Summary Letter

HARZA ENGINEERING COMPANY
CONSULTING ENGINEERS
POWER. WATER AND LAND RESOURCES
150 SOUTH WACKER DRIVE
CHICAGO, ILLINOIS 60606
TELEPHONE (312) 855-7000
CABLE ADDRESS: HARZENG CHICAGO TELEX 25-3540

January 15, 1971

Mr. John P. Badalich, P.E.
Executive Director
Minnesota pollution Control Agency
717 Delaware Street,S. E.
Minneapolis, Minnesota 55440

We are pleased to submit our report on a program for preserving the quality of Lake Minnetonka. Closely associated.with us in these studies were Barr Engineering Co., Eugene A. Hickok and Associates, and the Limnological Research Center of the University of Minnesota. The contributions of the Lake Minnetonka Conservation District were of great assistance to the successful completion of the project.

The lake in recent years has undergone a continued decline in quality. This is indicated by the increasing use of algicides in portions of the lake, and by complaints about algae decaying along the shores of the lake. Objectively, comparison of the measurements of algal concentrations which were made during 1968, 1969, and 1970 with those which were made during 1937 reveal that the lake now supports a crop of algae about twice that which it supported some 30 years ago. The quality of the lake has declined in terms of diversity of animal species and other aspects, but the principal problem is its accelerated eutrophication, or aging, due to increased production of aquatic plants.

The lake also has been deficient in water quantity from time to time. The most notable of these periods occurred during the 1930's. The lake overflowed in the fall of 1929 and did not overflow again until 1942, a period of 13 years. The minimum lake level was over 6 feet below the crest at Grays Bay dam. This decline in lake elevation resulted in many boat docks being left high and dry, and along shorelines with gradually sloping bottoms, the bottom was exposed for as much as a thousand feet.

While Lake Minnetonka would not have the same azure appearance that Lakes Tahoe or Superior have even if no one lived in the watershed, the decline in its quality is due to the presence of people living and working in the watershed. Storm runoff from farms carries a nutrient load greater than the runoff from forests or prairies, and the runoff from urban lands carries still more nutrients. It is the increase in nutrient input to the lake which has caused the increase in algal growth. Of the nutrients required by algae, phosphorus has been found to be the key nutrient in Lake Minnetonka during that portion of the year when nuisance growths of algae occur.

In addition to the urbanization of the watershed, wastewater treatment plants have been built to serve some of the urban population of the watershed. Originally, these plants were operated with inadequate disinfection of the effluent. It became apparent in the late 1950's that the bacteriological quality of the lake required greater protection than the treatment plants were providing at that time.

After facilities for disinfecting the effluent were added, treatment plants have been expanded, and new plants have been built. The treatment plants currently in the watershed contribute most of the nutrients to the lake. At the present time, wastewater from treatment plants is the largest single source of phosphorus to the lake.

We have, therefore, examined the desirability of chemically treating the wastewater to minimize the phosphorus it will contribute to the lake. We have also examined the desirability of diverting the wastewater from the watershed to the treatment plant planned for construction at Blue Lake on the Minnesota River.

We have found that the facilities required to convey the wastewater and provide secondary treatment at Blue Lake will be less expensive than providing similar facilities for secondary treatment within the Lake Minnetonka watershed, even assuming the continued use of existing plants. We estimated the capitalized cost of the construction and operating costs for the diversion alternative to be $38,700,000 as opposed to an estimated $39,600,000 for a comparable alternative including treatment within the watershed.

While these two alternatives would provide a comparable degree of secondary biological treatment to wastewater, they would not provide comparable degrees of nutrient protection to the lake. To remove 80% of the phosphorus from the wastewater, the estimated capitalized cost of the alternative including treatment within the watershed would be increased to $44,000,000. For the removal of 90% of the phosphorus, the capitalized cost would be $45,000,000, and the removal of phosphorus in the range 98-99%, which is about the maximum attainable with present technology, the capitalized cost would be $50,100,000.

It is more difficult to express the other costs which should be weighed in the selection of one alternative over the other. Of the more important are the questions of manpower requirements and reliability. The estimated costs of treatment within the watershed include the wages for competent operators. It is not certain, however, that manpower is available to adequately staff the several plants which would be required. Since phosphorus removal efficiencies depend in part on the skills of operating personnel, this consideration is closely allied to that of reliability. Diversion of wastewater from the watershed should be very reliable. Emergency power sources can be provided which will minimize any need for by-passing.

Our studies have shown that with diversion of wastewater, a drought similar to that of the 1930's beginning in the year 2000 would not result in lower lake levels than actually occurred during 1930's. This finding is based in part on our assumption that future wells for municipal water supplies would be developed in the Jordan formation. The Jordan formation is not directly connected to Lake Minnetonka and is recharged partly from sources outside the watershed. Most of the large capacity wells in the region utilize the Jordan sandstone.

We have therefore concluded that the decision to divert wastewater from the Lake Minnetonka basin for treatment at Blue Lake is fully justified by the costs of the alternatives and the potential effectiveness of the two alternatives.

During the course of our studies, we have found that the phosphorus carried to the lake by the major streams of the watershed is less than the amount which would be predicted from our observations and from research conducted by others. We conclude that much of this phosphorus is trapped in the many marshes and small lakes within the watershed. For this reason and others, these areas should be preserved in their natural state. In addition, future storm water drainage systems should be designed to encourage storm water to diffuse through the marshes. The development of facilities which retain stormwater in the marshes and small lakes of the watershed will also assist the infiltration of surface water into the subsurface formations until it is needed for domestic or industrial use.

Our studies indicate that this will be vital to avoid excessive draft on the groundwater supplies closely connected with the lake in meeting the needs of the growing population. We recommend that future water supply wells be developed in the Jordan formation or its hydraulic equivalent rather than the shallow formations.

During' the development of these wells, the opportunity should be taken to improve the existing knowledge of the underground formations through careful logging and pumping tests which will enable the hydraulic characteristics of the formations to be determined more accurately than they are known at present.

The hydrologic observation programs of the Minnehaha Creek Watershed District provide a vital back stop to these measures. Their observations of stream flows and water tables will help provide a warning if, despite the development of deeper wells in the future, diversion of wastewater from the lake causes an excessive decline in the regional water resources.

The recommendations above are relatively long range measures to improve the quality of the lake, preserve the quantity of water available for the lake, and detect changes in the quantity of water available. Diversion of wastewater can probably be instituted within 2 to 3 years. The others are continuing programs.

We also suggest actions suitable for immediate implementation by citizens in the watershed. Homeowners should be encouraged to purchase detergents which have a low content of phosphorus. The present practice of placing portable toilets around the lake should be continued. It may be possible to supplement these with publicly owned facilities in certain locations.

The owners of boats with toilets should be encouraged to use storage tanks in their boats. People should be encouraged to minimize the use of fertilizers on their lawns. Those people who live adjacent to the lake can help by leaving an unfertilized strip adjacent to the shore.

We appreciate this opportunity to assist in the preparation of plans for the protection of Lake Minnetonka. We trust our contribution will assist in the long range protection of the quality and quantity of the waters of the lake.

Very truly yours,

HARZA ENGINEERING COMPANY
Kenneth E. Sorensen
Vice President