Comparison of earthworm and chemical assays of the bioavailability of aged 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene, 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane, and heavy metals in orchard soils

Abstract

Orchard soils can contain elevated concentrations of 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (p,p′-DDE), 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (p,p′-DDT), and heavy metals as a result of historical agrichemical applications. The bioavailability of p,p′-DDE, p,p′-DDT, As, Cd, Cu, and Pb from five field-aged New Zealand orchards and three grazing soils was assessed by using a 28-d bioassay with Aporrectodea caliginosa and chemical assays. Significant relationships were found between total soil and earthworm tissue concentrations for p,p′-DDE (p < 0.001), p,p′-DDT (p < 0.001), Cu (p < 0.001), and Pb (p < 0.01). Two neutral salt solutions, 0.01 M CaCl2 and 1 M NH4NO3, were used as surrogate measures of the bioavailability of heavy metals. Copper was the only heavy metal for which significant correlations were found between neutral-salt-extractable and earthworm tissue concentrations (p < 0.001). Up to 28% of the aged DDT residues were released from the soils by Tenax over a 24-h extraction period. Significant relationships (p < 0.01) between the Tenax-extractable and earthworm tissue concentrations for p,p′-DDE and p,p′-DDT showed that Tenax provides a good surrogate measure of the bioavailability of these compounds to A. caliginosa. Surprisingly, there was a similarly significant relationship (p < 0.001) between the total soil and earthworm tissue concentrations for p,p′-DDE and p,p′-DDT, suggesting that total soil concentrations alone were sufficient to predict uptake by A. caliginosa. These results demonstrate that the aged agrichemical residues in orchard soils, and particularly p,p′-DDE and p,p′-DDT, remain highly bioavailable to A. caliginosa despite decades of weathering and continue to present an environmental risk