Vadnais Lake is located in northern Ramsey County, Minnesota. The lake is managed by the St. Paul Water Utility for storage of municipal water supplies that are provided to residents of St. Paul and surrounding communities. In recent years, algal blooms in Vadnais Lake have caused taste and odor problems in St. Paul's municipal water supply. This problem has potentially been exacerbated by phosphorus enrichment of the lake from surface-water transport and from nutrient recycling between the lake and bottom sediments. Phosphorus loading in Vadnais Lake has been linked to increased algal growth. Surface-water drainage from supply lakes and, during wet years, from wetlands in Lambert Creek watershed, which extends over about 20 square miles east of the lake, is known to be a significant source of phosphorus. The role of ground water in the phosphorus budget of the lake was unknown. The results of this study indicate that ground-water transport of phosphorus into Vadnais Lake is a small part of the phosphorus budget of the lake.

Seepage into and out of Vadnais Lake was estimated by the following methods: (1) interpretation of the hydraulic conductivities of glacial deposits around the lake and of the hydraulic gradients between ground water in these deposits and the lake; and (2) interpretation of the hydraulic conductivities of lakebed materials and of the hydraulic gradients between ground water in the lakebed and the lake. The estimated net annual seepage out of the lake determined by the second method of 9.56 x 10⁷ feet³ per year was greater by an order of magnitude than the value determined by the first method, which was 7.63 x 10⁶ feet³ per year.

The net seepage out of Vadnais Lake was a small part of the lake water budget. Inflow to the lake is mostly streamflow from Sucker Creek, which is water from the Mississippi River routed through a series of lakes. Outflow from the lake is mostly withdrawals by the St Paul Water Utility. The net seepage out of the lake between spring and winter was from less than 1 to about 4 percent of the total loss attributed to withdrawals and evaporation.

The quality of the ground water in the study area was similar to the quality measured in shallow glacial-drift aquifers throughout Minnesota. Calcium magnesium bicarbonate water was the dominant type. Concentrations of volatile organic compounds in ground-water samples were below detection limits. The concentration of total phosphorus in ground-water samples generally was below 0.15 milligrams per liter.

The results of the seepage analysis and ground-water quality evaluation indicate that the effect of the quality of the surrounding ground water on the quality of Vadnais Lake probably was small. Ground water that discharged to the lake generally had lower concentrations of calcium, magnesium, bicarbonate, and total dissolved solids than the lake. The mixing of ground water with the lake slightly diluted the lake with respect to these constituents.