5.7 Phosphorus Load Reduction

Long Lake

One of the water quality goals for this subwatershed is the achievement of the Long Lake in-lake phosphorus concentration goal of 40-50 ?g/L. Reduction of phosphorus loads from the subwatershed to achieve an interim goal of 50 ?g/L will require the combined efforts of the regulatory program, operational programs, and capital projects. 
Table 16 below sets forth a summary plan for how this could be accomplished.  It is important to note that this Plan does not reach Long Lake's long-term goal.  As improvements are completed, the lake, its goals, and this Plan will have to be assessed to determine if additional improvements can be made or if the goal should be reconsidered.

The table breaks down modeled phosphorus loading in the subwatershed 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, the reduction of existing loads from the subwatershed, and control of internal load.  Application of the current regulatory program to new development would reduce the expected load from ultimate development conditions.   As Table 10 above indicates, more stringent regulation of new development could significantly reduce the need for or size of capital projects. 

It is important to note that a significant share of the modeled phosphorus load to Long Lake is from sources that have not been specifically identified.  The phosphorus load contributed by wash off from land within 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.  An alum treatment was previously applied to Long Lake; an additional treatment may be warranted to control sediment sources.  Control of aquatic vegetation may alleviate some internal loading, as may rough fish management.  A diagnostic study would be required to determine the most appropriate internal load reduction option. 

Table 16.   Phosphorus load reduction plan for Long Lake (interim goal = 50 ?g/L).

Source 

Reduction 

Ultimate Phosphorus Load [lb/yr]

 

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric

 Atmospheric Deposition

NA

63

NA

63

 

External Loads

 External Load Determined

 From Modeling Land Use

 

1,065

 

 

  

 

 LGU load reduction allocation  (Table 15)

 

118

 

 

 

 No increase in load

 from upstream lakes

 

75

 

 

 

 Existing Regulations

 

113

 

 

 

 Additional Regulation

 

56

 

 

 Total After Reductions

 

 

 

703

 

Internal / "Unknown" Loads

 Internal / "Unknown"

 Loads Determined from   

 Modeling Land Use

 

412

 

 

  

 

 Vegetation management

 

288

 

Est 70% reduction of internal loading

 Total After Reductions

 

 

 

124

 

Total Load

 TOTAL 

 

1,540

650

890

  

 LOAD GOAL  

 

 

 

906

 

 DIFFERENCE 

 

 

 

(-16)

 

Tanager Lake

One of the water quality goals for this subwatershed is the achievement of the Tanager Lake interim in-lake phosphorus concentration goal of 70 ?g/L. Reduction of phosphorus loads from the subwatershed to achieve a goal of 70 ?g/L will require the combined efforts of the regulatory program, operational programs, and capital projects.  Table 17 below sets forth a summary plan for how this could be accomplished.

The primary means of addressing external loading are through the regulation of new loads generated by development, the reduction of existing loads from the subwatershed, and control of internal load.  Application of the current regulatory program to new development would reduce the expected load from ultimate development conditions.  

It is important to note that a significant share of the modeled phosphorus load to Tanager Lake is from sources that have not been specifically identified.  The phosphorus load contributed by wash off from land within 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 export from the many wetlands upstream of the lake.  A diagnostic study would be required to determine the most appropriate internal load reduction option. 

Table 17.  Phosphorus load reduction plan for Tanager Lake (interim goal = 70 ?g/L).

Source

Reduction 

Ultimate Phosphorus Load [lb/yr]

 

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric

 Atmospheric deposition

NA

12

NA

12

 

External Loads

 External load determined

 from modeling land use

 

749

 

 

  

 

  LGU load reduction allocation (Table 15)

 

38

 

 

 

  Load reduction from

  meeting upstream lake

  goal

 

114

 

 

 

 Existing regulations

 

27

 

 

 

 Additional regulation

 

14

 

 

 

 Subw LLC-48 Pond

 

49

 

 

 

 Subw LLC-51 Pond

 

23

 

 

 Total After Reductions

 

 

 

484

 

Internal / "Unknown" Loads

 Internal / "unknown"

 loads determined from   

 modeling land use

 

939

 

 

  

 

 Internal/wetland load management

 

658

 

Est 70% reduction of internal loading

 Total After Reductions

 

 

 

281

 

Total Load

 TOTAL 

 

1,700

923

777

  

 LOAD GOAL  

 

 

 

832

 

 DIFFERENCE 

 

 

 

(-55)