Using radiometric tools to track sediment and phosphorus movement in an agricultural watershed

Increased levels of phosphorus (P) in freshwater systems generally cause eutrophication leading to algal blooms, fish kills, and decreased biodiversity. Point sources have been fairly well characterized; however, non-point sources (NPS), such as agricultural fields, require further study to ascertain the origin and physicochemical forms of P. During a single storm event in June 2008 in a small Wisconsin agricultural watershed (12.4 km2), a comprehensive study was performed to characterize sediment and P transport dynamics. In addition to standard analytical techniques to quantify sediment and P transport, the atmospheric fallout radionuclides (7Be, 210Pbxs, and 137Cs) were employed to determine sediment origin and in-stream transport parameters. Sediments originated primarily from surficial upland soils, or cultivated fields, with minor contributions of resuspended streambed sediments and no discernable stream bank contributions. Sediments were deposited onto the streambed during this event, creating a temporary store, which could be resuspended during subsequent flow events. While for this moderate storm event the stream channels exhibited a short-term depositional behavior they appeared erosional in nature over longer time periods. Particulate-bound P was found to be 33–46% of the total P (TP) transported in the stream channel. The mean dissolved P and TP concentrations at the two stream sites ranged from 0.99 ± 0.32 mg L−1 to 1.14 ± 0.63 mg L−1 and 1.77 ± 0.78 mg L−1 to 1.83 ± 0.78 mg L−1, respectively. During baseflow conditions, the mean dissolved reactive P (DRP) and TP concentrations were quite high, 0.27 ± 0.02 mg L−1 and 0.33 ± 0.04 mg L−1, respectively, exceeding recommended USEPA TP levels (0.08 mg L−1; ) for eutrophication threshold. Overall, significant transport of P in both dissolved and particulate forms occurred during this moderate stormflow event. We assert that improved upland conservation practices are necessary to minimize P export from this agricultural watershed.