The Natural Attenuation of Fatty Acid Methyl Esters in Soil and Groundwater (report no. 5/16)
With the increasing use of fatty acid methyl esters (FAME) in automotive fuels there is growing interest in the fate and effects of biodiesel, and biodiesel/ petroleum distillate blends in the environment. While biodegradation of FAME has been documented, uncertainty remains regarding how FAME partitions and degrades in the subsurface either alone, or in conjunction with other fuel components (e.g. hydrocarbons). A review of technical literature has therefore been completed to bring together the available data in this area. The low solubility of FAME in groundwater means it is likely to behave as a light nonaqueous phase liquid (LNAPL) source with a relatively small region of influence. In this context, B5 or B20 biodiesel/petroleum blends may be expected to behave similarly to petroleum diesel in the subsurface. FAME does not appear to enhance the solubility of hydrocarbons as a whole, or individual components such as poly or mono-aromatic hydrocarbons. Individual FAME compounds have low aqueous solubility, low volatility and low mobility but the mechanisms of autoxidation and hydrolysis may result in the generation of more mobile but equally biodegradable components. FAME is widely reported to be readily biodegradable under both aerobic and anaerobic conditions, although rates may vary from site to site as a function of ground conditions, electron acceptor concentrations and biodiesel composition. FAME appears to enhance the biodegradability of petroleum diesel at concentrations of B20 and higher, but this effect has not been demonstrated at field scale in the context of a subsurface release. At sites with limited electron acceptors and macronutrients (nitrogen and phosphate), microorganisms that degrade FAME have the potential to deplete available electron acceptors and nutrients resulting in an extended time for diesel biodegradation. As with other labile biofuels such as ethanol, anaerobic biodegradation of FAME has the potential to produce significant quantities of methane, which should be taken into account during the risk assessment of biodiesel and biodiesel/ petroleum diesel blend release sites. Overall, natural attenuation would appear to be significant in controlling the fate, behaviour and potential risks posed by biodiesel and biodiesel/ petroleum diesel blends. Significant attenuation mechanisms include sorption, auto-oxidation and biodegradation via a variety of redox processes: the exact role and contribution of each will depend on the nature of the release, the characteristics of the biodiesel and the environmental setting.