Aluminum-based water treatment residuals (WTR) are a waste product of drinking water purification.  The material is relatively inert, comprised primarily of amorphous, non-crystalline aluminum compounds.  Due to its amorphous nature, it has great surface area for reaction to occur, and tends to readily adsorb anions such as phosphorus (PO₄^3-). In 1991, biosolids were surface applied to rangeland research plots owned by the city of Fort Collins, CO to help improve plant growth.  Then, WTR were co-applied to reduce excessive P concentrations added with biosolids application.  Biosolids were applied at a constant rate of 10 Mg ha^-1, and WTR were applied at increasing rates of 5, 10, and 21 Mg ha^-1.  In 2002, with the help of volunteers, both materials were re-co-applied at rates identical to 1991 to one-half of each plot.  The long-term effects of a single application and short-term effects of a repeated application on rangeland soil phosphorus dynamics were studied, finding the majority of inorganic soil phosphorus associated with the WTR.  However, questions arose as to particulate-bound P fate with potential off-site movement.  Therefore, the project goal was to identify the potential for phosphorus release from WTR-biosolids amended soil if the soil particles moved off-site into water bodies of varying pH (4, 5, 6, 7, 8).  Surface 5 cm soil samples were collected, introduced to various pH buffered solutions, and shaken for periods of up to 90 days.  Phosphorus release was measured using the ascorbic acid method.  Results will be discussed.