3.6 Impacts of Future Growth

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 11 illustrates how land use change such as the expected conversion of vacant land to other uses could be expected to impact water quality in the lakes in the lake bays currently experiencing the poorest water quality.  The table also illustrates the impact of a regulatory program managing these impacts.

?Ultimate development? in Table 11 is defined as the conversion of all agricultural lands, and one-half the upland forested area 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 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 11 contrasts three loading reduction scenarios.  Scenarios 1 and 2 contrast the required load reductions if there were no regulatory program to the requirements under the existing regulatory program.  The HHPLS assumed that there would be no load increase from future development; the third scenario in Table 11 indicates that even with a stringent regulatory program that strictly prohibits any new phosphorus loading, additional reductions would be necessary to achieve the desired phosphorus concentration goal within the three bays that exhibit the poorest water quality: Jennings Bay, Stubbs Bay, and Halsteds Bay.

Table 11.  Jennings, Stubbs, and Halsteds Bays on Lake Minnetonka modeled 2020 and ultimate development water quality and the total phosphorus loading reduction necessary to achieve in-lake total phosphorus concentration goals.

 Jennings Bay Goal = 50 ?g/L

2000

2020

Ultimate

Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (?g/L)

 

154

156

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

5,189

Scenario 2: Current Regulatory Program

Predicted in-lake TP (?g/L)

94

 

136

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

3,992

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

 (As assumed in HHPLS) 

Predicted in-lake TP (?g/L)

 

126

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

3,398

 

Stubbs Bay  Goal = 50-55 ?g/L

2000

2020

Ultimate

Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (?g/L)

 

67

68

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

336

Scenario 2: Current Regulatory Program

Predicted in-lake TP (?g/L)

63

 

65

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

294

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

 (As assumed in HHPLS) 

Predicted in-lake TP (?g/L)

 

62

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

238

 

Halsteds Bay  Goal = 50 ?g/L

2000

2020

Ultimate

Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (?g/L)

 

124

124

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

5,795

Scenario 2: Current Regulatory Program

Predicted in-lake TP (?g/L)

122

 

109

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

4,370

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

 (As assumed in HHPLS) 

Predicted in-lake TP (?g/L)

 

107

P load decrease needed to achieve 50 ?g/L (lbs/year)

 

4,224