Characterizing the Oxygen Isotopic Composition of Phosphate Sources to Aquatic Ecosystems

The oxygen isotopic composition of dissolved inorganic phosphate (delta O-18(p)) in many aquatic ecosystems is not in isotopic equilibrium with ambient water and, therefore, may reflect the source delta O-18(p). Identification of phosphate sources to water bodies is critical for designing best management practices for phosphate load reduction to control eutrophication. In order for delta O-18(p) to be a useful tool for source tracking, the delta O-18(p) of phosphate sources must be distinguishable from one another; however, the delta O-18(p) of potential sources has not been well characterized. We measured the delta O-18(p) of a variety of known phosphate sources, including fertilizers, semiprocessed phosphorite ore, particulate aerosols, detergents, leachates of vegetation, soil, animal feces, and wastewater treatment plant effluent We found a considerable range of delta O-18(p) values (from +8.4 to +24.9 parts per thousand) for the various sources, and statistically significant differences were found between several of the source types. delta O-18(p) measured in three different fresh water systems was generally not in equilibrium with ambient water. Although there is overlap in delta O-18(p) values among the groups of samples, our results indicate that some sources are isotopically distinct and delta O-18(p) can be used for identifying phosphate to aquatic systems.