Natural remediation processes: bioavailability interactions in contaminated soils

Metals are very common contaminants at ‘Superfund’ sites in US, and similarly contaminated sites worldwide. Long-term deposition of metals in soils can lead to accumulation, transport and biotoxicity/zootoxicity caused by mobility and bioavailability of significant fraction of the metals. Because of their immutable nature strict natural remediation processes alone may not be successful in mitigating the risks from metals at these sites. However, accelerating these processes, i.e., accelerated natural remediation that ultimately immobilizes the metals might be a viable option. Applications to soils of certain soil amendments that enhance key biogeochemical processes in soils that eventually immobilize metals have already been demonstrated in Europe and North America on a field scale. Case studies using lime, phosphate and biosolid materials have demonstrated, under field conditions, accelerated natural remediation, resulting in substantially improved vegetation growth, enhanced microbial population and diversity, and reduced off site metal transport. Depending on soil/hydrogeochemical properties, source term and metal form/species, and land use, the immobilization efficacy induced by such accelerated natural remediation may be enduring. While long-term stability of certain metal complexes such as metal pyromorphites has been shown in model systems, the influence of plant roots and its microbial and mycorrhizal association on such stability is unknown. A suite of chemical and biological tests is available to monitor the efficacy of accelerated natural remediation.