showing that aluminum interferes with orthophosphate corrosion control. Given our results, this is likely due to both increased solubility and particle-generating mechanisms. And while the full picture is complex, the effect of aluminum on lead solubility-neglecting particles and surfaces-can be modeled by allowing coprecipitation of aluminum and orthophosphate (here as variscite, AlPO4 • 2H2O) in the presence of hydroxypyromorphite (Figure ). We applied this model over a grid of aluminum and orthophosphate concentrations (Figure and) and, neglecting other sources of variation, to the aluminum concentrations measured in the distribution system (Figure ). Consistent with the experimental results, aluminum phosphate precipitation increased lead solubility by decreasing the concentration of orthophosphate in solution. This was predicted to occur except at very low aluminum concentrations (e.g., approximately 50 µg Al L -1 at 1 mg PO4 L -1 , Figure ). At the aluminum levels characteristic of the distribution system, significant seasonal variation in lead solubility is predicted (Figure ). Predicted lead solubility by day of the year, using distribution system aluminum data, pH 7.3, 5 mg L -1 of dissolved inorganic carbon, and 0.5 mg PO4 L -1 as inputs. N.B., one anomalously high record with 0.96 mg Al L -1 is omitted from the plot.