Fate of perchlorate in natural systems: Intrinsic biodegradation, plant uptake, and remediation potential of wetlands
Perchlorate (CIO4) has recently become a nationwide concern due to contamination in soil, sediment, surface water, and groundwater. Laboratory and field studies were initiated (1) to examine the potential of wetland systems to treat perchloratecontaminated waters, (2) to determine the intrinsic degradation kinetics in sediments and soils from multiple sites, and (3) to determine the fate of perchlorate in streams and near-surface sediments, including temporal and spatial distribution, plant uptake, and biological transformation potential. Results indicated that artificial wetlands might be a promising technology to treat perchlorate-contaminated waters, especially suitable for groundwater plumes or non-point surface water at lower perchlorate concentration levels. Wetland treatment efficiency was mainly controlled by nitrate concentration, organic substrate availability, and reoxygenation near the surface. Microbial degradation played a more important role than plant uptake and ftansformation in this type of wetland system. Perchlorate degradation rates and lag times in sediments and soils were site-specific and dependent on numerous environmental conditions, such as organic substrate availability, nitrate, initial C104' concentration, and prior exposure. Perchlorate degradation rates were mainly affected by organic substrate availability; lag time was mainly controlled by nitrate concentration and organic substrate availability. Rapid natural attenuation of perchlorate in sediments of natural habitats was demonstrated using in-situ dialysis samplers (peepers). Perchlorate penetration and persistence were found seasonally variable, with colder season showing the highest penetration. Biodegradation of perchlorate occurred over a depth of only 1-10 cm, and this active depth changed seasonally. Perchlorate distribution closely mirrored nitrate distribution in sediments. There was a large potential of perchlorate uptake in a variety of aquatic and terrestrial plants. Perchlorate uptake in aquatic plants from streams was up to two orders of magnitude higher than the bulk water concentration. Perchlorate uptake in tertestrial plants was dependent on exposure duration, species, and accessibility of perchlorate source. Plants may become the source and sink of perchlorate. This research has elucidated the fate of perchlorate in natural systems and highlighted the potential of using wetlands to treat perchlorate-contaminated water.