Public meeting to discuss phosphorus inactivation treatment at Silver Lake in Delaware County

• Tuesday, March 17, 2026

• News Release

DELHI - The Iowa Department of Natural Resources (DNR) will host a public meeting on March 30 at 5:30 p.m. at the Maquoketa Valley High School auditorium to discuss an alum treatment planned to improve water quality at Silver Lake in Delaware County.

DNR staff will present an overview of the proposed treatments planned for the fall of 2026 and spring of 2027, how these treatments will help reduce the frequency and intensity of algae blooms in the lake, and the results of a dredging feasibility study and risk assessment.

Alum (aluminum sulfate), a nontoxic substance used in everything from cooking to treatment of drinking water, is commonly used to reduce phosphorus concentrations in lakes. Phosphorus, a key nutrient for algae growth, stems from runoff in the watershed. Much of the phosphorus that ends up in a lake is buried in the lake sediments, but under the right environmental conditions it can also be recycled within the lake and released from the lake sediments when bottom waters have no oxygen.

These “internal releases” of phosphorus often fuel large algae blooms, especially in the late summer and early fall. Alum binds with the phosphorus in the lake and settles as a thin flocculant on the lake bottom, where it will continue to inactivate phosphorus that is released from the sediments. This process can significantly reduce the likelihood of harmful algae blooms (HABs), which can affect ecosystems and limit recreation.

Silver Lake has experienced periodic algae blooms and challenges with both rooted and filamentous plants over the past few years due to high watershed inputs and internal phosphorus recycling. These algae blooms have impacted water clarity and limited recreational opportunities.

Several research efforts show that a series of alum applications will likely improve Silver Lakes’s water quality. The DNR is also considering other future restoration practices, such as dredging.

The first application, planned for October 2026, will use specialized barges to apply liquid alum and sodium aluminate to Silver Lake for one week, depending on weather conditions. DNR staff will be on location to monitor the application. Lake users may see boats, barges, heavy trucking equipment, and storage tanks around the lake and park during the application dates. The DNR requests that you maintain a safe distance from the equipment to allow the crews to work efficiently. There should be minimal disruption to regular activities on the water. The application dates were selected to minimize interference with spring fishing activities.

This treatment is one part of a broader effort to continue to improve water quality and recreational opportunities at Silver Lake. An additional treatment is scheduled for spring of 2027.

Any person attending the public meeting and has special requirements such as those related to mobility or hearing impairments should contact the DNR or ADA Coordinator at 515-725-8200, Relay Iowa TTY Service 800-735-7942, or Webmaster@dnr.iowa.gov, and advise of specific needs.