respectively. The aluminum residual in treated water is generally highest when water temperature is lowest, due largely to the inverse temperature dependence of aluminum hydroxide solubility at the median coagulation pH of 5.75. Median water temperature in these two months was 5 and 20°C. Orthophosphate also exhibited a seasonal pattern. This is due primarily to variation in the applied corrosion inhibitor dose (Figure ), but seasonal variation in the reversion rate of polyphosphate may have also been a factor. Minimum and maximum orthophosphate concentrations occurred in February and May respectively (130 and 170 µg P L -1 ), approximately opposite those of aluminum (Figure ). Orthophosphate also varied spatially: concentrations were 11% lower in the distribution system compared to the treatment plant, as estimated by a parametric term in the generalized additive model (Table ). Aluminum precipitates with orthophosphate as AlPO4, which may contribute to this difference and to the seasonal pattern in the distribution system.