With lead in 0.45 µm filtrate as the response, several effect estimates in the linear model were notably different. Adding orthophosphate, for instance, caused a much larger percentage decrease in filtrate lead levels (78%, Figure ). This is consistent with orthophosphate's expected effect on lead solubility, while effective control of particulate lead by orthophosphate requires that lead phosphate precipitates become immobilized in corrosion scale. Here, P:Pb molar ratios were much greater than one, a threshold that has been noted previously to promote formation of dispersed lead phosphate particles. Moreover, the dispersive effect of orthophosphate may be especially pronounced at the relatively low hardness characteristic of our experimental water (3.9 mg CaCO3 L -1 , Table ). Dispersion is also enhanced in the presence of humic and fulvic acids. And while coagulation here would have removed the majority of the hydrophobic acid fraction, 70 natural organic matter may still have played a role in dispersing particulate lead. In contrast to its effect on total lead release, aluminum decreased lead in filtrate by 21%