Temperature-driven lead seasonality has been described in previous work, and sampling guidance is often designed to control for temperature effects. But water quality parameters other than temperature can contribute to seasonal lead concentrations, and aluminum is an important example. Aluminum concentrations can vary seasonally when water is treated to remove particles and dissolved organic matter by coagulation. This is because the solubility of the aluminum hydroxide (Al(OH)3) that precipitates during coagulation with aluminum salts is highly temperature-dependent.