Indices for bioavailability and biotransformation potential of contaminants in soils.

Bioavailability is an important consideration in risk assessment of soil contaminants and in the selection of appropriate remediation technologies for polluted sites. The present study examined the bioavailability and biodegradation potential of phenanthrene with respect to a pseudomonad in 15 different soils through separate measurements of mineralization, transformation, and desorption to a polymeric infinite sink (Tenax) after 180-d sterile pre-equilibration with phenanthrene. Fractions strongly resistant to desorption and mineralization at long times were evident in all cases. After correcting for bioconversion (moles mineralized per mole transformed) determined in aqueous particle-free soil extracts, a correlation was found between the biotransformation-resistant fraction and the Tenax desorption-resistant fraction. Indices are proposed to assess bioavailability (BAt) and biotransformation potential (BTPt) of a compound in a soil based on parallel desorption and degradation studies over a selected period t. The BAt is the ratio of moles biotransformed to moles desorbed to an infinite sink, and it reflects the biotransformation rate relative to the maximal desorption rate. Values of BA30 (30-d values) ranged from 0.64 (for dark gray silt loam) to 1.12 (Wurtsmith Air Force Base [AFB] 2B, Oscoda, MI, USA). The BTPt is the ratio between moles biotransformed and moles of contaminant remaining sorbed after maximal desorption. The BTPt provides an indication of the maximum extent of biotransformation that may be expected in a system, assuming desorption is a prerequisite for biodegradation. Values of BTP30 ranged between 0.3 (Wurtsmith AFB 1B) and 13 (Mount Pleasant silt loam, NY, USA). The combination of BAt and BTPt provides insights regarding the relationship between physical availability (desorption) and biological processes (biotransformation kinetics, toxicity, other soil factors) that occur during biodegradation and are suggested to represent the remediation potential of the chemical. The BA30 values less than 0.9 and BTP30 values less than five indicate poor potential for site remediation.