It’s hard to imagine now, but for most of the 20th century more than a half-dozen wastewater treatment plants discharged effluent into Lake Minnetonka and its tributaries. The enormous group effort to decommission these plants in the 1970s and 1980s is among the most important events in the history of the lake and the Minnehaha Creek watershed.

Seven different wastewater treatment plants discharged into Lake Minnetonka beginning as early as 1927 and continuing as late as 1986: Excelsior, Long Lake, Maple Plain, Mound, Orono, Victoria and Wayzata. 

Although it fell short of raw sewage, the discharge from these plants was still a lot more polluted than what we’d like going into a lake. It contained high amounts of phosphorus, which is the primary driver of algae and other water quality issues. One pound of phosphorus can lead to 500 pounds of algae, and in the early 1970s these treatment plants were discharging an estimated 50,000 pounds of phosphorus per year. That would equal about 25 million pounds of algae per year! The treatment plants accounted for more than double the phosphorus loading than all other sources combined. 

The degraded water quality, demonstrated by floating algae blooms on main parts of Lake Minnetonka, became a highly charged political issue. With water quality at an all-time low, the plants began to be phased out in 1971. Sanitary interceptors were constructed around the lake to collect and transport the sewage flow to central plants for treatment and discharge into the Minnesota and Mississippi Rivers. 

Though Lake Minnetonka saw a significant rebound in water quality from this diversion, the long-term effects of these practices are still being felt, especially in the western portion of the lake. A significant amount of the phosphorus that entered the lake from the treatment plants remains on the lake bottom and is recycled each year in a process known as “internal loading.” This is exacerbated by a large population of common carp which stir up the lake bottom and release the trapped phosphorus. 

To address these historic pollution problems, the Minnehaha Creek Watershed District began a multi-year focus on the headwaters of Lake Minnetonka – the Six Mile-Halsted Bay Subwatershed. This work will concentrate on solving issues of internal loading and invasive carp, reversing the damage caused by decades of sewage treatment discharge. 

What I like about this story is that it shows that big things actually can change if enough people want them to. By working together, pooling our resources and integrating our water resource protection goals with our partners’ goals, we can keep Lake Minnetonka a healthy and beautiful resource to be enjoyed for generations to come.