5.7 Phosphorus Load Reduction

One of the water quality goals for this subwatershed is the achievement of the Dutch Lake in-lake phosphorus concentration goal of 40 μg/L.  Reduction of phosphorus loads from the subwatershed to achieve that goal will require the combined efforts of the regulatory program, operational programs, LGU load reductions, and capital projects.  Table 14 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 and the reduction of existing loads.  Application of the current regulatory program to new development would also reduce the expected load from ultimate development conditions.  As Table 9 above indicates, more stringent regulation of new development could reduce the need for or size of capital projects. 

It is important to note that a significant share of the modeled phosphorus load to Dutch 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 the wetlands upstream.  Quantification of those sources will require more detailed diagnostic study.  On completion of further diagnostic work, a feasibility study could more accurately identify specific improvements and their likely load reduction efficiencies.

Table 14.  Phosphorus load reduction plan for Dutch Lake.

Source 

Reduction

Ultimate Phosphorus Load [lb/yr]

Planned Reductions [lb/yr]

Final Loading [lb/yr]

 

Atmospheric

Atmospheric deposition

NA

38

NA

38

 

External Loads

External load determined from modeling land use

 

351

 

 

 

 

LGU load reduction (Table 13)

 

 28

 

 

 

Existing regulations

 

71

 

 

 

Additional regulation

 

36

 

 

 

Wetland detention pond

 

65

 

50%Removal at Outlet from DL-4

Total After Reductions

 

 

 

151

 

Internal / "Unknown" Loads

Internal /"unknown" loads determined from modeling land use

 

335

 

 

 

 

Internal load management project

 

235

 

70% reduction of internal loading

Total After Reductions

 

 

 

100

 

Total Load

 TOTAL

 

724

435

289

 

 LOAD GOAL

 

 

 

306

 

 DIFFERENCE

 

 

 

(-17)