Carbon isotope fractionation of organic contaminants due to retardation on humic substances: implications for natural attenuation studies in aquifers.

Experimental approaches were developed which permit the measurement of carbon isotope effects during partitioning of organic compounds between water and humic substances. Fractionation factors alpha(sorption) = K(OC)12C/K(OC)13C for carbon isotopomers of benzene (1.00044 +/- 0.00015) and toluene (1.00060 +/- 0.00010) were determined from a 10-step batch experiment. Similar fractionation factors were estimated for benzene (1.00017), 2,4-dimethylphenol (1.00035), and o-xylene (< or = 1.00092) from chromatographic experiments. The latter method is based on chromatographic amplification of the fractionation effect (deltadelta13C) in an HPLC column with humic acid (HA) as the stationary phase. Possible implications of the sorption-based isotope fractionation for assessment of natural attenuation processes in contaminated aquifers are discussed. Depending on the aquifer properties (organic carbon content, heterogeneity) together with the plume source, length, and status (stationary or expanding), scenarios may be constructed where sorption-based isotope fractionation competes significantly with that caused by chemical or microbial degradation processes.