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Your Environment. Your Health.

Progress Reports: Texas A&M University: Exposure and Risk Assessment of Complex Mixtures

Superfund Research Program

Exposure and Risk Assessment of Complex Mixtures

Project Leader: Robin L. Autenrieth
Grant Number: P42ES004917
Funding Period: 2000-2008

Progress Reports

Year:   2004  2003  2002  2001  2000 

Project 6 is designed to incorporate toxicology data into improved methods for risk characterization and remediation of complex mixtures at Superfund sites.   The risk characterization activities of Project 6 have focused on improving existing models for linking chemical concentrations in environmental media with exposures in humans and wildlife.  Studies are being conducted near a synthetic rubber plant in Sumgayit, Azerbaijan.   A comparison of data from soils, sediments, floor dust, window wipes, animal tissues, and human serum revealed important distinctions.  The soil, sediment, and dust samples were predominantly comprised of larger ring PAHs.  Initial data from human serum and mice tissues revealed the dominance of the two- and three- ring PAHs.  These results triggered implementation of an air monitoring field sampling effort initiated in December of this year.  The results from this initial air sampling event will be used to guide a more detailed series of samplings to occur in early 2005.  The relative similarity between the human serum and mice tissue PAH distributions indicate the potential to harvest mice in and around human habitats as surrogate samples to better approximate human exposures.

Biomarker data have also been obtained for ecological receptors from Azerbaijan.  An integrated method was developed for assessing the cumulative population impacts of chronic contaminant exposure by studying both population genetic and evolutionary effects.  Biomarker results documented elevated somatic genetic damage in marsh frogs from the two most heavily contaminated sites.  Based on mitochondrial DNA control region sequence data, the Sumgayit region has reduced levels of genetic diversity, likely due to environmental degradation.  One of the most contaminated sites in Sumgayit appears to be a source of new mutations resulting from an increased mutation rate.  Therefore, the Sumgayit region acts as an ecological sink, with levels of gene flow into the region exceeding gene flow out of the region. 

Remediation studies have focused on investigating the effect of alkyl substitution on degradation of PAHs.  The effect of molecular structure and properties on the biodegradability of selected PAHs was evaluated for PAHs containing two to four aromatic rings and a number of methylated analogues. Resting cells of acclimated Sphingomonas paucimobilis strain EPA505 under extant conditions degraded all 21 PAHs tested. The Monod or Andrews model was numerically fitted to the depletion curves to determine the specific maximum degradation rates, qmax, the half saturation constants, KS, their ratio qmax/KS, and the inhibition constant, KI. In general, qmax decreased with the size of the PAH and with the degree of methylation. Also, it was found that qmax/KS decreased with the degree of methylation. Microbial growth on the PAHs was indirectly quantitated by a cell proliferation assay measuring reduction of WST-1, a tetrazolium salt, at 450 nm {Δ(NetA450)}. Nine of the PAHs tested supported growth. These included naphthalene and its two monomethylated congeners, phenanthrene and two monomethylated congeners, acenaphthylene, fluorene, and fluoranthene. Methylation either inhibited or prevented growth.

Field data have also been obtained from a Superfund site in EPA Region 8.  A biological sample was taken from the bioreactor that is used to treat groundwater contaminated with pentachlorophenol and PAH mixtures. The biodegradation kinetics of this sample were compared to the S. paucimobilis culture for the biodegradation of phenanthrene. The same protocol used for the sole substrate experiments was followed, the difference being in the culture used for the experiment.  The ‘wild’ culture degraded approximately 30% of the phenanthrene that S. paucimobilis degraded in a period of 4 hours.

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