGLS/LUST) investigates sites where petroleum-contaminated groundwater has been found in springs or private wells but a source of the contamination is unknown. These investigations start at the regional scale by determining the recharge area of the contaminated groundwater and by identifying the potentially responsible parties (PRPs). A review of DGLS documents and well logs supplemented by field measurements of water levels in private wells throughout the area is used to construct a potentiometric map and determine regional groundwater flow direction. Water tracing is used to confirm or deny the validity/accuracy of the potentiometric map and to establish hydrogeologic connections between the potential surface source of the contamination and the affected groundwater. Fluorescent dyes are injected into the subsurface, recovered in packets containing activated carbon and analyzed by a synchronously scanning spectrofluorometer. These results are compared to background fluorescence conditions established at the site prior to injection. The background conditions and dye recovery results must be submitted to the Missouri Water Trace Committee for approval as an independent quality assurance/quality control measure. Each of the PRPs found to be within the recharge area of the contaminated groundwater are investigated to determine the source, or sources, of the contamination. Initial field screening techniques are followed up by confirmatory sampling of soil and groundwater. Geophysical methods such as resistivity and electromagnetic conductivity are used to locate the bedrock surface, fracture patterns and areas of extensive petroleum contamination. On-site drilling activities are performed at each of the PRPs with on-site contamination screening methods such as analyzing soil gas within the borehole with a photoionization detector (PID) and colorimetric tubes. Soil samples are collected and can be analyzed on-site with immunoassay kits with confirmatory samples sent to the lab. Temporary monitoring wells are installed in boreholes to collect on-site groundwater data. Depth-to-water levels are measured, and the relative elevations of the temporary wells are surveyed so that a more detailed potentiometric map can be constructed. Water samples are collected from the temporary wells, and the boreholes are abandoned per Missouri Law. Petroleum fingerprinting by fluorescence is another tool used to investigate PRPs and determine the source of contamination. Petroleum is extracted from soil or water samples collected during on-site drilling and compared to the petroleum which is extracted from a sample collected at the contaminated spring or well.