Excessive soluble P in runoff is a common cause of eutrophication in fresh waters. Abundant evidence indicates that drinking-water treatment residuals (WTRs) can reduce soluble P concentrations in P-impacted soils in the short term (days to weeks). The long-term (years) stability of WTR-immobilized P has been inferred, but validating field data are scarce. This study was undertaken at two Michigan field sites with a history of heavy manure applications to study the longevity of alum WTR (Al-WTR) effects on P solubility over time (7.5 yr). Amendment with Al-WTR reduced water-soluble P (WSP) concentration by ≥ 60 % of those of the control plots, and the WTR immobilized P (WTR-P) remained stable 7.5 yr after WTR application. We also utilized rainfall simulation techniques to investigate P losses in runoff and leachates from surface soils of the field sites, 7.5 yr after the one-time Al-WTR application. Amendment with WTR reduced dissolved P and bioavailable P (BAP) by > 50 % from both sites, showing that even 7.5 yr after WTR amendment of the sites, the WTR-immobilized P remained non-labile. Thus, WTR-immobilized P would not be expected to dissolve into runoff and leachates to contaminate surface waters or groundwater. Even if WTR-P erodes to surface waters, the bioavailability of the immobilized P will be minimal and will have negligible effects on water quality.