Our data imply that treatment facilities applying aluminum-based coagulants should ensure that residual aluminum in treated water remains low to limit seasonal variation in the performance of orthophosphate. In the water system we studied, a recent increase in coagulation pH to 6.2 has decreased the median April aluminum concentration at the treatment plant by a factor of more than four relative to the 2003-2016 study period. At the more recent concentrations, predicted aluminum phosphate precipitation is minimal (<1 µmol), even at a higher orthophosphate dose of 1 mg PO4 L -1 . The predicted effect of aluminum on equilibrium lead solubility, then, is also much smaller.