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.  The development of vacant land or conversion of developed land to other uses can be expected to affect water quality in the lakes. 

Table 12 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 eight lakes in the subwatershed.  The table also illustrates the impact of a regulatory program managing these impacts.  Powderhorn Lake was not modeled in the HHPLS so no scenarios for that lake are included here.

Ultimate development is defined as the conversion of all agricultural lands, and one-half the upland forested area identified in the 2020 land use plans of the respective local governments as remaining undeveloped after 2020.  This conversion may take place by 2030 or require significantly more time; 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.

Table 12 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 third scenario illustrates the expected result of a stringent regulatory program that strictly prohibits any new phosphorus loading.

Table 12.  Modeled 2020 and ultimate development water quality and the total phosphorus loading reduction necessary to achieve in-lake total phosphorus concentration goals.

 Brownie Lake Goal = 35 μg/L 

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

40

40

P load decrease needed to achieve 35 μg/L (lbs/year)

 

12

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

38

 

38

P load decrease needed to achieve 35 μg/L (lbs/year)

 

9

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

 (As assumed in HHPLS) 

Predicted in-lake TP (μg/L)

 

38

P load decrease needed to achieve 35 μg/L (lbs/year)

 

7

Cedar Lake HHPLS Goal = 25 μg/L 

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

23

23

P load decrease needed to achieve 25 μg/L (lbs/year) – no degradation

 

16

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

22

 

22

P load decrease needed to achieve 25 μg/L (lbs/year) – no degradation

 

6

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

P load decrease needed to achieve 25 μg/L (lbs/year) – no degradation

 

2

Lake of the Isles Goal = 40 μg/L 

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

42

42

P load decrease needed to maintain 38 μg/L (lbs/year) – no degradation

 

18

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

38

 

40

P load decrease needed to maintain 38 μg/L (lbs/year) – no degradation

 

18

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

 (As assumed in HHPLS) 

Predicted in-lake TP (μg/L)

 

39

P load decrease needed to maintain 38 μg/L (lbs/year) – no degradation

 

8

Lake Calhoun Goal = 25 μg/L 

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

23

23

P load decrease needed to maintain 21 μg/L (lbs/year) – no degradation

 

103

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

21

 

22

P load decrease needed to maintain 21 μg/L (lbs/year) – no degradation

 

56

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

P load decrease needed to maintain 21 μg/L (lbs/year) – no degradation

 

27

Lake Harriet Goal = 20 μg/L 

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

24

24

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

 

152

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

23

 

23

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

 

132

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

 (As assumed in HHPLS) 

Predicted in-lake TP (μg/L)

 

23

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

 

123

Lake Nokomis Goal = 50 μg/L 

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

65

65

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

 

310

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

 

65

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

 

295

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

 (As assumed in HHPLS) 

Predicted in-lake TP (μg/L)

64

 

64

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

 

288

Lake Hiawatha Goal = 50 μg/L TMDL = 61 μg/L

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

74

74

P load decrease needed to achieve 61 μg/L (lbs/year)

 

1,833

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

72

 

72

P load decrease needed to achieve 61 μg/L (lbs/year)

 

1,580

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

 (As assumed in HHPLS) 

Predicted in-lake TP (μg/L)

 

72

P load decrease needed to achieve 61 μg/L (lbs/year)

 

1,507

Diamond Lake Goal = 90 μg/L;  TMDL = 90 μg/L

2000

2020

Ultimate Development

Scenario 1:  No Regulatory Program

Predicted in-lake TP (μg/L)

 

149

149

P load decrease needed to achieve 90 μg/L (lbs/year)

 

180

Scenario 2: Current Regulatory Program

Predicted in-lake TP (μg/L)

141

 

145

P load decrease needed to achieve 90 μg/L (lbs/year)

 

166

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

 (As assumed in HHPLS) 

Predicted in-lake TP (μg/L)

 

143

P load decrease needed to achieve 90 μg/L (lbs/year)

 

159