Student researchers find Centerville Creek shows repeatedly high phosphorus; cover crops and targeted restoration cited as mitigation

Students and staff at the 2025 Lakeshore Water Summit presented summer monitoring results from 25 sampling sites across three Manitowoc County streams and warned that Centerville Creek showed persistently higher phosphorus levels than the other watersheds sampled.

"We sampled 25 sites across Little Manitowoc, Pine Creek, and Centerville Creek on Mondays and during rain events," Chi, the lab coordinator, said, describing the team’s sampling plan and laboratory methods. The team measured pH, turbidity, dissolved oxygen, temperature, flow, depth, E. coli and both total and total dissolved phosphorus to assess stream health.

The students said land use and rainfall together explained much of the variation in nutrient levels. "CS 13 is the most upstream site ... with over 81% of its land being agricultural," Chi said, identifying CS 13 (South Centerville branch, near South Union Road) as a persistent hotspot that spikes in phosphorus and ammonia after storms. Farley, the project QA/QC coordinator, reported statistically stronger land-use correlations in Centerville (r²≈0.4433 for agricultural percentage vs. non-dissolved phosphorus) than in Pine Creek (r²≈0.3679). Correlations between agricultural land and E. coli were weak (r²≈0.016–0.108 across sites), the presenters said.

Quentin, the field coordinator, explained the rain-event response: "After the 24 hours after the rain events, you can see how the phosphorus spikes, but when we go a few days later, it is mostly normalized." He also noted the team recorded several sampling dates below the 0.075 mg/L phosphorus threshold the presenters cited ("seven days, I believe, that were under the limit"). In contrast, Centerville averages were substantially higher; team members said one rain-event reading at a Centerville site reached nearly ten times the 0.075 mg/L threshold the presentation referenced.

The students also tested the effect of local activities. Aurora, the data analyst, pointed to a clear spike in total phosphorus following July 4 sampling dates and said fireworks-related sediment is a plausible contributor, while cautioning other factors (fertilizer application, manure spreading, heat and subsequent runoff) also influence results: "It's not necessarily from the fireworks ... there are many factors," she said.

Presenters emphasized land-management practices that reduce runoff. The team contrasted CS 13 with PIO5 in Pine Creek, which uses cover cropping. "Cover crops like cereal rye ... reduce sediment and nutrients runoff," Chi said while showing photos and graphs indicating PIO5 had much lower phosphorus and turbidity during rain events than CS 13. Presenters explained that cereal rye and similar species hold soil in place and can reduce nutrient export during storms, although termination methods (including herbicide use) and costs were noted as trade-offs.

The summit also reviewed a long-term restoration near Hika Park completed by the Lakeshore Natural Resource Partnership (LNRP). Quentin said the restoration—part of work that followed a dam removal in the 1990s—appears to stabilize phosphorus despite increases in turbidity during some rain events, suggesting the restoration is partially effective at reducing nutrient export even as some bank and gully erosion persists.

On technological responses, the team documented an experimental phosphorus-removal system observed near a sampling site (activated alumina, sometimes referenced by participants as "Poseidon pellets") and described ongoing work on an alternative they called for now "aluminum hydroxide," which the students said has shown promising lab results for higher phosphorus removal in small-scale tests. "We're trying to create a product which we're calling aluminum hydroxide for now ... we've discovered that it removes a higher amount of phosphorus than most other products," one presenter said.

Presenters said next steps include continued monitoring, sharing results with land managers and conservation partners, and using the data to target interventions such as cover crops, buffer strips and selective restoration. Organizer Sam Fronfeld, of the Lakeshore Natural Resource Partnership, said the students' data will be used with community partners including LNRP and Manitowoc Soil and Water Conservation to guide where to implement lower-cost practices and larger restoration projects.

The presentation underscored several consistent findings: upstream agricultural land increases the risk of post‑storm phosphorus and turbidity spikes; targeted management such as cover crops can reduce export at some sites; and restoration work can reduce nutrient export but may require continued maintenance and monitoring. The team cautioned that single-event spikes have multiple possible causes and that attribution to one source (for example fireworks) is not definitive based on the current data.

The summit closed with a public Q&A on cover-crop species, feasibility and costs, and reaffirmation that monitoring and collaboration with local land managers will continue.