Site contamination can involve either organic or inorganic fugitive compound(s) or element(s). This brief review will concentrate on possible bioremedial options for organic pollutants in soil or groundwater, or both.
Certain contaminated sites afford the opportunity, based upon an environmental impact assessment or an environmental risk assessment, to allow “natural processes” to take their course Over an extended time period. This is referred to as natural attenuation.
Undertaking an active in situ bioremedial treatment program can involve a number of options and varying levels of complexity. In its simplest form it might involve the injection of nutrients such as ammonium, nitrate, phosphate, or micronutrients at groundwater depths to stimulate the growth of microorganisms already present in this environment. In some cases air is injected at groundwater depths, or within the vadose (unsaturated) zone in an attempt to provide a more aerobic environment for the existing microbial population. In other cases alternate electron acceptors such as nitrate are injected within the contaminated zone to allow specific microbial groups (denitrifiers) to metabolize contaminants in a manner similar to aerobic organisms without the need for molecular oxygen to be present.
Phytoremediation is an emerging and increasingly recognized bioremedial option. It includes the use of phreatophytic plants to consume large volumes of groundwater, thus preventing net groundwater flow further downgradient. This, in turn, can help to prevent further downgradient movement of contaminant plumes. Phytoremediation also describes the essential role aquatic plants play in the bioremedial capability of constructed wetlands. Phytoremediation also refers to the use of plants (and their root systems) to act as primary hosts for microbial populations which, in turn, have the ability to catabolize contaminant organic compounds. It is theorized that as plant roots push forward in their soil environment the associated microbial population will remove soil organic contaminants which might otherwise prove toxic to these plants. Much still remains to be learned regarding this possible symbiotic activity in circumstances of severe soil contamination.