3.4.2 Impacts from Historical Municipal Wastewater Treatment Plant Discharges

Several lakes and bays of Lake Minnetonka continue to exhibit impacts from historical discharges from municipal wastewater treatment plants.  Eight municipal wastewater treatment plants (WWTPs) operated and discharged effluent to Lake Minnetonka between 1927 and 1986. The WWTPs were systematically decommissioned between 1971 and 1986 as sanitary sewer interceptors were constructed around the lake.  The interceptors now collect the sewage flow and transport it to central plants, for treatment and discharge to the Minnesota and Mississippi Rivers. In the years prior to the decommissioning of these plants, water quality in Lake Minnetonka was at an all time low. The degraded water quality, demonstrated by floating algae scums on main parts of the lake, became a highly charged political issue and there was concern for declining property values.

During the early 1970’s, when all the WWTPs were still in operation, an estimated 50,000 lbs of phosphorus was discharged annually, with high dissolved phosphorus content. The fate and transport of the phosphorus varied from plant to plant before reaching Lake Minnetonka.  For example, the Excelsior WWTP discharge was nearly 100% efficient in delivery of phosphorus because of the short and direct travel path.  In contrast, the Maple Plain WWTP discharge was far away and much less efficient.  An estimated 70% of the phosphorus discharged from the Maple Plain WWTP was retained in multiple receiving wetlands upstream and along Painter Creek, delivering only about 30% of the total load discharged to Jennings Bay. Together these point discharges represented an external load to Lake Minnetonka about twice as big as all the non-point sources, mainly from runoff, combined.

Decommissioning the plants and constructing the interceptor sewer system required a significant public investment, but upon completion of the projects there was an immediate and noticeable improvement in water quality in Lake Minnetonka and other receiving waters.  To quantify the impact of that change, existing water quality was compared to what it might have been if the waters were still receiving treatment effluent.  The table below shows that the continued operation of the WWTPs would have resulted in continued degradation of water quality and use in all cases except Halsteds Bay, where the loading from the Victoria plant reaching the bay is inconsequential and did not change predicted in-lake concentrations of nutrients.

These historic discharges may continue to have an impact on water quality in the receiving waters through sediment storage.  The Langdon Lake internal load diagnostic study proposed in this plan would investigate what role those historic loads may play on existing water quality in the lake.

Table 22.  Comparison of the current actual water quality of MCWD receiving waters historically impacted by wastewater treatment plants to current modeled water quality assuming continuation of historic wastewater discharges.

    

2000 (1997-2003 Avg)

2000 With WWTP Discharges

WWTP Location 

Primary Receiving Water 

Secondary Receiving Water 

Watershed Load (lbs)

Avg In-Lake TP Conc (ug/l)

Trophic Status Index

Watershed & WWTP Load (lbs)

Avg In-Lake TP Conc.(ug/l)

Trophic  Status Index

Excelsior

Gideon Bay

 

1,750

21

47

10,400

56

62

Long Lake

Tanager Lake

Browns Bay

315

101

71

3,570

221

82

Long Lake

 

Browns Bay

5,650

21

48

7,420

25

51

Wayzata

Peavey Pond

Browns Bay

190

56

62

13,990

960

100

Wayzata

 

Browns Bay

5,650

21

48

7,420

25

51

Maple Plain

Wetlands, Painter Creek

Jennings Bay

(30% Export Efficiency Through Interim Wetland Systems)

Maple Plain

 

Jennings Bay

1,780

68

65

3,020

117

73

Mound

Langdon Lake

Cooks Bay

150

87

69

18,050

568

96

Mound

 

Cooks Bay

860

28

52

3,460

64

64

Orono

Crystal Bay

 

2,340

28

52

3,950

41

58

Victoria

Lakes, Wetlands

Halsteds Bay

(10% Export Efficiency Through Interim Wetland Systems)

Victoria

 

Halsteds Bay

1,800

99

70

1,900

99

70

Source: MCWD.