Below the solubility minimum at pH 6-7, solubility decreases with increasing temperature, and above pH 6-7, solubility increases with temperature. Treatment facilities that coagulate below the pH of minimum solubility, then, tend to see high residual aluminum in winter. Facilities that coagulate at alkaline pH may see high aluminum in summer. This in turn may influence lead release, but the complex environment of a drinking water distribution system-and the possibility of multiple competing mechanisms-make it difficult to predict aluminum's net effect. Aluminum might precipitate at the scale-water interface as a hydroxide or silicate mineral that slows lead diffusion to the bulk water, but this is controversial. It might also precipitate as a phosphate mineral, diminishing the activity of orthophosphate and preventing formation of hydroxypyromorphite (Pb5(PO4)3OH) and other low-solubility phases that control lead release. Aluminum precipitation that results in suspended particles or colloids may generate a mobile sink for lead, facilitating lead transport from source to tap. Here, we consider aluminum and other seasonally-varying water quality parameters as drivers of seasonal lead release. We use a hybrid approach that combines statistical analysis of observational data, a factorial experiment, and a mechanistic model. We identify possible origins of periodic lead release in the distribution system of Halifax, a mid-sized North American city, and we isolate a subset of these-aluminum, orthophosphate, and temperature-for investigation using a lead coupon study and a geochemical solubility model. We find that interactions between orthophosphate and aluminum have an important effect on lead release and that variation in aluminum concentrations may play a key role in observed lead concentrations. In our view, mechanisms involving soluble, colloidal, and particulate lead are all relevant to this phenomenon.