5.7 Phosphorus Load Reduction

One of the water quality goals for this subwatershed is the reduction of phosphorus loading into the Creek and lakes that exceed their total phosphorus goal or that are subject to a TMDL load reduction requirement.

Tables 19, 20, 21, 22 and 23 break down modeled phosphorus loading to each lake by source: atmospheric deposition, external sources, and internal sources.  Atmospheric deposition is a regional issue and is not dealt with here.  The primary means of addressing external loading are through the regulation of new loads generated by development and the reduction of existing loads from the subwatershed.

In some cases the phosphorus load contributed from the subwatershed is not sufficient to explain the current in-lake phosphorus concentration.  The most likely sources for this discrepancy are internal loading from lake sediments or aquatic vegetation.  Internal load management such as alum treatment to control sediment sources coupled with control of aquatic vegetation often helps to alleviate some internal loading.  Rough fish management may also be required.  A feasibility study would determine the most appropriate internal load reduction options.   

Minnehaha Creek

The HHPLS identified a need to reduce phosphorus loading to Minnehaha Creek to achieve a goal of an in-creek total phosphorus concentration of 80 ug/L.  See Section 5.6.1 for a discussion of load reduction strategies.

Brownie Lake

Brownie Lake is at or very near its phosphorus, chlorophyll, Secchi transparency, and TSI goals, and is not expected to change significantly due to development.  The emphasis in the future will be on the prevention of degradation of water quality.

Cedar Lake

Cedar Lake is better than its phosphorus, chlorophyll, Secchi transparency, and TSI goals, and is not expected to change significantly due to development.  The emphasis in the future will be on the prevention of degradation of water quality.

Lake of the Isles

Lake of the Isles meets or is better than its phosphorus, chlorophyll, Secchi transparency, and TSI goals, and is not expected to change significantly due to development.  The emphasis in the future will be on the prevention of degradation of water quality.

Lake Calhoun

Lake Calhoun is better than its phosphorus, chlorophyll, Secchi transparency, and TSI goals, and is not expected to change significantly due to development.  The emphasis in the future will be on the prevention of degradation of water quality.

Lake Harriet

Lake Harriet water quality has been improving.  Although the long-term average of 23 μg/L is still slightly higher than its total phosphorus goal of 20 μg/L, water quality is very good.  Table 19 sets forth a summary plan for how load reductions could be accomplished.

Table 19.  Phosphorus load reduction plan for Lake Harriet (Goal = 20 μg/L).

Source

Reduction 

Ultimate Phosphorus Load [lb/yr]  

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric

Atmospheric deposition

NA

85

NA

85

 

External Loads

External load determined from modeling land use

 

543

 

 

  

 

 Existing Regulations

 

21

 

 

 

LGU load allocation (Table 18)

 

28

 

 

 Total After Reductions

 

 

 

494

 

Internal / "Unknown" Loads

 Internal/“unknown"

loads determined

from  modeling land use

 

37

 

 

  

 

 Internal load management

 

26

 

 

 Total After Reductions

 

 

 

11

 

Total Load

 TOTAL 

 

665

 

590

  

 LOAD GOAL  

 

 

 

562

 

 DIFFERENCE 

 

 

 

28

 Adaptive

 management

Lake Nokomis

A TMDL study including a phosphorus reduction plan is currently being developed for Lake Nokomis.  Proposed reductions include: reductions due to the new prohibition on the use of fertilizer with phosphorus; proper operation of the Lake Nokomis weir; application of neighborhood-level BMPs to increase infiltration and treat stormwater; and internal load management.  A diagnostic and feasibility study would be required to evaluate internal load management in more detail.  Table 20 sets forth a summary plan for how load reductions could be accomplished, in accordance with the TMDL.

Table 20.  Phosphorus load reduction plan for Lake Nokomis (Goal = 50 μg/L).

Source 

Reduction 

Ultimate Phosphorus Load [lb/yr]

 

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric

Atmospheric deposition

NA

49

NA

49

 

External Loads

External load determined from modeling land use

 

453

 

 

  

   Reductions

Phosphorus free fertilizer

 

31

 

 

   proposed in the

Operate weir

 

104

 

 

   draft TMDL

General BMP load reductions: sweeping,

neighborhood BMPs,

infiltration

 

66

 

 

 

 

 

 

 

 

 

 

 

 

   Regulation

Existing regulations

 

13

 

 

Total After Reductions

 

 

 

239

 

Internal / "Unknown" Loads

Internal/“unknown"

loads determined

from  modeling land use

 

406

 

 

  

 

 Internal load management

 

90

 

 

 Total After Reductions

 

 

 

316

 

Total Load

 TOTAL 

 

908

 

604

  

 LOAD GOAL  

 

 

 

599

 

 DIFFERENCE 

 

 

 

5

 

Lake Hiawatha

A TMDL study including a phosphorus reduction plan is currently being developed for Lake Hiawatha.  Lake Hiawatha receives direct inflow from Minnehaha Creek, and is greatly influenced by the creek’s water quality and the amount of flow, making developing specific load reductions difficult.   Proposed reductions from the TMDL include: reductions due to the new prohibition on the use of fertilizer with phosphorus; construction of the proposed Hiawatha Golf Course ponds and other BMPs; application of neighborhood-level BMPs to increase infiltration and treat stormwater; reduction of phosphorus loading in Minnehaha Creek through streambank stabilization and buffer restoration; and other BMPs throughout the watershed such as increased street sweeping, retrofits of infrastructure, etc.  Table 21 sets forth a summary plan for how the reductions could be accomplished, in accordance with the TMDL.

Table 21.  Phosphorus load reduction plan for Lake Hiawatha (Interim Goal = 61 μg/L).

Source 

Reduction 

Ultimate Phosphorus Load [lb/yr]

 

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric

 Atmospheric deposition

NA

15

NA

15

 

External Loads

External load determined

from modeling land use

 

10,917

 

 

  

   Reductions

Phosphorus free fertilizer

 

355

 

 

   proposed in the

Golf course, other ponds

 

46

 

Local projects

   draft TMDL

Increase infiltration

 

626

 

 

 

Creek improvements

 

626

 

 

 

Other specific BMPs

 

53

 

Local prpojects

   Regulation

Existing Regulations

 

302

 

 

Total After Reductions

 

 

 

8,909

 

Internal / "Unknown" Loads

Internal/“unknown"

loads determined

from  modeling land use

 

194

 

 

  

 

 Internal load management

 

----

 

 

 Total After Reductions

 

 

 

194

 

Total Load

 TOTAL 

 

11,126

 

9,118

  

 LOAD GOAL  

 

 

 

10,038

 

 DIFFERENCE 

 

 

 

(-920)

 

Diamond Lake

A TMDL study including a phosphorus reduction plan is currently being developed for Diamond Lake.  Proposed reductions from the TMDL to reach the lake’s interim goal of 90 ug/L include: reductions due to the new prohibition on the use of fertilizer with phosphorus; increased street sweeping; application of neighborhood-level BMPs to increase infiltration and treat stormwater; treatment of runoff from an industrial area in subwatershed MC-142; and lake biomanipulation to reduce internal loading.  A diagnostic and feasibility study would be required to evaluate internal load management in more detail.  Table 22 sets forth a summary plan for how load reductions could be accomplished in accordance with the TMDL.

Table 22.  Phosphorus load reduction plan for Diamond Lake (Interim Goal = 90 μg/L).

Source 

Reduction 

Ultimate Phosphorus Load [lb/yr]

 

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric

Atmospheric deposition

NA

15

NA

15

 

External Loads

External load determined from modeling land use

 

260

 

 

  

Reductions

proposed in the

draft TMDL

Phosphorus free fertilizer

 

26

 

 

General BMP load reductions: sweeping,

neighborhood BMPs,

infiltration

 

84

 

Local projects

MC-142 runoff improvement

 

18

 

Local project

   Regulation

Existing regulations

 

15

 

 

Total After Reductions

 

 

 

121

 

Internal / "Unknown" Loads

Internal/“unknown"

loads determined

from  modeling land use

 

211

 

 

  

 

 Internal load management

 

95

 

 

 Total After Reductions

 

 

 

116

 

Total Load

 TOTAL 

 

486

 

252

  

 LOAD GOAL  

 

 

 

265

 

 DIFFERENCE 

 

 

 

(-13)

 

Powderhorn Lake

A TMDL study including a phosphorus reduction plan is currently being developed for Powderhorn Lake.  Proposed reductions from the TMDL to reach the lake’s interim goal of 90 ug/L include: reductions due to the new prohibition on the use of fertilizer with phosphorus; addition of a filtration system on the CDSs on the outfalls; increased street sweeping; application of neighborhood-level BMPs to increase infiltration and treat stormwater; and lake biomanipulation to reduce internal loading.  A diagnostic and feasibility study would be required to evaluate internal load management in more detail.  Table 23 sets forth a summary plan for how load reductions could be accomplished in accordance with the TMDL.

Table 23.  Phosphorus load reduction plan for Powderhorn Lake(Interim Goal = 90 μg/L).

Source 

Reduction

Ultimate Phosphorus Load [lb/yr]

 

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric

 Atmospheric Deposition

NA

2

NA

2

 

External Loads

 External Load Determined

 from Modeling Land Use

 

126

 

 

  

   Reductions

Phosphorus free fertilizer

 

15

 

 

   proposed in the

Increased street sweeping

 

24

 

 

   draft TMDL

Increase infiltration

 

2

 

 

 

Filtration on CDSs

 

31

 

 

   Regulation

Existing Regulations

 

-

 

 

Total After Reductions

 

 

 

54

 

Internal / "Unknown" Loads

Internal/“Unknown"

Loads Determined

from  Modeling Land Use

 

139

 

 

  

 

 Internal load management

 

104

 

 

 Total After Reductions

 

 

 

35

 

Total Load

 TOTAL 

 

267

 

91

  

 LOAD GOAL  

 

 

 

90

 

 DIFFERENCE 

 

 

 

(-1)