COMPARISON OF SAMPLING METHODS FOR DETERMINATION OF REDOX ZONATION AND NATURAL ATTENUATION OF CHLORINATED SOLVENTS IN A WETLAND, ABERDEEN PROVING GROUND, MD

Biogeochemical reactions in wetland sediments can change over small vertical distances. Determining the fate of organic contaminants in wetland ground water thus requires obtaining samples at discrete small-spaced intervals. In addition to complex spatial heterogeneities and the unstable or volatile nature of many organic compounds and redox-sensitive species, contaminant fate characterization is complicated by the difficulty in obtaining sufficient porewater volumes without changing hydraulic gradients or mixing porewater from different zones. As part of a study of natural attenuation of chlorinated solvents discharging to a freshwater tidal wetland, porewater concentrations of volatile organic compounds (VOCs), iron, sulfide, and methane were compared at six sites using four different sampling methods at comparable depths. The sampling methods included: (1) 1.9-cm-diameter drive-point piezometers, (2) 0.64-cm-diameter tubing samplers with inverted screens at the bottom, (3) multilevel samplers that contain seven 1-cm-diameter screened intervals in one borehole, and (4) porous-membrane diffusion samplers (peepers).

Peepers offered the most reliable results and gave the best overall indication of porewater chemistry. Samples from the peepers generally showed higher concentrations of VOCs and sulfide than samples from the other three devices at the same depths. Methane concentrations in the peepers and multilevel samplers showed fairly good agreement. The lowest concentrations of these three constituents were found in the piezometers; however, the piezometers were generally consistent with the other devices in reduced iron concentrations. The higher concentrations of most constituents in the peepers may be attributed to the lower chance of sample aeration and volatilization because samples are passively collected, tubing insertion for sample withdrawal is unnecessary, and samples do not have to be pulled from depth as in the other devices. Because of their fine sampling resolution, the peepers generally showed more distinct trends in redox zonation and in the distribution of VOC metabolites than the other sampling devices, thus providing more detailed evidence of the occurrence of natural attenuation in the wetland porewater.