Environmental research specialist, Monsanto Co., Environmental Sciences Center, St. Louis, MO
Pages: 8 Published: Jan 1988
A spill of formaldehyde from a pipe was discovered in October 1983 at a manufacturing site. Groundwater investigation revealed that much of the formaldehyde was trapped within a clay till layer 8 ft below the surface at a concentration of up to 9% by weight. The formaldehyde was leaching out of the unsaturated zone and moving into the saturated zone. A remedial action plan was begun that involved the oxidation of formaldehyde with injection of hydrogen peroxide coupled with pumping withdrawal and reuse of surrounding contaminated groundwater.
In the development of any hazardous waste site/groundwater remedial program, one issue that must be addressed is “When is the remediation finished?” or “How clean is clean?” In the case of the above formaldehyde spill, the specific question was how much formaldehyde can remain in the clay till such that leaching of the formaldehyde to groundwater with subsequent degradation, dispersion, and transport to a down gradient surface water with dilution would result in toxicologically safe concentrations for both humans and aquatic life? A remedial action endpoint of 100 ppm formaldehyde was predicted to achieve these human and environmental health goals using classical hazard assessment methodologies. These hazard assessment procedures are discussed in terms of their relative contribution and usefulness in the overall remedial action process.
Two-dimensional solute transport groundwater modeling was used to predict concentrations that may discharge into a down gradient river. Hydrogeologic features, including water level gradients, dispersivities, hydraulic conductivities, porosity, and aquifer material density, were measured with a calculated leaching rate and formaldehyde biodegradation rates for estimating down gradient solute transport times and concentrations. Formaldehyde entering surface waters would be diluted by appropriate stream flows to yield instream formaldehyde exposure concentrations. Resultant formaldehyde brook concentrations were combined with the conservative no impact concentration of 40 ppb to yield a margin of safety (MS). The instream formaldehyde concentration yielding an MS < 1.0 was used to back calculate the formaldehyde level that could be left in groundwater for dispersion and biodegradation by natural processes (here, 100 ppm). These modeling calculations were used to guide the appropriate choice of the target remedial action endpoint by using conservative assumptions and parameter values. Model results were considered sufficiently valid by local regulatory personnel to be of use as guidance for setting the remedial action target endpoint while the model predictions are still being fully verified. Final verification by measuring formaldehyde instream concentrations is part of the longer term remediation plan (<3 years).
clean, assessment, groundwater, remedial action
Paper ID: STP34062S