William Street Pond

Reduced Phosphorus

William Street Pond is a stormwater collection pond located in Roseville. Originally converted from a wetland in 1990, the pond receives stormwater from the surrounding neighborhood and discharges directly into Lake McCarrons.

The Challenge

In the last 15 years, CRWD has launched several projects to reduce the phosphorus load to Lake McCarrons, one of which included improvements to William Street Pond. The goal was to prevent the excessive growth of algae and improve the pond’s ability to settle suspended solids. Within the Lake McCarrons subwatershed, William Street pond accounts for the second-highest phosphorus input to the lake.

“Stormwater ponds often go unnoticed by residents in the District, but they play a critical role in managing water quality for most of the District’s water resources. William Street Pond is an example of how best management practices at a relatively unknown body of water can have a huge impact on a more recognizable lake like McCarrons.” – Bob Fossum, Monitoring & Research Division Manager

The Solution

CRWD and its partners prioritized several improvements to William Street Pond that would have a direct impact on water quality. First, sediment was removed from the pond to restore its full water storage capacity. Next, a new baffle device and pond inlet pipe were installed to trap large debris and dissipate the flow energy of the water entering the pond, improving water clarity and reducing turbidity.

Finally, the District created two iron-enhanced sand filtration benches near the pond outlet. These are flat areas next to the water line filled with a mixture of sand and iron filings. Sand filtration is a common solution that remove particulate matter and pollutants from stormwater. When stormwater flows through the sand, dissolved pollution bonds to the iron and prevents it from entering Lake McCarrons. Pond improvements were completed in September 2011.


Water flowing to Lake McCarrons has had better water quality since CRWD improved William Street Pond. The iron-enhanced sand filters are successfully reducing the amount of phosphorus entering the lake. According to the final William Street Pond performance report in 2017, the William Street pond improvements accounted for a four-year reduction of 67.5 percent in total phosphorus.

However, while 2014 exhibited decreased total phosphorus and other contaminants in Lake McCarrons, 2015 and 2016 values were closer to being in line with previous years. This highlights the need to better understand what is contributing to total phosphorus, especially the relationship between William Street Pond and Lake McCarrons.

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