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

摘要

The growing population in the Missouri Ozarksrely on the abundant supply of potable groundwater found in thefractured bedrock underlying the region. Protection of thisvaluable resource from surface, or near-surface contamination isessential to the health and safety of the citizens of Missouri.Petroleum products stored in underground storage tanks pose athreat to groundwater if a releases occurs. "Pools" of freeproduct of petroleum can become trapped in voids commonlyfound on the karst bedrock of the Ozarks. These pools becomeunderground sources of contamination by slowly dissolving intothe groundwater. Although the free product rarely travels farfrom the surface source, dissolved-phase contamination canmigrate thousands of meters through underground conduits.Dissolved phase petroleum degasses and partitions out of thegroundwater in these conduits. Consequently, contaminatedgroundwater is not Commonly found more than a kilometer ortwo (few thousand feet) from the source.The Missouri Department of Natural Resources, Division ofGeology and Land Survey's Leading Underground Storage TankUnit (DGLS/LUST) investigates sites where petroleum-contaminated groundwater has been found in springs or privatewells but a source of the contamination is unknown. Theseinvestigations start at the regional scale by determining therecharge area of the contaminated groundwater and by identifyingthe potentially responsible parties (PRPs). A review of DGLSdocuments and well logs supplemented by field measurements ofwater levels in private wells throughout the area is used toconstruct a potentiometric map and determine regionalgroundwater flow direction. Water tracing is used to confirm ordeny the validity/accuracy of the potentiometric may and toestablish hydrogeologic connections between the potentialsurface source of the contamination and the affected groundwater.Fluorescent dyes are injected into the subsurface, recovered inpackets containing activated carbon and analyzed by asynchronously scanning spectrofluorometer. These results arecompared to background fluorescence conditions established atthe site prior to injection. The background conditions and dyerecovery results must be submitted to the Missouri Water TraceCommittee for approval as an independent quality assurance/quality control measure.Each of the PRPs found to be within the recharge area of thecontaminated groundwater are investigated to determine thesource, or sources, of the contamination. Initial field screeningtechniques are followed up by confirmatory sampling of soil andgroundwater. Geophysical methods such as resistivity andelectromagnetic conductivity are sued to locate the bedrocksurface, fracture patterns and areas of extensive petroleumcontamination. On-site drilling activities are performed at each ofthe PRPs with on-site contamination screening methods such asanalyzing soil gas within the borehole with a photoionizationdetector (PID) and colorimetric tubes. Soil samples are collectedand can be analyzed on-site with immunoassay kits withconfirmatory samples sent to the lab. Temporary monitoringwells are installed in boreholes to collect on-site groundwaterdata. Depth-to-water levels are measured, and the relativeelevations of the temporary wells are surveyed so that a moredetailed potentiometric map can be constructed. Water samplesare collected from the temporary wells, and the boreholes areabandoned per Missouri Law.Petroleum fingerprinting by fluorescence is another tool usedto investigate PRPs and determine the source of contamination.Petroleum is extracted from soil or water samples collectedduring on-site drilling and compared to the petroleum which isextracted from a sample collected at the contaminated spring or well.