Technical Report No. 180
Donna S. Buxton and Richard E. Green
Sorption processes for Hawaii soils which contain highly sorbed dibromochloropropane (DBCP) residues were analyzed to assess the leachability of the residues and their potential as a source for goundwater contamination. Leaching potential was inferred from measurements of equilibrium distribution coefficients and rates of desorption considered in the context of convective and diffusive transport by water moving in structured soils. Two methods that exploit the volatile nature of DBCP were developed to quantify the partitioning of the nematicide into the sorbed, solution, and vapor phases. The indirect sorption method determined equilibrium distribution coefficients; the air-purge system measured the rate and extent of desorption. Both methods involved analysis of only the vapor phase of a soilpesticide system. Effective pollutant diffusion coefficients and diffusion times were calculated for DBCP movement out of soil aggregates.
The tightly bound sorbed DBCP residues are extremely resistant to desorption. The amount of pesticide from the sorbed phase that can contribute to groundwater contamination during any one rainfall event is on the order of a thousandth to a millionth the amount of the sorbed phase. Even under sustained rainfall or irrigation, it may take from days to months for DBCP to be released from soil aggregates. Thus, movement of surface-soil DBCP residues is not expected to result in future groundwater contamination; the residue concentrations are low, the desorption rate is low, and large travel times to groundwater enhance the likelihood of dispersion and degradation of DBCP.