Unexpected Mobility and Bioaccumulation of Effluent- and Biosolids-Borne PBDEs and Chlorinated Paraffins in an Organic-Rich Soil System

Land application of wastewater residuals introduces hydrophobic contaminants into soils, where high organic matter (OM) is widely assumed to ensure contaminant immobilization. We tested this assumption in greenhouse mesocosms using an organic-rich soil (50% OM, 50% sand) receiving selected polybrominated diphenyl ethers (PBDEs) and chlorinated paraffins (CPs) from biosolids or effluent. The soil's composition allowed us to test the conflict between high potential for contaminant sorption (to OM) and high hydraulic conductivity (from sand). Despite high soil retention of contaminants (>70%), response curves indicated significant leaching from both sources with up to 11.5% of a short-chain CP (SCCP) being mobilized. While phytoaccumulation was minimal (BAF_dw < 1), earthworms accumulated high loads (BAF_dw up to 23), suggesting a potent trophic transfer pathway. Both mobility and bioaccumulation were source-dependent. For instance, effluent-irrigated soils showed more than 6-fold greater medium-chain CPs (MCCPs) leaching (6.7% vs ∼1%) and higher bioavailability than biosolids-amended soils. These findings suggest hydraulic flux can override the sequestration capacity of organic-rich soils, highlighting the limitations of OM's role as an effective sink under dynamic, nonequilibrium conditions. The study provides critical data urging a re-evaluation of risk assessment models for land-applied wastes.