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Natural Attenuation of Chlorinated Solvent Groundwater Plumes Discharging Into Wetlands

By Michelle M. Lorah, David R. Burris, and Linda Jo Dyer

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A substantial number of chlorinated solvent Groundwater plumes at hazardous waste sites are currently discharging, or may potentially discharge, into ecologically sensitive wetland environments. Although chlorinated solvents tend to be relatively resistant to degradation within most aquifer systems, rapid and complete transformations can occur within the organic-rich reducing environment typical of wetland sediments, indicating that monitored natural attenuation may be an effective remediation option for discharges of chlorinated solvent plumes into wetlands. A previous study by the U.S. Geological Survey in a freshwater tidal wetland at Aberdeen Proving Ground (APG), Maryland, showed complete attenuation of chlorinated solvents before the wetland surface or creek was reached. A collaborative study by the U.S. Geological Survey and the Air Force Research Laboratory under the Department of Defense Environmental Security Technology Certification Program was conducted to determine if the natural attenuation of chlorinated solvents that occurs at the APG wetland site can occur at wetland sites in different hydrogeologic environments and to assist in the transfer of this technology to other potential users. The objectives of the demonstration reported here were (1) to assess and compare the extent of natural attenuation of chlorinated solvents at the APG wetland site to an inland forested bog in the Colliers Mills Wildlife Management Area, near McGuire Air Force Base, New Jersey; (2) to demonstrate and compare different methods of sampling and analysis for collecting the site data needed to evaluate natural attenuation in wetlands; and (3) to develop a technical protocol for the assessment of natural attenuation of chlorinated solvent plumes discharging into wetlands. The APG site was used for the second objective of evaluating different methods of sampling and analysis appropriate for assessing natural attenuation in wetlands.

Field and laboratory evidence showed that anaerobic biodegradation of chlorinated volatile organic compounds is less efficient in the Colliers Mills wetland sediment than in the APG wetland sediment. Unlike the APG site, the wetland at the Colliers Mills site responded rapidly to precipitation events, which altered Groundwater flow and chemistry during the spring compared to the drier conditions in the summer and fall. Groundwater flow was predominantly vertically upward from the aquifer and through the wetland and stream-bottom sediments in the summer and fall. Methanogenic or mixed iron-reducing and methanogenic conditions occurred in the shallow wetland sediments during the summer, and biodegradation of the trichloroethene discharging from the aquifer to 1,2- dichloroethene was evident in the wetland sediments under these favorable reducing conditions. When recharge was high in the spring, Groundwater flow was predominantly vertically downward in a large area of the wetland. The influx of oxygenated rainwater was evident in the change to iron-reducing conditions in the shallow wetland sediments, and production of 1,2- dichloroethene was not evident. Vinyl chloride production was not observed at the Colliers Mills site during any season, indicating that biodegradation of trichloroethene was incomplete in the Colliers Mills wetland sediments even when methanogenic conditions occurred.

Similarly high concentrations of dissolved organic compounds in the wetland and streambed sediments at the APG and Colliers Mills sites indicated that organic substrate concentration is not a limiting factor in TCE degradation at the Colliers Mills site. In addition to the high natural organic carbon in the wetland porewater, toluene and benzene, which also can provide a carbon substrate for reductive dechlorination of chlorinated solvents, were present in the wetland porewater at some sites in the Colliers Mills wetland. Although the sites with benzene and toluene had favorable reducing conditions with high methane concentrations, degradation of trichloroethene past 1,2-dichloroethene did not occur.

The less efficient chlorinated solvent degradation in the wetland sediments at the Colliers Mills site most likely can be attributed partly to differences in groundwater residence time in the wetland sediment, and partly to differences in the microbial communities that are active in the wetland. Estimated average linear velocity along upward flowpaths at the Colliers Mills site is about 1.6 meters per year, whereas the upward Groundwater flow velocity is between 0.6 to 0.9 meters per year at the APG site. The thinner wetland sediments at Colliers Mills (0.3 to 1.2 meters) compared to the wetland sediments at APG (1.8 to < 3.6 meters) also would result in a lower residence time for the contaminants in the Colliers Mills wetland sediments, allowing less time for degradation to occur. Macropore flow, which could allow contaminants to bypass anaerobic zones favorable for reductive dechlorination, also was indicated by the relatively high trichloroethene concentrations observed in the stream even when transformation of trichloroethene to 1,2-dichloroethene was observed in the shallow stream-bottom sediments under methanogenic conditions.

If the difference in natural attenuation efficiency at the APG and Colliers Mills sites was caused solely by hydrologic factors, however, similar biodegradation rates would be expected in laboratory microcosms constructed with wetland sediment from the two sites. Trichloroethene degradation in the Colliers Mills microcosms was insignificant over a 30-day period compared to the APG microcosms, indicating that microbial species or groups critical for rapid, complete biodegradation were lacking or inactive at the Colliers Mills site. The periodic increase in redox state of the wetland porewater from influx of oxygenated rainwater most likely is a dominant factor controlling the microbial communities in the Colliers Mills wetland sediments. Factors such as the acidic nature of the wetland porewater at Colliers Mills compared to APG sediments and the different type of vegetation and, therefore, the different available organic substrates, also could result in different microbial communities in the wetland sediments at the two sites.

The comparison of the Colliers Mills and the APG wetland sites shows that natural attenuation of chlorinated solvents may not be efficient at all wetland sites, despite organic-rich characteristics of the sediment. Insufficient supply of organic substrates (electron donors) often has been cited as the primary reason for incomplete reductive dechlorination of chlorinated solvents, but the results of this wetland study indicate that other hydrologic and microbial factors are equally important. This demonstration indicates that precipitation- dominated wetlands may not provide suitable conditions for natural attenuation of chlorinated solvents. Additional studies of wetland sites in other hydrogeomorphic settings that include in-depth characterization of microbial communities would be valuable in understanding the criteria leading to efficient degradation in wetland sediments.

Although anaerobic biodegradation in the Colliers Mills wetland sediments is not complete, sorption and plant uptake also attenuate TCE as it moves from the aquifer and through the wetland sediments. The retardation factor for the Colliers Mills wetland sediment was estimated to be nearly 70, compared to about 10 in the less organic-rich APG wetland sediments. Treecoring data showed that uptake of TCE also occurred by trees, although hydrologic data indicate that this attenuation mechanism probably is significant during only about 6 months of the year. Because of the seasonal and recharge effects on biodegradation and plant uptake, a more complete seasonal and storm-related study would need to be completed at this wetland site to fully evaluate the feasibility of natural attenuation as a remediation method for the TCE plume.

Wetland porewater concentrations were compared for volatile organic compounds, iron, sulfide, and methane using four different sampling devices placed at comparable depths. The sampling devices included (1) drive-point piezometers with 15-centimeter (cm)- long screened intervals, (2) tubing samplers that have inverted screens at the bottom, (3) porous-membrane diffusion samplers (peepers) with sampling chambers spaced 3 cm to 6 cm apart, and (4) multi-level samplers that contain seven screened intervals in one borehole. Samples from the peepers generally showed higher concentrations of ferrous iron, sulfide, and volatile organic compounds than samples from the other devices. The generally higher concentrations of these constituents in the peepers than the other devices may be attributed largely to the lower chance of sample aeration and volatilization in the peepers because samples are collected passively into chambers filled with water, rather than needing to wait for recovery and physically pulling water samples from depth as in the other sampling devices. Peepers showed more distinct trends in redox-sensitive species and in the distribution of intermediate degradation products of the chlorinated VOCs than did the other sampling devices. Thus, the peepers provided the most substantial evidence for the occurrence of degradation in the shallow wetland porewater. Between 100 and 220 cm below land surface (below where the peepers reached), the tube wells and multi-level samplers generally showed higher concentrations of most constituents than were detected in the drive-point piezometers. The results indicate that samples from drive-point piezometers, a common groundwater sampling device, must be interpreted with caution to accurately characterize porewater quality in anaerobic wetland sediments.

A microelectrode system with an electrochemical analyzer that can measure multiple redox-sensitive constituents was tested as a possible method for obtaining measurements by either direct push of the microelectrode into shallow wetland sediments or by insertion of the microelectrode in water samples. The primary advantages of the microelectrode system are: (1) analyses are rapid, taking approximately 3 minutes at each depth or peeper cell to scan for dissolved oxygen, iron, manganese, and sulfide; (2) the sample is minimally disturbed from its natural setting (particularly in the direct push mode of operation), which is important for limiting oxidation or volatilization of redox-sensitive species; and (3) multiple constituents can be measured at extremely small-scale depth intervals without removing porewater or sediment for analyses. In situ microelectrode measurements made in the sediment adjacent to a peeper before its removal from the sediment showed the same general trends in iron and sulfide concentrations with depth as microelectrode measurements made in the peeper cells after removal of the peeper from the sediment. In situ measurements for redox constituents with the microelectrode system also compared well with results of standard chemical analyses on the peeper water. Calibration curves made with the microelectrodes before direct push into the wetland sediment and after their removal from the sediment agreed, indicating that the epoxy-filled glass microelectrodes could be pushed to at least 50 cm below land surface without damage. Much of the contaminant biodegradation occurred within the upper 50 cm of the wetland sediment and withdrawal of porewater samples with devices other than peepers was difficult at these shallow depths. Thus, further development of this in situ tool for redox characterization in wetlands may be warranted.

The current protocol for natural attenuation of chlorinated solvents in Groundwater, which was developed initially by the Air Force Center for Environmental Excellence and then formalized in a U.S. Environmental Protection Agency document, does not address sites where contaminated Groundwater discharges to surface water. As part of this study, an addendum to the natural attenuation protocol for chlorinated solvents was drafted to address unique considerations in developing a site conceptual model and in selecting field methodologies for characterizing natural attenuation processes in wetland environments. Natural attenuation tends to occur in wetlands at a much smaller spatial scale than in aquifers; thus, site characterization and monitoring methods require greater spatial resolution. The complex hydrology and logistical difficulties associated with most wetland work also require special consideration in selection of field methodologies. The technical methodologies included in the natural attenuation protocol addendum for wetland discharges include collection of soil/sediment borings, reconnaissance methods and strategies, installation of multi-level piezometer (or Groundwater sampler) transects, and characterization of the hydrogeology and biogeochemistry. These methods sometimes include equipment that is not commercially available and drilling and analytical methods that are non-standard. Because wetland systems are complex and highly variable, characterization methods appropriate for one site may not be appropriate for another site.

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