Lead toxicity to Lemna minor predicted using a metal speciation chemistry approach

In the present study, predictive measures for Pb toxicity and Lemna minor were developed from bioassays with 7 surface waters having varied chemistries (0.5–12.5 mg/L dissolved organic carbon, pH of 5.4–8.3, and water hardness of 8–266 mg/L CaCO₃). As expected based on water quality, 10%, 20%, and 50% inhibitory concentration (IC10, IC20, and IC50, respectively) values expressed as percent net root elongation (%NRE) varied widely (e.g., IC20s ranging from 306 nM to >6920 nM total dissolved Pb), with unbounded values limited by Pb solubility. In considering chemical speciation, %NRE variability was better explained when both Pb hydroxides and the free lead ion were defined as bioavailable (i.e., f{OH}) and colloidal Fe(III)(OH)₃ precipitates were permitted to form and sorb metals (using FeOₓ as the binding phase). Although cause and effect could not be established because of covariance with alkalinity (p = 0.08), water hardness correlated strongly (r² = 0.998, p