Column: Using geology to read the past

Angie Hong

At first glance, Shields Lake in the city of Forest Lake appears natural and relatively untouched.

There is a tiny city park with a fishing pier at the northeast corner of the lake and a golf course to the north. The rest of the shoreline is undeveloped, fringed by cattails and trees.

Surprisingly, the lake has some of the worst water quality in Washington County. During 2015, water monitoring data showed the lake to have an average phosphorus concentration of 349 µg/L (micrograms per liter), which is eight times higher than the state’s threshold for impairment — 40 µg/L. The water is green, murky and choked with invasive curlyleaf pondweed. In its annual report card, the Met Council gave Shields Lake an F+.

In an effort to help understand what went wrong with little Shields Lake, researchers from the University of St. Thomas and the St. Croix Watershed Research Station recently collected and analyzed sediment core samples from Shields and two other nearby lakes. Sam Duncanson, a recent St. Thomas graduate now working as a seasonal technician for the Comfort Lake – Forest Lake Watershed District presented findings from the project at a district board meeting on May 25.

Sediment accumulates on the bottom of lakes and slow-moving rivers over time as sand, silt and organic debris are washed into the water from the surrounding landscape. The sediment settles to the bottom in layers, creating a storyline that geologists can study to learn about events in the past. For example, sediment cores collected from the St. Croix River include a layer of wood dust accumulated during the height of the logging era.

In Shields Lake, the sediment cores collected showed a dramatic change in the rate of sedimentation around 1950, the year the Forest Hills Golf Course was constructed. In addition, it appears the lake became anoxic (greatly deficient in oxygen) around that time (likely due to an algae bloom) and phosphorus that had been bound to sediment on the lake bottom was released into the water. Since then, water quality in the lake has remained poor, with low oxygen levels and lots of suspended algae.

At the other end of Washington County, the South Washington Watershed District is looking at bedrock and surficial geology (the sediment beneath the soil that’s been deposited on top of bedrock) to determine the best route to bring floodwater from Woodbury and Cottage Grove down to the Mississippi River.

During glacial times, a stream ran from what is now northern Woodbury, down through Cottage Grove Ravine Park, and into the glacial River Warren (now the Mississippi River). Over time, loose rock and sediment filled the bedrock channel until eventually the landscape became flat, as it is today. Now, the watershed district is working to partially re-create that ancient watercourse so that homes and neighborhoods don’t flood during large rainstorms. The multi-stage project includes large storm water pipes underground, as well as dry streams above ground through Cottage Grove Ravine Park and other low-lying areas.

Back in Forest Lake, the Comfort Lake – Forest Lake Watershed District received a Clean Water Legacy grant from the Minnesota Board of Water and Soil Resources to address runoff pollution in Shields Lake. The district will help the Forest Hills Golf Course to install a storm water reuse system so that the golf course can capture and use runoff from the surrounding landscape for irrigation.

The project will keep nutrient-rich runoff from flowing into Shields Lake, and will also reduce the amount of groundwater being pumped by the golf course. It will take several years to reverse the damage done to Shields Lake, but district managers hope the project will eventually improve water quality in both Shields Lake and Forest Lake downstream.

Though the goals of these two watershed projects are quite different — one hopes to reduce flooding, while the other aims to improve lake water quality — both use geologic studies to read the past and help devise solutions for the future. The past is buried underground, but the future of our communities and our water resources is ours to create.

Angie Hong is an educator for East Metro Water, mnwcd.org/emwrep. Contact her at 651-330-8220 ext. 35 or [email protected]