Stuart Forbes and James Dawick Shell Technology Centre, Thornton, UK
The main study currently used to assess the biodegradation of chemicals in soil is the OECD Guideline 307 for testing chemicals: Aerobic – Anaerobic Transformation in Soil (OECD, 2002b). This test was originally designed to provide degradation rate data for crop protection products but this is now being undertaken to provide data for other ‘chemicals’ under the EU Reach regulations. Many do not believe the current guidelines are suitable to assess the fate of complex substances. For example, the current approach recommended by CONCAWE (Conservation of Clean Air and Water in Europe) for complex hydrocarbon substances is to model persistence of the constituent hydrocarbon blocks. However, as part of the registration of a new substance the European Chemicals Agency (ECHA) have stipulated to the registrants that an OECD 307 study should be undertaken to determine potential persistent hydrocarbon components of a gas to liquid (GTL) fuel.
A series of OECD 307 studies were undertaken on GTL fuel which consists predominantly of branched and linear aliphatic hydrocarbons having carbon numbers in the range of C8 to C26. In an initial study the GTL fuel was applied to a single soil and although it was feasible to undertake an OECD 307 study the test was unable to assess whether losses were due to biodegradation alone. In a second study the GTL fuel was once again added to a single soil. However, in addition three individual n-alkanes (dodecane, hexadecane and eicosane) and a C15 iso-alkane were applied separately to a single soil type to monitor their respective degradation rates. The n-alkanes were added to the soil at a concentration comparable to their respective concentrations in the GTL fuel. An important addition to the second series of studies was that sterile (abiotic) soil systems were also treated with GTL fuel, the individual n-alkanes and the iso-alkane to assess the losses by abiotic factors (e.g. volatilisation and/or non-extractable residues).
By incorporating sterile controls the OECD 307 test has potential to improve the understanding of the fate of components of complex substances like GTL fuel in soil. In particular, it has potential to identify recalcitrant components which may warrant further investigation. However, analytical constraints, different physic-chemical properties of components and dose rates at which the test can be conducted will differ significantly to those of individual substances and all of these factors will complicate interpretation of results. Furthermore, it should be recognised that when using an OECD 307 type soil study to assess the fate of components of complex substances the objectives will differ to those for ‘standard’ OECD 307 studies.
The results of the current studies indicate that, although sterile controls can provide an indication of physical losses, the OECD 307 test will ultimately determine ‘disappearance’ as opposed to biodegradation of components of a complex substance like GTL fuel. The results of the studies indicate that predicted half-lives are conservative and, that no additional bioaccumulation assessments of the components of GTL fuel are warranted based on the premise that even if some remain in soil they will not be bioavailable to soil organisms because they cannot be extracted using acetone and hexane.
Once again the issue regarding persistence was related to another key factor with the lack of persistent components negating the requirement for further bioconcentration studies.