5.7 Phosphorus Load Reduction

One of the water quality goals for this subwatershed is the achievement of the Langdon Lake in-lake phosphorus concentration goal of 55-70 μg/L.  Reduction of phosphorus loads from the subwatershed to achieve that goal will require the combined efforts of the regulatory program, operational programs, and capital projects.  Table 13 below sets forth a summary plan for how this could be accomplished to achieve either the 55 or 70 μg/L goal.

Table 13 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 and the reduction of existing loads.  The HHPLS identified a 10 pound upstream load reduction that could be achieved through the general application of BMPs to subwatershed unit LL-5, such as phosphorus-free fertilizer, rain gardens, infiltration basins, agricultural BMPs, shoreline restorations, and retrofit projects.

Application of the current regulatory program to new development would also reduce the expected load from ultimate development conditions.  As Table 8 above indicates, more stringent regulation of new development could significantly reduce the need for or size of capital projects.  Redevelopment of parts of Mound’s central business district, known as the Mound Harbor Renaissance, is an example of how redevelopment can provide unique opportunities to improve water quality.  Much of the redevelopment area is tributary to Lost Lake downstream of Langdon Lake, but the plan also includes redevelopment of an area tributary to Langdon Lake.  Through negotiation between the developer, the City of Mound, and the District, an agreement was reached to achieve a 70 percent reduction in total phosphorus from storm runoff, well in excess of the 50 percent required as of 2005.  The agreement provides an incentive for the developer to achieve even greater removal efficiencies and receive credit towards meeting the removal goal in later developments.  The suite of innovative BMPs proposed by the developer would also result in no runoff from a 1 inch or lesser storm event, significantly reducing runoff volume.

It is important to note that a significant share of the modeled phosphorus load to Langdon 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 of phosphorus from wetlands upstream of the lake.  Quantification of those sources will require more detailed diagnostic study.  This summary plan sets forth estimated reductions that could be achieved through typical capital projects.  On completion of further diagnostic work, a feasibility study could more accurately identify specific improvements and their likely load reduction efficiencies.

Table 13.  Phosphorus load reduction plan for Langdon Lake.

Source

Reduction

Ultimate Phosphorus Load [lb/yr]

Goal = 70 μg/L

Goal = 55 μg/L

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

Planned Reductions [lb/yr]  

Final Loading [lb/yr]  

Atmospheric 

 Atmospheric Deposition

NA

35

NA

35

NA

35

External Loads 

 External Load Determined

 from Modeling Land Use

 

223

 

 

  

 

 

  

 

 LGU load reduction allocation (Table 12)

 

20

 

20

 

 

 Existing regulations

 

34

 

34

 

 

 Additional regulations

 

 

 

 

 

 

 Wetland detention pond at former WWTP site

 

45

 

45

 

 Total After Reductions

 

 

 

124

 

124

Internal / "Unknown" Loads 

 Internal / "Unknown"

 Loads Determined from    

 Modeling Land Use

 

352

 

 

  

  

 

 Internal Load

 Management Project

 

148

 

273

 

 Total After Reductions

 

 

 

204

 

79

Total Load 

 TOTAL 

 

610

 

363

 

 

238

 LOAD GOAL  

 

 

 

382

 

258

 DIFFERENCE 

 

 

 

(-19)

 

(-20)