Temporal Changes in Indicators of Natural Attenuation and Physical Controlling Factors for a Freshwater Tidal Wetland Contaminated With Chlorinated Volatile Organic Compounds, West Branch Canal Creek, Aberdeen Proving Ground, Maryland, 1995-2001
By Michelle M. Lorah, Tracey A. Spencer, and Angela L. McGinty
Since 1992, the U.S. Geological Survey has studied natural attenuation processes occurring as chlorinated volatile organic compounds (VOCs) discharge to a freshwater tidal wetland along West Branch Canal Creek, Aberdeen Proving Ground, Maryland. Field and laboratory studies have shown that anaerobic biodegradation is a major natural attenuation process in the wetland sediments and that monitored natural attenuation could be an effective Groundwater remediation method for the VOCs at this site. In this study, temporal changes in the contaminated aquifer and wetland porewater were characterized using water-quality and physical data collected during 1995-2001 to assist in defining natural attenuation efficiency throughout the year and in developing long-term monitoring plans.
The major contaminants in the aquifer within the natural attenuation study area are 1,1,2,2-tetrachloroethane and trichloroethene. The major contaminants in the wetland porewater are the anaerobic daughter compounds of 1,1,2,2-tetrachloroethane and trichloroethene, including 1,2-dichloroethene and vinyl chloride. A consistent seasonal pattern of higher VOC concentrations in the summer/fall (June-November) than in the winter/spring (December-May) was observed in the aquifer in the upland area. Little seasonal change was observed in VOC concentrations in the aquifer and deeper wetland sediments within the wetland boundary, however. Major ions and constituents indicative of oxidation-reduction (redox) reactions did not show a consistent seasonal variation in the aquifer in either the upland or within the wetland, except for elevated chloride and other inorganic constituents in the uplands during 1996. Porewater collected at 3-centimeter intervals along upward flowpaths through the wetland sediments showed that complete natural attenuation of the VOCs typically occurred before land surface was reached, although low VOC concentrations sometimes were detectable within the upper 10 centimeters during the summer (June-August). A distinct temporal variation in VOC concentrations in the shallow porewater was observed, with VOC concentrations low in the winter/spring and increasing by a factor of 2 to 4 in the summer/fall. VOC concentrations were highest in June 2000. Concentrations of redox-sensitive constituents, including methane, ferrous iron, and sulfide, in porous-membrane diffusion samplers (peepers) indicated that the shallow wetland porewater was anaerobic at all times of the year. Methane concentrations were higher in the summer/fall than in the winter/spring, whereas ferrous iron and sulfide concentrations showed an opposite temporal pattern of lower concentrations in the summer/fall compared to the winter/spring.
Major physical factors that could potentially control temporal variations in the wetland and aquifer geochemistry include Groundwater head gradients and flow patterns, surface-water levels and tidal fluctuations, and sediment temperature. Water levels measured in the aquifer in the upland indicated a consistent seasonal pattern of rising water levels with increased recharge to the aquifer in the late winter and spring, and decreasing water levels in the summer. The lower winter/spring VOC concentrations detected in the aquifer in the uplands could be associated with dilution caused by increased recharge. In contrast, water levels and horizontal and vertical head gradients were remarkably constant throughout the study within the wetland area. Upward discharge from the aquifer was sufficient to maintain watersaturated or nearly saturated (within 10 centimeters of land surface) conditions in the wetland sediments throughout the sampling period, despite summer evapotranspiration demands and below normal precipitation during 1997-1999. Thus, the water-saturated, anaerobic conditions needed for reductive dechlorination and efficient natural attenuation of the VOCs were prevalent throughout the study. Vertical head gradients from the aquifer to the wetland sediments were relatively constant-around 0.2 meter/meter from 1995 through 2000. Horizontal head gradients in the wetland porewater were lower, typically ranged from about 0 to 0.04 (meter/meter), and also were relatively constant throughout much of the study period. During March and June 2000, however, a strong negative horizontal gradient (-0.18 meter/meter) indicated a prolonged period of wetland porewater flow in an inland direction from the creek channel. Mean creek stage was 20 to 25 percent higher in the spring and summer of 2000 compared to the previous 2 years, which probably caused the prolonged flow reversal and the unusual increase in shallow porewater VOC concentrations in 2000.
Although temperatures varied seasonally in the wetland sediments, changes were greatest in the upper 0.15 meters of sediment and were dampened greatly with depth. The temperature fluctuations apparently did not significantly decrease biodegradation because VOC concentrations in the shallow wetland porewater were substantially lower in the winter/spring compared to the summer without breakthrough of parent VOCs to the wetland surface. Overall, the results of this 6-year temporal study indicate that anaerobic conditions and efficient biodegradation of the VOCs is maintained throughout the year in the wetland sediments, supporting earlier conclusions that monitored natural attenuation could be an effective Groundwater remediation method at this site. The relatively constant hydrologic and temperature conditions in the wetland sediment appear to assist in maintaining efficient natural attenuation of the VOCs throughout the year. Although a clear cause of the annual, transient increases in VOC concentrations in the shallow porewater was not determined, the most extreme increases in concentrations of inorganic constituents in 1996 and of VOCs in the summer of 2000 were associated with unusual hydrologic events-above normal precipitation in 1996 and prolonged high creek stage in 2000. Monitoring these physical factors could provide an indication of when unusual concentrations might occur in the wetland porewater; a sampling event could then be initiated.