Land Contamination: Technical Guidance on Special Sites: Acid Tar ...

6.3 Natural AttenuationNatural attenuation refers to the reduction of a contaminant concentration via naturallyoccurring physical, chemical or biological processes. For petroleum products, depending onspecific site conditions, this can be an important factor in reducing contaminantconcentrations, the most important processes being (i) biodegradation by micro-organismsnaturally present in the soil and groundwater; (ii) weathering of surface spillages; and, (iii)volatilisation. Additional processes include retardation, sorption, hydrodynamic dispersionand dilution.The main factors that dictate the rate of biodegradation are the oxygen supply (degradation isgenerally more rapid under aerobic conditions) and the molecular structure of the petroleumproducts. As described in TPHCWG (1998), the rate of biodegradation according tomolecular structure is typically affected as follows:• n-alkanes especially in the C10 – C25 range (e.g. diesel and gasoline) degrade moreeasily;• isoalkanes degrade more slowly;• alkenes degrade more slowly than alkanes;• BTEX degradation occurs if they are in concentrations not toxic to the microorganismspresent;• PAHs degrade more slowly than monoaromatics; and• degradation of higher molecular weight cycloalkines may be slower.However, consideration should be given to those chemicals which degrade to more toxicdaughter products.Natural attenuation will not be applicable as a remediation option on all sites, and will only beacceptable if it can be demonstrated that the aforementioned processes are reducing thepollutant load such that there is no current or future unacceptable risk to receptors or areexpected to do so over an acceptable timescale. A thorough site investigation andcharacterisation is required to understand the hydrogeology and geology of the site, and todetermine whether these processes are active. In addition, natural attenuation will require ongoingmonitoring (not only of the pollutant, but also for example, oxygen, carbon dioxide andmethane concentrations if biodegradation is occurring) until it can be demonstrated that thereis no unacceptable risk to receptors.The main benefits of natural attenuation are (i) treatment plants (and their associated costs)are not required; (ii) avoidance of utilisation of landfill capacity; and (iii) relatively littlesurface disturbance. The main disadvantages are (i) a long time periods and (ii) the need forcontinuous monitoring and a robust scientific argument. Further information can be obtainedfrom the Model Procedures, Environment Agency R&D publication 95 (2000) and TPHCWG(1998).R&D <strong>Technical</strong> Report P5-042/TR/04 39