Author(s): DC Adriano,
Metals are common contaminants 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. Contaminant bioavailability is increasingly being used as a key indicator of potential risk that contaminants pose to both environmental and human health. However, the definition of bioavailability and the concepts on which it is based are still unclear, the methods adopted for its measurement vary and as such there is no single standard technique for the assessment of either plant availability of contaminants or their ecotoxicological impacts on soil biota. Moreover, bioavailability is often assumed to be static in nature where most decisions on risk and remediation are based on laboratory estimations of the bioavailable fraction, which may vary with time, nature of species as well as with temporal variation in environmental factors. Because of their immutable nature, strict natural attenuation processes alone may not be sufficient in mitigating the risks from metals. However, accelerating these processes with human interference (i.e., assisted natural remediation) that effectively immobilizes metals might be a viable option. Application to soils of certain amendments that enhance key biogeochemical processes in soils that effectively immobilize metals have already been demonstrated in Europe and North America on a field scale. Case studies using lime, phosphate and biosolid amendments have demonstrated, under field conditions, enhanced natural remediation resulting in substantially improved vegetation growth, invigorated microbial population and diversity, and reduced offsite metal transport. Depending on soil/hydrogeochemical properties, source term and metal form/species, and land use, the immobilization efficacy induced by such assisted natural remediation may be enduring. The use of green plants as a remediation tool in environmental cleanup has also offered some potential. Plants can uptake and bioaccumulate (phytoextraction) as well as immobilize (phytoimmobilization) certain trace elements, in conjunction with their rhizospheric processes. 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 are available to monitor the efficacy of assisted natural remediation.