2.5.1 Lakes

Langdon Lake is the primary receiving water within the subwatershed, and is classified by the DNR for shoreland management purposes as a Recreational Development lake.  It has a surface area of 144 acres, a mean depth of 8 feet, and a maximum depth of 38 feet.  Two other receiving waters within the subwatershed carry the informal designation of lake: Saunders Lake and Black Lake.  Saunders Lake is a large, Type 5 wetland, classified as a Natural Environment lake while Black Lake is a multi-type wetland with a small area of Type 5 open water.

Langdon Lake has consistently been scored in the D-F grade range on the District’s annual lake report cards.  A sewage treatment facility operated for decades on the west side of the lake and the treatment lagoon discharged into the lake.  In 1974 this practice was discontinued, and lake water quality slowly improved.  Although there was a marked reduction in total phosphorus concentration following an alum treatment in 1998, clarity improved only slightly.  In 2004 the mean summer phosphorus concentration was 138 μg/L, well in excess of the 55-70 μg/L interim goal established in the HHPLS.   It also exceeds the standards developed by the State of Minnesota and is in an impaired use condition, although it has not been formally designated an Impaired Water.  Water clarity as measured by Secchi disk was 0.4 meters and the Trophic State Index was 74.  No monitoring water quality data is available for Saunders or Black Lakes.  As estimated by University of Minnesota Water Resources Center satellite imagery, water clarity in Saunders Lake is less than 0.5 meters and in Black Lake is 2-4 meters.

Table 4.  Selected water quality goals and current conditions in the Langdon Lake subwatershed.

Lake

1997 TP Goal(μg /L)

HHPLS TP Goal (μg /L)

1994-2004 Average TP (μg /L)

2004

TP (μg /L)

Chl-a (μg /L)

Secchi (m)

TSI

Langdon

50

55-70*

136

138

99

0.4

74

Saunders

N/A

N/A

N/A

N/A

N/A

<0.5**

N/A

Black

N/A

N/A

N/A

N/A

N/A

2-4**

N/A

*In the development of the HHPLS, this subwatershed fell under the review of two different Regional Teams, which established two different goals for Langdon Lake: 55 μg/L and 70 μg/L.  **Clarity as estimated by the University of Minnesota using satellite imagery.

Source: MCWD and Minnesota DNR/University of Minnesota

According to standards established by the MPCA, water quality in Langdon Lake does not support swimming.  Modeling performed for the HHPLS and for this plan indicates that phosphorus loading from runoff in the watershed cannot explain the total phosphorus concentrations in the lake.  Internal loading from lake sediments or phosphorus exported from the extensive wetlands that drain to the lake is likely the cause of the higher than expected in-lake TP concentrations.  No water quality modeling was performed for Saunders or Black Lakes.