3.6 Impacts of Future Growth

Water Quantity and Quality

Land use change impacts downstream water quality by increasing the volume of runoff and the concentration and load of nutrients and sediment transported to receiving waters.  Table 8 illustrates how land use change such as the expected conversion of agricultural and vacant land to low-density residential could be expected to impact water quality in Langdon Lake.  The tables also illustrate the impact of a regulatory program managing these impacts.

?Ultimate Development? is defined as the conversion of all agricultural lands and one-half of the upland forested area outside the regional park that remains undeveloped in the 2020 local government land use plans.  This conversion may take place by 2030 or require significantly more time; but it is simply assumed that at some point in the future these conversions will occur.  More detail regarding this modeling can be found in Technical Appendix A.

During the development of the HHPLS, this subwatershed was reviewed by two Regional Teams which recommended two different phosphorus goals for Langdon Lake: 55 ?g/L and 70 ?g/L.  Table 8 contrasts three loading reduction scenarios for each of these goals.  Scenarios 1 and 2 contrast the expected results if there were no regulatory program to the results under the existing regulatory program.  The HHPLS assumed that there would be no load increase from future development; the third scenario in Table 8 indicates that even with a regulatory program that strictly prohibits any new phosphorus loading, additional reductions would be necessary to achieve the desired phosphorus concentration goal of 55-70 ?g/L.

Table 8.  Langdon Lake modeled 2020 and ultimate development water quality and the total phosphorus loading reduction necessary to achieve an in-lake total phosphorus concentration goal of 55 ?g/L.

 

2000

2020

Ultimate

Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (?g/L)

 

93

93

P load decrease needed to achieve 55 ?g/L (lbs/year)

 

352

Scenario 2: Current Regulatory Program

Predicted in-lake TP (?g/L)

87

 

89

P load decrease needed to achieve 55 ?g/L (lbs/year)

 

312

Scenario 3: Regulatory Program That Prohibits A Net Increase in Loading from New Development

 (As assumed in HHPLS)

Predicted in-lake TP (?g/L)

 

85

Additional P load decrease needed to achieve 55 ?g/L (lbs/year)

 

273

Langdon Lake modeled 2020 and ultimate development water quality and the total phosphorus loading reduction necessary to achieve an in-lake total phosphorus concentration goal of 70 ?g/L.

 

2000

2020

Ultimate

Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (?g/L)

87

93

93

P load decrease needed to achieve 70 ?g/L (lbs/year)

 

228

Scenario 2: Current Regulatory Program

Predicted in-lake TP (?g/L)

 

89

P load decrease needed to achieve 70 ?g/L (lbs/year)

 

188

Scenario 3: Regulatory Program That Prohibits A Net Increase in Loading from New Development

 (As assumed in HHPLS)

Predicted in-lake TP (?g/L)

 

85

Additional P load decrease needed to achieve 70 ?g/L (lbs/year)

 

149

Other Impacts

While most of the eastern Langdon Lake subwatershed has been impacted by development, the western, upper subwatershed is yet only lightly developed and much is located within a regional park reserve.  The area outside the park reserve faces varying threat levels from degradation as a result of development pressure, rapid urbanization, and subsequent channelization of stream conveyances.  Those impacts could exacerbate impacts already affecting natural resource integrity in the downstream, developed part of the subwatershed.  Prior to the encroachment of additional development, the opportunity exists to create a connection between ecosystems within the Langdon Lake, Dutch Lake, Painter Creek, and Lake Minnetonka Subwatersheds to improve water quality, preserve natural conveyances, and facilitate the movement and proliferation of native species as well as enhance recreational opportunities.

The opportunity exists to restore approximately 150 acres of priority wetland complexes and 180 acres of adjacent high priority uplands.  This will improve the characteristics of the aquatic ecosystem and the water quality within the subwatershed as well as areas downstream.  Key Conservation Areas and the corridor plan for this subwatershed are illustrated on Figure 19 of this document.