5.7 Phosphorus Load Reduction

One of the water quality goals for this subwatershed is the reduction of phosphorus loading into the bays that exceed their total phosphorus goal.  Actions in this plan are focused on three bays that exceed their goal – Jennings, Halsteds, and Stubbs -and that may be negatively influencing water quality in other adjacent bays.  The District lacks are a whole-lake model that would help understand how the bays interact.  Until that model is in place it is assumed that improvement in the ?upstream? bay will lead to improvement in the ?downstream? bay. 

Table 15, Table 16 and Table 17 below break down modeled phosphorus loading to each of the three bays of concern 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.  

Jennings Bay

As discussed in the subwatershed plan for the Painter Creek subwatershed, phosphorus and sediment loading from that subwatershed is conveyed by Painter Creek to Jennings Bay.   The Painter Creek Feasibility Study set forth a plan of action to reduce phosphorus form the subwatershed, and recommended internal load management in Jennings, West Arm, and Harrisons Bay.  Aquatic vegetation management will also likely be required to bring the in-lake phosphorus concentration to the desired goal.  A feasibility and diagnostic study would be necessary to evaluate the various options for internal load management prior to implementation of specific improvements.

Table 15.  Phosphorus load reduction plan for Painter Creek/Jennings Bay.  (interim goal = 70 ?g/L TP.)

(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

 

Painter Creek (from Feasibility Study)

 

3,137

 

1,309

 

 

PC-6 & 7 pond  (2)

 

111

 

 

 

Lake Katrina 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 wetland restoration

 

293

 

 

 

PC-25 wetland treatment

 

24

 

 

 

Carp gate at creek mouth

 

-

 

 

 

Subtotal

 

1,827

1,310

 

Total After Reductions

 

 

 

1,669

 

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

2,179

 

 LOAD GOAL

 

 

 

1,900

 

 DIFFERENCE

 

 

 

279

 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 development 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. 

Stubbs Bay

Previous diagnostic work including the Stubbs Bay Feasibility Report and the Stubbs Bay Diagnostic Report indicates that internal loading contributes about two-thirds the phosphorus load to Stubbs Bay, while Classen Creek contributes a significant phosphorus and sediment load.  Some actions have been undertaken in the subwatershed, including aeration in the bay.  Recommended actions to reduce the in-lake phosphorus concentration in Stubbs Bay include a diagnostic and feasibility report to assess contributions from Classen Creek and identify options for improvement, including correction of eroded areas in Reach 1 of the Creek; and internal load management in Stubbs Bay.  Aquatic vegetation management will also likely be required to bring the in-lake phosphorus concentration to the desired goal.  A feasibility and diagnostic study would be necessary to evaluate the various options for internal load management prior to implementation of specific improvements.

Table 16.  Phosphorus load reduction plan for Stubbs Bay.  (interim goal = 50 ?g/L TP. )

Source

Reduction

Ultimate Phosphorus Load [lb/yr]

Planned Reductions [lb/yr]

Final Loading
[lb/yr]

 

Atmospheric

Atmospheric Deposition

NA

47

NA

47

 

External Loads

External Load Determined from Modeling Land Use

 

406

 

 

 

 

LGU load reduction allocation (table 10)

 

37

 

 

 

Existing regulation

 

57

 

 

 

Additional regulation

 

28

 

 

 

Classen Creek project

 

50

 

Estimated reduction

Total After Reductions

 

 

 

234

 

Internal / "Unknown" Loads

Internal/"Unknown"

Loads Determined from  Modeling Land Use

 

438

 

 

 

 

 Internal load management project

 

307

 

Est 70% reduction of internal loading

 Total After Reductions

 

 

 

131

 

Total Load

 TOTAL

 

891

479

412

 

 LOAD GOAL

 

 

 

554

 

 DIFFERENCE

 

 

 

(-142)

 Adaptive

 management

Halsteds Bay

The water quality of Halsteds Bay is heavily influenced by phosphorus and sediment loads delivered from the shallow lakes and wetlands at the downstream end of Six Mile Creek: Parley Lake, Mud Lake, and Six Mile Marsh.  Drainage from the Six Mile Marsh subwatershed is conveyed by Six Mile Creek into Parley Lake.  The outflow of Parley Lake and Mud Lake contains significantly higher loads of phosphorus and sediment than the inflow.  It appears that even if the upstream lakes all attain their water quality goals (even Parley), it is likely that Halsteds Bay will not be able to meet its water quality goal without addressing this issue.

Background

Phosphorus concentrations vary upward and downward along the channel and lakes of Six Mile Creek.  The HHPLS existing condition lake phosphorus concentrations are depicted in downstream order below:

Due to increasing runoff volumes and large internal loads, the loads from upstream lakes become a large portion of the load to Halsteds Bay.  Model results suggest a large internal load in the lowest portions of Six Mile Creek, particularly in Parley and Mud Lakes, as well as in Halsteds Bay and Six Mile Marsh just upstream of Halsteds Bay. 

A detailed investigation of the phosphorus load and internal loading dynamics from Lunsten Lake to Halsteds Bay should be conducted. The focus would be to diagnose the actual magnitude and causes of internal loading along lower creek.  Once this is determined solutions to reduce loading could be investigated.  It may be that some intensive management could have an effect, but it is likely to be difficult due to carp and other factors.  (Carp control is not likely to be effective due to the connections to Lake Minnetonka, several upstream lakes and extensive wetland areas.)  

Potential solutions such as the diversion of runoff from upstream of Parley Lake to avoid much of the internal loading that occurs in the lower reaches and dredging Parley and Mud Lakes to provide greater storage and treatment were preliminarily investigated.  These options could provide some improvement – although not enough for Halsteds Bay to reach its goal - at a very significant cost.  The most effective option for improving Halsteds Bay is likely a combination of alum injection treatment at the discharge of into the Bay and internal load management of the Bay itself.

Table 17.  Phosphorus load reduction plan for Halsteds Bay.  (interim goal = 50 ?g/L TP. )

Source

Reduction

Current Phosphorus Load [lb/yr]

Planned Reductions [lb/yr]

Final Loading
[lb/yr]

 

Atmospheric

Atmospheric deposition

NA

130

NA

130

 

External Loads

External load determined from modeling land use

 

2,690

 

 

 

 

LGU load reduction allocation (Table 14)

 

16

 

 

 

Reduction due to upstream lakes at goal

 

1,275

 

 

 

Existing regulations

 

146

 

 

 

Additional regulation

 

73

 

 

 Total After Reductions

 

 

 

1,180

 

Internal / "Unknown" Loads

 Internal/?unknown"

 loads determined from  modeling land use

 

4,998

 

 

 

 

 Internal load management

 

NA

 

Achievable reduction to be identified in a diagnostic study

 Total After Reductions

 

 

 

4,998

 

Total Load

 TOTAL

 

7,818

1,510

6,308

 

 LOAD GOAL

 

 

 

2,027

 

 DIFFERENCE

 

 

 

4,281

 

Other Lakes

Forest Lake has experienced a recent trend of decreasing water quality.  That trend may be a result of development within the watershed or may be a result of some other force.  The goal for Forest Lake is no degradation over current conditions.  If water quality continues to decline, then a more detailed analysis of that lake should be undertaken to assess its condition and the cause or causes of this trend.

Peavey Pond has been evaluated in the past.  It is a sheltered, deep lake that in the past received sewage treatment plant discharge.  Past study suggests that the lake may be more or less permanently stratified, and that runoff from the watershed likely only minimally mixes with the deeper, more phosphorus-rich waters.  No action was recommended in the past, and this plan recommends no future action beyond continued monitoring.