Dane County to Pursue “Legacy” Phosphorous Removal Project

Dane County Executive Joe Parisi has proposed a $12 million project to remove “legacy” phosphorus-laden sediment from tributary streams to the Yahara chain of lakes, in an attempt to improve the lakes’ water quality.

Research on Dorn Creek, a Lake Mendota tributary, revealed not only that some of the dense, mucky sediments in the streambed were up to 150 years old, but also were releasing phosphorus into the stream at a rate equal to the state phosphorus water quality criterion of 0.075 mg/L. In other words, if all phosphorus inputs from the heavily agricultural landscape of Dorn Creek were halted altogether, the stream would still be generating so much pollutant phosphorus from its own sediments that water quality would still be impaired.

Dane County’s rationale is that by using a floating dredge to remove legacy sediments from polluted stretches of Dorn and five other Yahara watershed streams (Sixmile, Door, Token, and Nine Springs creeks, as well as the Yahara River), 33 miles of stream in total, water quality will improve. The County also anticipates additional habitat and recreational benefits to the stretches of stream where improvements are made, as streambank and floodplain restoration are part of the project and will improve habitat conditions for fish and wildlife.

So let’s break out the dredges and start digging up legacy sediments in all of our lakes and streams!!! Not so fast. Dane County has been working with farmers in the Dorn Creek watershed for decades to address a major source of phosphorus pollution, and most of the farming community has participated in some fashion. Of the 6,000 acres of cropland in the watershed, 90% (or 5,400 acres) feature conservation practices (grassed waterways, conservation tillage, etc.) that are designed to improve water quality. Additionally, three-quarters of the watershed’s farms have nutrient management plans that detail soil-saving conservation practices (reducing run-off and erosion); these farms lose on average three pounds of phosphorus per acre, per year—half of the state allowable limit of six pounds lost per acre per year.

Despite ample conservation on the landscape, though, Dorn Creek’s water quality wasn’t showing a corresponding improvement. Dane County then discovered that 55% of the phosphorus in Dorn Creek was dissolved, as opposed to “particulate” phosphorus that is attached to soil particles, as is found in higher concentrations in parts of Wisconsin with more terrain than Dorn Creek. So the majority of phosphorus was in the creek already, which led to evaluating how to remove it. It’s also worth noting that most of the streams slated for phosphorus removal in this project are small headwaters streams – source waters, without other streams or rivers draining into them – which stand a better chance of sustaining reduced phosphorus levels post-removal than do main-stem rivers.

Dane County’s legacy phosphorus removal project is slated to cost an efficient $14 per pound of phosphorus removed. Over that time, it’s projected to remove 870,000 pounds of phosphorus from the streams. Assuming it receives funding from Dane County’s elected officials in the coming weeks, we’ll watch with interest to see how it helps improve water quality, and if so, whether it’s a project that is replicable elsewhere in the state (or gets tagged with the oft-heard “only in Dane County…” label).