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 | |
