Bioavailability of Copper and Zinc in Mining Soils

The soil-contact exposure pathway can be the main driver of ecological risk assessments. There is currently no standard method to measure bioavailability of metals in soil to ecological receptors, yet the influence of metal bioavailability on toxicity has been known for decades and is a major factor influencing risk to ecological receptors. Bioavailability is to a large degree governed by varying soil characteristics within and among sites, yet ecological screening benchmarks are often derived on a total-concentration basis. We compared a calcium chloride (CaCl₂) extraction, cyclodextrin extraction, simulated earthworm gut (SEG) test, earthworm kinetic bioaccumulation test, and metal residues in plant tissues with a battery of invertebrate and toxicity tests using mining soils consisting of high organic-matter content cocontaminated with copper (Cu) and zinc (Zn). Earthworm (Eisenia andrei) tissue concentrations of Cu and Zn were regulated and were not predictive of invertebrate toxicity. All chemical measures of bioavailability correlated with several biological responses; however, CaCl₂-extractable Cu and SEG-extractable Cu and Zn best predicted effects to E. andrei. Total Cu concentrations in soil best correlated with effects to plants. Overall, a chemical measure was the best predictor of toxicity to each organism compared with biological measures, although the exact measure was dependent on organism and end point. Chemical-extraction techniques provide relatively quick, inexpensive indicators of essential metal bioavailability compared with biological measures; however, no single measure was indicative of all effects to all organisms.