SEASONAL VARIABILITY IN REDOX CONDITIONS IN SHALLOW POREWATER IN THREE CONTAMINATED WETLANDS

High variability in redox conditions often occurs in shallow wetland environments from fluctuating ground-water and surface-water levels. Where organic contaminants discharge to wetlands, elucidation of changing redox conditions and their controlling factors is critical because biodegradation is greatly affected. The shallow (upper 30 to 60 cm) porewater chemistry was determined in small-scale (3.0-cm interval) vertical profiles at three wetland sites— a slough in Norman, Oklahoma, contaminated with landfill leachate; an inland forested wetland at McGuire Air Force Base, New Jersey, contaminated with chlorinated volatile organic compounds (CVOCs); and, a freshwater tidal wetland at West Branch Canal Creek, Aberdeen Proving Ground (APG), Maryland, contaminated with CVOCs. At the Norman and McGuire sites, shallow porewater redox chemistry fluctuated greatly through varying seasonal conditions. Ferrous iron, ammonium, and dissolved organic carbon concentrations increased by a factor of two or more in the shallow porewater at the Norman landfill site during high recharge conditions in the spring, apparently due to increased discharge of these landfill leachate components to the slough. The high concentrations of organic carbon and reduced inorganic contaminants in the landfill leachate plume made it difficult to discern the natural redox cycling in the slough sediments. At the McGuire site, shallow porewater conditions changed from predominantly methanogenic during the summer/fall to predominantly iron-reducing during high recharge conditions in the spring. Ground-water flow and chemistry in the thin, relatively permeable wetland sediments at the McGuire site was largely influenced by rapid influx of oxygenated recharge water. No evidence of biodegradation of the CVOCs was observed during high recharge conditions at the McGuire site. In contrast to the McGuire site, the hydrology and geochemistry in the wetland sediments at West Branch site did not respond to recharge events, and strongly reducing conditions and efficient CVOC biodegradation were maintained throughout wet and dry seasons. This study shows that hydrogeology is critical in controlling shallow redox conditions even in naturally organic-rich wetland sediments.