Sediment nutrient dynamics in selected Milwaukee metropolitan area streams, Wisconsin, 2022

The U.S. Geological Survey and Milwaukee Metropolitan Sewerage District in Wisconsin have an ongoing partnership to monitor water quality in streams in the Milwaukee metropolitan area and to assess the effects of stream restoration on habitat and water quality. Because sediment nutrient dynamics can improve or further impair water quality, we measured sediment nitrogen and phosphorus concentrations, potential nitrogen removal, and potential phosphorus retention at 32 streams sites in the Milwaukee metropolitan area in summer 2022. Four of the sites were in rehabilitated stream reaches. Based on the results from this project, we provided a rating of good, fair, or poor for the sediment nutrient dynamics at each site.

Sediment nitrogen removal and phosphorus retention in stream reaches increased as the proportion of fine particles increased. Bioavailable nitrogen and phosphorus concentrations increased in stream reaches as particle size decreased, especially at locations with more silt and clay. Particle size typically decreased from upstream to downstream, and several of the sites with finer particles were in downstream parts of the study area, especially in the Milwaukee Estuary sites. The sites that had more fine sediment and higher bioavailable nutrients also had elevated rates of nitrification and denitrification enzyme activity, which is a measurement of the potential nitrogen removal when nitrogen and carbon do not limit the denitrifying bacteria. Additionally, sites with fine sediment typically had the highest potential to retain phosphorus as fine sediment provides many binding sites for phosphorus. The binding sites can become saturated with phosphorus, however, increasing the potential for the phosphorus to be released to stream water. Five of the 32 sampled sites were potentially saturated with phosphorus. Sites that contained more agricultural land in their drainage areas were at higher risk of having sediment that was saturated or near-saturation; however, the sites that had more agricultural land in their drainage areas also had higher nitrification rates. Results from this study indicate that stream rehabilitation projects that promote sediment deposition and accumulation of organic matter in the stream channel can increase nitrogen removal and phosphorus retention.