5.7 Phosphorus Load Reduction

One of the water quality goals for this subwatershed is the reduction of phosphorus loading into Jennings Bay by reducing phosphorus loading from Painter Creek.  Table 15 below sets forth a summary plan for how this could be accomplished.   This plan was described in detail in the Painter Creek Feasibility Study.

The table breaks down modeled phosphorus loading to Jennings Bay 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 loading from the stream channel itself through streambank restoration and erosion control.    The Feasibility Study identified a number of improvements throughout the subwatershed to consider that are described in more detail below.

It is important to note that the movement of phosphorus in the Painter Creek subwatershed has been studied extensively but is still not completely understood.  The phosphorus load contributed by the creek to Jennings Bay 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, although wetland export from the subwatershed may also contribute.  Internal load management in Jennings Bay such as alum treatment to control sediment sources coupled with control of aquatic vegetation in the Bay may alleviate some internal loading.  Rough fish management in Katrina Lake and the creek may also be required.  A feasibility study would determine the most appropriate internal load reduction options.   

This load reduction plan would not achieve the upper limit of the Jennings Bay HHPLS total phosphorus concentration goal of 50-70 ?g/L.  Because of the unknowns associated with phosphorus loading in and from the subwatershed and experimental nature of some of the wetland restorations identified in the Feasibility Study, this plan assumes the District will continue to monitor performance and may make changes to this plan to adapt to new information or changing conditions.

Table 15.  Phosphorus load reduction plan for Painter Creek/Jennings Bay (based on the Painter Creek Feasibility Study.)

Source 

Reduction 

2000 (1) Phosphorus Load [lb/yr]  

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

  

Atmospheric

Atmospheric Deposition

NA

71

NA

71

 

External Loads

Dutch Lake Subwatershed

 

284

95

189

  

Lake Minnetonka Direct

 

253

83

170

 

 

LGU load reduction allocation  (Table 10)

 

157

 

 

Painter Creek (from Feasibility Study)

 

3,137

 

 

 

 

PC-6 & 7 pond  (2)

 

111

 

 

 

Katrina Lake in-lake load management options

 

97

 

 

 

South Katrina Marsh improvements

 

384

 

 

 

Additional PC-13 storage

 

-

 

 

 

Stabilize PC-13 stream

 

69

 

 

 

?Potato Farm? wetland improvements

 

271

 

 

 

Treatment at Painter Marsh inflow

 

54

 

 

 

Wetland corridor improvements

 

-

 

 

 

Painter Marsh restoration

 

524

 

 

 

Hwy 26 (Pond 937) wetland restoration

 

293

 

 

 

PC-25 wetland treatment

 

24

 

 

 

Carp gate at creek mouth

 

-

 

 

 

Subtotal

 

1,827

1,310

 

Total After Reductions

 

 

2,162

1,512

 

Internal / "Unknown" Loads

Internal/Unknown

Loads Determined from  Modeling Land Use

 

1,463

 

 

  

Internal load management

 

1,024

 

70% reduction

 Total After Reductions

 

 

 

439

 

Total Load

 TOTAL 

 

5,208

3,029

2,022

  

 LOAD GOAL  

Interim goal of 70 ug/L

 

 

1,900

 

 DIFFERENCE 

 

 

 

122

 Adaptive

 management

(1)  Phosphorus loads and estimated reductions resulting from improvements are from the Painter Creek Feasibility Study, which updated and refined the HHPLS loadings for this subwatershed.  Ultimate land use conditions are not available.

(2) Proposed projects are presented in an upstream to downstream order.  Estimated reductions assume a ?treatment train? approach whereby downstream projects and associated reductions assume that upstream projects and reductions are in place