5.7 Phosphorus Load Reduction

One of the water quality goals for this subwatershed is the achievement of the Gleason Lake in-lake phosphorus concentration interim goal of 80 Mu-g/L.  Reduction of phosphorus loads from the subwatershed to achieve that goal will require the combined efforts of the regulatory program, loading reduction allocations to LGUs, operational programs, and capital projects.  Table 15 below sets forth a summary plan for how this could be accomplished.

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.   The Gleason Lake Management Plan identified a number of potential improvements that could be considered which together would result in a substantial decreasing in phosphorus load form the subwatershed.    In addition, a new detention pond on Ditch #15 just north of CR 6 could provide a significant phosphorus load reduction. 

It is important to note that a significant share of the modeled phosphorus load to Gleason 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.  This lake is not a good candidate for application of an alum treatment to control sediment sources.  There is a significant amount of curlyleaf pondweed in the lake, and control of that invasive plant often can significantly reduce internal phosphorus.  

Table 15.  Phosphorus load reduction plan for Gleason Lake. 

(interim goal = 80 Mu-g /L TP.)

Source 

Reduction 

Ultimate Phosphorus Load [lb/yr]

 

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric 

Atmospheric deposition

NA

37

NA

37

 

External Loads 

External load determined from modeling land use 

 

879

 

 

  

  

 

Existing regulations

 

6

 

 

 

LGU load reduction allocation (Table 14)

 

115

 

 

Cooperative projects from the Gleason Lake Management Plan

 

Cooperative projects: Rehab ponds per GLMP

 

60

 

Improve ponds 2,3,6, and bank erosion site E3

Cooperative projects: Wayzata Central Middle School

 

7

 

Wetland treatment and infiltration

Cooperative projects: Rehab ponds per GLMP

 

17

 

Improve pond 8, and Cimmaron Ponds subd

Other projects

Gleason Lake inlet pond

 

53

 

Pond at lake inlet under design in 2006

Ditch #15 CR 6 detention pond

 

116

 

Proposed in-line pond upstream of CR 6

Total After Reductions

 

 

 

505

 

Internal / "Unknown" Loads 

Internal / "unknown"

loads determined from  modeling land use

 

 

392

 

 

  

 Internal load management

 

196

 

Est. 50% reduction of internal loading

Total After Reductions

 

 

 

196

 

Total Load 

 TOTAL 

 

1,309

456

738

  

 LOAD GOAL  

 

 

 

694

 

 DIFFERENCE 

 

 

 

44

 Adaptive

 management