On-site evaluation of the effects of carbonaceous amendments on the bioavailability of aged organochlorine pesticide residues in soil

Abstract

Reducing the bioavailability of persistent organic contaminants in soil by incorporating carbonaceous material (CM) has been investigated and confirmed by numerous laboratory studies. However, the efficacy of these methods under more complex field conditions needs exploration. We conducted an 18-month, small-scale plot study (n = 7) to evaluate the ability of two CMs, a compost aged for four months and a compost aged for two years, to reduce the bioavailability of the highly aged organochlorine pesticides (OCPs) dichlorodiphenyltrichloroethane (DDT), its metabolites (together as DDx), and dieldrin in soil. The study was carried out in a former orchard where OCPs were routinely applied until they were banned in the early 1970s. Soil and earthworms were collected, contaminant concentrations were measured, and bioaccumulation factors (BAF) of the OCPs were calculated for the 28 subplots at multiple timepoints. A decrease in the uptake and bioaccumulation of the OCPs was observed with increasing soil concentration, i.e., the relationship between soil and worm concentrations was non-linear. In addition, substantial spatial variability in soil and earthworm concentrations were observed, which resulted in variable BAFs across the site. The soil treated with CMs, 14 subplots in total, showed varying effects on bioaccumulation: for DDx 8 showed a reduction, 4 showed no difference, and 2 showed an increase; and for dieldrin 6 showed a reduction, 5 showed no difference, and 3 showed an increase. Although this study showed some evidence that CM amendments may reduce bioaccumulation over time, the results were not statistically significant due to the spatial and temporal variability. This work illustrates the challenges of extrapolating laboratory results to onsite determinations and suggests that more robust methods are needed in calculating ecological risk assessments for indigenous animals, particularly for legacy sites with substantial spatial variability.