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 10 and Table 11 illustrate how land use change such as infill development and the conversion of agricultural and vacant land to low- and medium-density residential could be expected to impact water quality in Lakes Minnewashta and Virginia.  These tables also illustrate the impact of a regulatory program managing these impacts.  The HHPLS did not model Tamarack Lake and Lake St. Joe.

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

Table 10 and Table 11 contrast three loading reduction scenarios.  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 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.

Table 10.  Lake Minnewashta modeled 2020 and ultimate development water quality and the total phosphorus loading reduction necessary to achieve in-lake total phosphorus concentration goal of 20 μg/L.

 

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

23

24

P load decrease needed to achieve 20 μg/L (lbs/year)

 

114

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

22

 

23

P load decrease needed to achieve 20 μg/L (lbs/year)

 

81

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

 (As assumed in HHPLS)

Predicted in-lake TP (μg/L)

 

22

Additional P load decrease needed to achieve 20 μg/L (lbs/year)

 

49

Table 11.  Lake Virginia modeled 2020 and ultimate development water quality and the total phosphorus loading reduction necessary to achieve its in-lake total phosphorus concentration goal of 40 μg/L.

 

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

51

51

P load decrease needed to achieve 40 μg/L (lbs/year)

 

95

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

46

 

48

P load decrease needed to achieve 40 μg/L (lbs/year)

 

63

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

 (As assumed in HHPLS)

Predicted in-lake TP (μg/L)

 

44

Additional P load decrease needed to achieve 40 μg/L (lbs/year)

 

35

Other Impacts

The Lake Virginia subwatershed ecosystem faces varying threat levels from degradation as a result of development pressure, urbanization, and subsequent channelization of stream conveyances that go beyond impacts to water quality and hydrology.   Development can directly or indirectly degrade and fragment habitat, and reduce or eliminate the opportunities for natural stormwater management provided by minimally disturbed grasslands, forests, woodlands, and wetlands.

The establishment of the connectivity between ecosystems will become increasingly difficult as development encroaches on the corridor.  Currently about one-third of the subwatershed is urbanized.  It is expected that about half the existing agricultural and one-third the existing forested lands will be converted to low-density residential development by 2020.