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Final Progress Reports: University of California-Davis: Transport, Transformation, and Remediation of Contaminants in the Environment

Superfund Research Program

Transport, Transformation, and Remediation of Contaminants in the Environment

Project Leader: Kate M. Scow
Grant Number: P42ES004699
Funding Period: 1995-2015
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Final Progress Reports

Year:   2014  2009  2004  1999 

The antimicrobial chemical triclocarban (TCC) is used widely in consumer products such as soap and toothpaste and are consequently widely detected in treated waste streams such as wastewater effluent and treated sludge from wastewater treatment plants (biosolids). The effects of TCC were studied by exposing New Zealand mudsnails to concentrations ranging from 0.05 to 10.5 µg/L dissolved TCC and subsequently counting and classifying the embryos as shelled or unshelled. Environmentally relevant TCC concentrations of 1.6 to 10.5 µg/L resulted in statistically significant increases in the number of unshelled embryos. The median effective concentration (EC50) for unshelled effects was 2.5 μg/L. Given the widespread occurrence of TCC in the environment and effects shown at environmentally relevant concentrations, these results indicate that TCC may be causing reproductive effects in the environment. Concentrations we measured of TCC and triclosan (TCS) in wastewater sludges before and after treatment indicated very limited transformation of these chemicals in most biosolid treatment methods and provided another line of evidence supporting the environmental persistence of these compounds.

The effects of TCS on soil microbial communities were measured in soil exposed to biosolids containing the antimicrobial chemical at environmentally relevant concentrations. TCS had little effect on populations of Archaea but initially increased total numbers of Bacteria; this enrichment was no longer evident after one month. Nitrifying, but not denitrifying, bacteria showed strong reductions in population size and activity on exposure to TCS though the effects were less pronounced after one month. Exposure to TCS also caused microbial community composition to shift in comparison to the composition of unexposed communities. These results indicate that TCS entering soil in land-applied biosolids may alter microbial communities and important ecosystem functions.

Field detection of natural attenuation of groundwater contaminants is often key to estimating the impact of the contamination and to determining clean up strategies. High-resolution modeling of groundwater contaminant plume migration in typically complex media was used to investigate the effects of sparse monitoring networks on apparent natural attenuation. Results show that the commonly applied approach of monitoring in a string of wells along the plume “centerline” failed to characterize the plume geometry and extent in nearly all cases. Apparent plume lengths often were substantially less than true plume length, and over time could appear to stabilize, which would lead to false indications of plume mass loss by natural attenuation. A better approach is to make hundreds of point measurements of concentration in several transects oriented normal to the direction of plume movement. This scheme can often correctly capture the total mass flux of contaminants with distance from the source and time from release; however, the transect scheme consistently indicated decreasing mass flux with distance. In the field this would be attributed to actual mass loss of the contaminant, but, to the contrary, the modeling shows it is due to sequestration of contaminant mass in non-aquifer media.

Field methods that both account for the above physical phenomena and for chemical reactions (e.g., compound-specific isotope fractionation) are needed for field detection of natural attenuation. We have begun developing a new random-walk solute transport modeling model capable of modeling reactive transport as well as the above-described physical phenomena. In related work, we have developed a new, highly efficient computer code for determining the connectivity (probability of fast, connected paths for contaminant transport) of 3D, heterogeneous systems.

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