Researchers say Polk County rivers exceed recreational E. coli thresholds about 20%–30% of the time

Researchers leading a Central Iowa source-water research assessment reported preliminary findings to Polk County officials, saying monitoring sites in the Des Moines and Raccoon River watersheds exceeded EPA recreational E. coli thresholds about 20%–30% of the time and that the patterns vary by water-body type.

Project manager Jen, who is overseeing the two-year study, told the briefing the team has logged "almost 4,000 hours of research" and has worked with about 70 organizations to gather data; she said the full technical report (about 250 pages) and a 2–4 page executive summary will be released in March. "We don't want this report to be something that just sits on a shelf," she said, stressing outreach and targeted materials for different audiences.

The study's scientist Elliot Anderson (IIHR Hydrology Institute, University of Iowa) explained that the health risk the team is tracking is associated with recreation — activities such as swimming or kayaking in untreated water — and that researchers use E. coli as an indicator of broader pathogen risk. "E. coli is what's known as an indicator bacteria," Anderson said, noting the U.S. Environmental Protection Agency sets thresholds used to evaluate recreational risk.

Anderson summarized mapped monitoring results for the Polk County watersheds, saying "we tend to see that around 20 to 30% of the time our waters ... have exceeded this threshold." He said exceedances are more common in rivers and streams than at beaches or in larger reservoirs, and that hydrology matters: sites downstream of wastewater treatment plants, such as Beaver Creek at Grimes, have had persistent exceedances, while water just downstream of Saylorville Lake shows much lower exceedance rates because the reservoir's retention reduces pathogen concentrations.

The researchers also reviewed public-health data. Anderson said Polk County (and state) records for waterborne diseases over the last 30 years show some illnesses — for example, campylobacter and giardia — spike in summer months, consistent with increased recreational exposure.

On pesticides, Anderson noted common agricultural products such as glyphosate and atrazine are present in watershed monitoring and often show a ‘‘spring flush’’ pattern: "after a big rainfall event ... you get a large spike in the pesticides that are in the water," typically in May and June following early-season application.

Calvin Walter (GIS specialist with the State Geological Survey) presented modeling of manure nutrient loading and land application. He said that, under the assumptions used by the project (manure applied at typical nitrogen application rates), it would take roughly 1,400,000 acres within the watershed to fully utilize the manure nitrogen; applying at phosphorus rates would require more than 2.5 million acres, and "257,000 acres are receiving more than double the rate of phosphorus." Walter and the team flagged that current manure-application rules that allow application at nitrogen rates — rather than phosphorus-based limits — contribute to phosphorus buildup that can increase algal growth and related oxygen-loss problems in streams.

Walter also showed lidar-derived erosion maps indicating some reaches have experienced hundreds of feet of channel change over a 10–12 year period, and he said keeping runoff on the landscape through wetlands, rain gardens, riparian buffers and similar measures is the primary way to reduce ongoing erosion.

Jen closed by emphasizing the project's next steps: researchers will present a final report and recommendations in March, and the team plans outreach such as an anglers advisory group and partnerships with county, state and federal agencies to translate findings into local action.

Question-and-answer highlights included a committee member's question about whether Saylorville Lake acts like a natural treatment plant; Anderson replied it is not an official treatment plant but that reservoir retention can reduce some pathogens and that researchers are exploring management strategies that could improve downstream water quality.

The meeting concluded with thanks to the presenters and adjournment.