5.7 Phosphorus Load Reduction

One of the water quality goals for this subwatershed is the achievement of in-lake phosphorus concentration goals.  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.  Tables 15 and 16 below set forth a summary plan for how this could be accomplished.

The tables break 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 upstream load reductions that could be achieved through the general application of BMPs to residential and agricultural land, 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 10 in Section 3.6 above indicates, more stringent regulation of new development could assist in maintaining good water quality in Lake Minnewashta and in meeting Lake Virginia water quality goals as limited opportunities are available for larger capital projects.

Table 15.  Phosphorus load reduction plan for Lake Minnewashta.

Source  Reduction Ultimate Phosphorus Load [lb/yr] Planned Reductions[lb/yr] Final Loading [lb/yr]  

Atmospheric

 Atmospheric deposition

NA

178

NA

178

 

External Loads

External load determined

 from modeling land use 

 

 

 

276

 

 

 

LGU load reduction allocation (Table 14)

 

28

 

 

Existing regulations

 

31

 

 50% removal

Additional regulation

 

15

 

 25% addl removal

Targeted BMPs

 

15

 

 In Arboretum, etc.

 Total After Reductions

 

 

 

187

 

Internal / "Unknown" Loads

Internal/unknown

Loads determined from  modeling land use

 

0

 

 

 

NA

 

-

 

 

 Total After Reductions

 

 

 

0

 

Total Load

 TOTAL

 

454

89

365

 

 LOAD GOAL

 

 

 

341

 

 DIFFERENCE

 

 

 

24

 Adaptive Management

Lake Virginia

A TMDL study including a phosphorus reduction plan is currently being developed for Lake Virginia.  Proposed reductions from the TMDL include: reductions due to the new prohibition on the use of fertilizer with phosphorus; achievement of upstream water quality goals; shoreline restoration; treatment of runoff from Highway 7 and from Smithtown Road; application of more stringent regulations prohibiting new net phosphorus loading from development; and reduction of internal loading through internal load management such as biomanipulation or vegetation management.   Table 16 below sets forth a summary plan for how this could be accomplished in accordance with the TMDL.  The TMDL calculates reductions from the current conditions rather than future conditions.

Table 16.  Phosphorus load reduction plan for Lake Virginia.

Source

Reduction

Ultimate Phosphorus Load [lb/yr]

Planned Reductions [lb/yr]

Final Loading [lb/yr]

 

Atmospheric

Atmospheric Deposition

NA

29

NA

29

 

External Loads

External Load Determined

 from Modeling Land Use

 

253

 

 

 

Reductions

proposed in the

draft TMDL

Phophorus free fertilizer

 

20

 

 

Lake Virginia lakescaping

 

3

 

 

Treat Hwy 7 runoff

 

4

 

Local project

Treat Smithtown Rd runoff

 

6

 

Local project

Reduction from upstream

 

5

 

 

 

Regulations

 

 

 

 

 

    No new increase

 

 

 

 

 Total After Reductions

 

 

38

215

 

Internal / "Unknown" Loads

 Internal / "Unknown"

 Loads Determined from   

 Modeling Land Use

 

161

 

 

 

 

 

9

 

Biomanipulation

 Total After Reductions

 

 

 

152

 

Total Load

 TOTAL

 

443

97

396

 

 LOAD GOAL

 

 

 

397

 

 DIFFERENCE

 

 

 

(-1)