DRASTIC SEDIMENTATION CHANGES IN A TWIN CITIES METRO AREA WATERSHED ON THE URBAN-RURAL BOUNDARY

 

The Comfort Lake-Forest Lake Watershed District includes numerous lakes located in both Chisago and Washington County, MN. Several of these lakes were deemed impaired, due largely to higher than recommended phosphorus levels. Three of these impaired bodies: Shields, Comfort, and Moody lakes, are being considered for remediation and long-term management. Before such steps are taken, however, an understanding of the lakes’ historical conditions needs consideration. We collected sediment cores from each lake in February 2016. Afterwards, the cores underwent several geochemical analyses, including loss on ignition (LOI), magnetic susceptibility, x-ray fluorescence (XRF), and ²¹⁰Pb age dating. Here we focus on data collected from a 274cm core collected from Shields Lake and a 214cm core collected from Comfort Lake.

Our Shields ²¹⁰Pb-dated record indicates a rise in inorganic matter (35-57%), as well as increases in magnetic susceptibility that correspond to European settlement ca. 1860-1875. Increases in Fe and Mn concentrations also occurring reflect sediment sourced from outside the lake. The introduction of industrial agriculture in the watershed post World War 2, followed by the construction of a golf course in 1960 adjacent to Shields Lake had significant impacts on sedimentation in the lake. LOI results show a large increase in organic matter (48-87%), magnetic susceptibly decreases, and sedimentation rates double to .8515 g/cm² /yr at this time. Based on our data, an alum treatment to remediate the impaired lake in 1990 appears to have had a significant impact on production and burial of organic matter in the lake. Similar geochemical changes are reported from Comfort Lake, with ²¹⁰Pb results providing a chronology of historical changes.

These results reflect environmental changes that have occurred in and around Shields and Comfort Lakes over the past >200 years. Based on the these geochemical changes, as well a sevenfold increase in sedimentation rate in Shield's watershed, we connect these changes in the lake sedimentation to both general and specific land-use changes in the post- European settlement of the area.