Variation in orthophosphate, pH, and alkalinity predicted a complex seasonal pattern in equilibrium lead solubility, with two prominent peaks (Figure ). The first occurred in March, corresponding to the minimum seasonal orthophosphate concentration and the first of two seasonal alkalinity maxima. The second occurred in July, corresponding to the second seasonal alkalinity maximum. Both peaks in alkalinity yielded corresponding peaks in calculated dissolved inorganic carbon (Figure ), and at circumneutral pH, equilibrium solubility increases with dissolved inorganic carbon in the presence of orthophosphate. Maximum and minimum predicted lead solubility occurred in March and May, respectively, with mean concentrations of 39 and 27 µg L -1 .