Fate of perchlorate in natural systems: Intrinsic biodegradation, plant uptake, and remediation potential of wetlands

Perchlorate (ClO₄−) has recently become a nationwide concern due to the contamination in soil, sediment, surface water, and groundwater. Laboratory and field studies were initiated to examine the potential of wetland systems to treat perchlorate-contaminated waters, to determine the intrinsic degradation kinetics in sediments and soils from multiple sites, and to investigate 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 source at lower perchlorate concentration level. Wetland treatment efficacy was mainly controlled by nitrate concentration, organic substrate availability, and reoxygenation zone near the surface. Microbial degradation played a more important role than plant uptake and transformation in ClO₄− degradation in this type 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 ClO₄− concentration, and prior exposure. Perchlorate degradation rates were mainly affected by organic substrate availability; lag time was mainly controlled by nitrate concentration although organic substrate amendment could also reduce the lag time. Rapid natural attenuation of perchlorate in sediments of natural habitats was demonstrated by using in-situ dialysis sampler-peeper, although perchlorate penetration and persistence were found seasonally variable, with colder season showing the highest penetration. Biodegradation of perchlorate occurred over a depth of only a 1–10 cm although 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 higher than bulk water concentration. Perchlorate uptake in terrestrial plants was dependent on exposure duration, species, and accessibility of perchlorate source. Plants may become the source and sink of perchlorate.