Formatted Title
Abiotic Degradation Can Account for Rate Constants for Natural Attenuation of TCE and cDCE in Groundwater
Background/Objectives
Background/Objectives. When the concentration of a contaminant is plotted against distance along the flow path in a plume, the concentrations along the plume centerline often follow a log-normal relationship. If the natural logarithm of concentrations is plotted against distance, the data plot on a straight line. The slope of the line is the value of a first-order rate constant describing attenuation in concentration with distance along the flow path. In the past, most hydrologists and groundwater scientists attributed the attenuation to dispersion along the flow path. However, in recent years, the contribution of dispersion to attenuation has been re-evaluated. The expected contribution of dispersion cannot explain the bulk rate of attenuation seen in many plumes of chlorinated alkenes. This study evaluated three plumes of chlorinated alkenes in groundwater to determine whether abiotic degradation could explain the bulk rate constant for attenuation of concentrations with distance along the flow path. At one other site, active remediation had impacted concentrations at the head of the plume before the tail of the plume was fully delineated. At this site, abiotic degradation was evaluated to determine whether it could explain the rate constants for attenuation in concentrations over time in individual wells.
Approach/Activities
Approach/Activities. Bulk rates of attenuation were extracted from monitoring data at four sites: (1) a release of TCE and jet fuel at a fire training site at the former Plattsburgh AFB in New York, (2) a release of PCE and other solvents during uncontrolled land disposal of solvents at Site A on the former Twin Cities Army Ammunition Plant (TCAAP) near St Paul, Minnesota, (3) a release of PCE and TCE at fire training site on the former Otis AFB, on Cape Cod, Massachusetts, and (4) a release of TCE at OU3 of the Motorola 52nd Street Superfund Site in Phoenix, Arizona. Microcosms containing sediment from the sites were amended with 14C-labelled TCE or cDCE. The rate of degradation of the TCE or cDCE was determined by measuring the rate of accumulation of 14C-labeled carbon dioxide and 14C labelled polar transformation products. The rate constants for degradation over time in the microcosms were corrected to reflect the rate constants that would be expected at field scale at natural water contents. Then the rate constants over time in the microcosms were divided by the seepage velocity of water at the sites to calculate a rate constant for abiotic degradation with distance along the flow path in the aquifer.
Results/Lessons Learned
Results/Lessons Learned. At the former Plattsburgh AFB, the field scale bulk rate constant was 0.00088 per foot. The rate constant expected from abiotic degradation was 0.0017 per foot. At the former TCAAP, the field scale rate constant was 0.0021 per foot. The rate constant expected from abiotic degradation was 0.0016 per foot. At the site on Cape Cod, the field scale rate constant was 0.00024 per foot. The rate constant expected from abiotic degradation was 0.00026 per foot. At the Phoenix site, the average rate constant for attenuation of TCE over in six wells far downgradient of the pump-and-treat system was 0.13 per year. The rate constant for attenuation over time expected from abiotic degradation was 0.4 per year. In each of the sites, abiotic degradation could account for most or all of the bulk rate constants for attenuation of TCE or cDCE.
At these sites, biodegradation of TCE or cDCE was not expected. Either the water was aerobic, or anaerobic microbial biodegradation products did not accumulate. Degradation was probably carried out through a reaction with magnetite in the aquifer sediment. There was enough magnetite in the sediment to explain the rate constants.