Dutch Lake Sand/Iron Filter Project
(Updated May 6, 2013) -- Beginning in 2013, the MCWD will begin collecting water quality data in the system to measure its performance.
The Minnehaha Creek Watershed District (MCWD) used an innovative new filter system to improve water quality in Dutch Lake and downstream wetlands.
The MCWD used a sand/iron filter to reduce the amount of phosphorus that enters Dutch Lake, which is considered impaired for its nutrient concentration. It’s a relatively new technique in stormwater management.
“This is an exciting technology that will allow us to protect water quality in ways that stormwater ponds and other traditional stormwater management approaches do not,” said MCWD Project Manager Renae Clark. “The filter uses sand mixed with iron filings to both react with dissolved phosphorous and remove it from the water.”
Traditional stormwater ponds allow phosphorus particles to settle before the water moves into a water resource, but don’t capture the phosphorus that is dissolved.
The new system diverts a portion of the flow of a nearby nutrient-laden stream through the filter and return the water to the stream before it flows into Dutch Lake. The MCWD also is managing buckthorn and installing native plantings in the area.
Dutch Lake flows into Lake Minnetonka’s Jennings Bay, which tied for the lowest MCWD lake grade among Lake Minnetonka’s 30 bays in 2011. The MCWD will monitor the filter’s effect on Dutch Lake and downstream resources to determine whether to use it in other parts of the watershed.
“The Minnehaha Creek Watershed District is known for innovative approaches to protecting water quality in our District,” said MCWD Administrator Eric Evenson. “The project is another way we are taking the lead and creating a model that we hope others who are just as passionate about water quality can use.”
The filter is comprised of two separate 20-foot wide rectangular basins: one 80 feet long and the other 160 feetlong. They are parallel to a stream that connects two large wetland complexes. During high flows, the inlets take on the high water at the north end and connect it to the two large filtration cells, then return the water back to the stream channel via a pipe outlet. The filters are a mix of sand and iron, with about 7 percent iron for this project. The basins have asix inch clay base and an additional six inches of clay for the ground profile to contain the media. Riparian buffers will also be established to reduce erosion within the project area.
Performance and Maintenance
As the iron rusts, dissolved phosphorus binds to the oxides by surface absorption. For this to be effective, the system cannot be permanently inundated. The system is gravity fed and is designed to drain inbetween storm events. Tests on sand/iron filters show that they can remove between 30 and 90 percent of dissolved phosphorus. Maintenance requirements for this typeof system vary, but this particular project is designed to allow for simple access to filter media for easyreplacement, in addition to the construction of a permanent easement road creating a convenient entrypoint to access the system.
The feasibility study for this project focused on the Dutch Lake subwatershed DL-3 (see Dutch Lake Subwatershed Map below). The goals of the project are to conserve ecological integrity within the subwatershed, protect the high-value wetland that outlets to Dutch Lake, and improve water quality in Dutch Lake. Initially, the draft feasibility study focused on the possibility of a wetland restoration/infiltration project within the Turner/Dutch Lake wetland south of Game Farm Road (see DL-3 Project Area Map below). Site visits and ecological assessments indicated that this wetland complex contains high quality wetland types, including poor fen and tamarack swamp. The focus of the feasibility study was then broadened to include an upstream project that would protect both Dutch Lake and the high quality Turner/Dutch Lake wetland. Water quality samples have consistently showed that about 74% of the phosphorus in the water flowing through the Game Farm Road culvert into the Turner/Dutch Lake wetland is soluble, which means that it is dissolved in the water rather than being attached to sediment particles. The proposed project would, therefore, have to remove soluble phosphorus in order to have a meaningful reduction in phosphorus load to downstream resources. As a result, the project was changed to an assisted filtration project which would route water through a filter media (limestone or iron filings) that will react with soluble phosphorus and pull it out of solution.
MCWD staff started monitoring the downstream high-quality wetland in late May of 2010. Construction of the proposed project in conjunction with long term water quality, hydrology, and vegetation monitoring of the Turner/Dutch Lake wetland will provide an excellent opportunity to evaluate the results of using an innovative assisted filtration practice for soluble phosphorus removal.