Copper Speciation and Impacts on Bacterial Biosensors in the Pore Water of Copper-Contaminated Soils

Knowledge of heavy metal speciation and its relationship with biological responses is important for the derivation of effects-based soil quality criteria. We determined soluble Cu concentrations and free Cu2+ activities in the pore waters from 22 soils with total Cu varying from 19 to 8645 mg kg-1. Pore water pCu2+ (= −log (Cu2+ activity)) varied from 3.9 to 10.5 and was controlled by soil pH and total Cu concentration. The percentage of free Cu2+ in total soluble Cu varied from 0.02 to 96% and was influenced strongly by pore water pH and, to a lesser extent, by dissolved organic C. In the pore waters with pH >6, the percentage of free Cu2+ in total soluble Cu was lower than 1%. Using the default data base and with the fulvic acid content of DOC optimized at 69%, the equilibrium speciation program WHAM/Model VI gave estimates of pCu2+ that agreed closely with measured values. Pore water samples were analyzed by two bioluminescence-based bacterial biosensors: Escherichia coli HB101 pUCD607 and Pseudomonas fluorescens 10586r pUCD607. The response of P. fluorescens correlated more closely with soil pore water pCu2+ than with soluble Cu concentration, whereas pCu2+ and soluble Cu fitted the response of E. coli equally well. The effect concentrations (EC25 and EC50 values) of pCu2+ for the two biosensors were about 5.8 and 5.0, respectively. This is the first time that threshold values for Cu have been obtained for bacterial biosensors exposed to soil pore water from well-equilibrated contaminated soils.