Copper speciation and impacts on bacterial biosensors in the pore water ofcopper-contaminated soils

Knowledge of heavy metal speciation and its relationship with biological re sponses is important for the derivation bf effects-based soil quality crite ria. We determined soluble Cu concentrations and free Cu2+ activities in th e pore waters from 22 soils with total Cu varying from 19 to 8645 mg kg(-1) . Pore water pCu(2+), (= -log (Cu2+ activity)) varied from 3.9 to 10.5 and was controlled by soil pH and total Cu concentration. The percentage of fre e Cu2+ in total soluble Cu varied from 0.02 to 96% and was influenced stron gly by pore water pH and, to a lesser extent, by dissolved organic C. In th e pore waters with pH >6, the percentage of free Cu2+ in total soluble Cu w as lo;ver than 1%. Using the default data base and with the fulvic acid con tent of DOC optimized at 69%, the equilibrium speciation program WHAM/Model VI gave estimates of pCu(2+) that agreed closely with measured values. Por e water samples were analyzed by two bioluminescence-based bacterial biosen sors: Escherichia coli HB101 pUCD607 and Pseudomonas fluorescens 10586r pUC D607. The response of P. fluorescens correlated more closely with soil pore water pCu(2+) than with soluble Cu concentration, whereas pCu(2+) and solu ble Cu fitted the response of E. coli equally well. The effect concentratio ns (EC25 and EC50 values) of pCu(2+) 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.