Natural attenuation of high concentrations of organic pollutants by biodegradation in soils

In view of its relatively low cost, monitored natural attenuation by biodegradation is increasingly relied upon to clean up pollution of soils caused by landfills, industrial activities and major transport and storage related spills. Current policy tends to aim at reducing soil pollution to levels reflecting tolerable risk for specific recipients within a reasonable time frame. Natural attenuation by biodegradation of high concentrations of major pollutants that preferentially partition to the particle fraction of soils tends to be poor. These pollutants include hazardous hydrocarbons, highly halogenated hydrocarbons and nitro-organics. Relying on natural attenuation of these compounds, when feasible at all, leads to exceeding tolerable risk levels for a long time. Better perspectives exist for natural attenuation by biodegradation of pollutants that partition to a significant extent to the aqueous phase, especially for low molecular weight organic aromatics and chlorinated solvents. Dependent on conditions in the aquifer and the presence of suitable micro-organisms, there can be substantial biodegradation of a variety of hydrocarbons, organochlorines and oxygenates. In some cases natural attenuation has been found to bring down high levels of pollution to levels meeting current standards of tolerable risk. Predictions whether in the future levels reflecting tolerable risk can be attained, are uncertain. Uncertainty is especially large in case of expanding plumes. There are uncertainties that beset current modelling to predict future concentrations and there is uncertainty about what in the future will be considered tolerable risk. Even when it is supposed that current standards will be applied indefinitely, uncertainties related to present modelling and sampling often prevent certainty that these standards will be met in the future. In practice, natural attenuation is often falling short of attaining promised outcomes. This means that often interventions aimed at enhanced remediation will be necessary to achieve tolerable risk within a reasonable time frame. These may include enhanced biodegradation.