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Sixteen lakes in the city of Eagan, Minnesota, were sampled during 1982-83 to detect water-quality changes that might have occurred because of urbanization since a previous study conducted during 1972-78. Each of the lakes was sampled five times to determine pH, specific conductance, dissolved oxygen, water temperature, transparency, and concentration of dissolved chloride. Three determinations of chlorophyll were made for each of the lakes near the end of the study, and additional determinations were made for a few lakes of particular interest. Most of the lakes have been incorporated into the city's storm-runoff system for use as retention basins during large storms. The chemistry of the lakes appears to be degraded by urban runoff. Chloride concentrations were significantly higher in six lakes than during the previous study, and remained elevated (about 30 milligrams per liter) in four other lakes. The association between increased chloride and specific conductance with runoff from urban areas suggests that the lakes are subject to contamination by chloride commonly present in urban runoff, and chloride concentrations harmful to aquatic life may have been associated with high specific conductance measured in one of the lakes. Analysis of the data collected for this study indicate that the chemistry of the lakes changes, adjusting to a variety of influences including (1) alternate loading and flushing by runoff, (2) excessive average-annual precipitation, (3) changing ground-water and lake interactions, and (4) changing land use in the lake watersheds. Some lakes affected by urban runoff had reduced concentrations of total phosphorus; however, other lakes unaffected by urban runoff also had significantly lower phosphorus, suggesting that increased precipitation may have diluted the lake water. Ten phosphorusloading models tested or developed during the previous study generally were found to be inadequate for describing the results of this study. The trophic status of 12 lakes improved but declined in the other 4 lakes, and productivity increased 38 percent in what had been the least-eutrophic lake. INTRODUCTION The city of Eagan, with a 1980 population of 20,700 (U.S. Department of Commerce, 1982), is a rapidly growing suburb of the Twin Cities Metropolitan Area in southeastern Minnesota. The evolution of this area from an agricultural community to one dominated by light industry and extensive residential development has led to concerns about the effects of urbanization on the quality of the numerous lakes and ponds in the city. Seventeen lakes and ponds were sampled during 1972-78 to establish the baseline quality of the Eagan lakes. Findings from the first 2 years of this study were presented in a report by Have (1975). Ayers and others (1980) presented the lake-quality data determined from 6 years of sampling, explained changes in lake quality observed during the study, and evaluated several phosphorus-loading models that might be used to determine changes in lake quality that result from urbanization. Sixteen of the 17 lakes were resampled in 1982-83. During the last several years, many of the lakes and ponds in Eagan have been incorporated into the storm-sewer system to serve as retention basins for storm-water runoff according to the plans of Bonestroo and others (1978). Concern over the effect of storm water and continued urbanization on the quality of the Eagan Lakes resulted in a follow-up study performed by the U.S. Geological Survey in cooperation with the city of Eagan. Although most of the work had been completed, installation of the storm-sewer system continued during this study. Purpose and Scope The purposes of this report are to summarize the results of the 1982-83 study and to compare the results with those obtained during the previous study to detect changes in lake quality. Trends in lake-water quality are evaluated to determine their relation to land-use changes in the watersheds. Lakequality models developed during the previous study (Ayers and others, 1980) are retested using the expanded data base and updated values for the parameters in the models. Physical Setting and Storm-Sewer System The city of Eagan (fig. 1) is located in the northwestern part of Dakota County, Minnesota, and borders on the Minnesota River to the northwest. The mean annual precipitation for this area is about 27.5 inches, most of which occurs during April through September. The lakes included in this study are situated in clay, sand, and gravel deposited by glaciers about 10,000 years ago. None of the lakes have natural outlets at normal stages, indicating that lake stages depend on precipitation and interactions with the local ground water; normal lake elevations probably reflect the changes in precipitation and the elevation of the water table in the immediate vicinity of the lakes. The lakes, ponds, and marshes in this region probably are major points of recharge to the ground-water system following heavy rains and snowmelt. Regional ground-water flow is towards the Minnesota River.

Effect of urban runoff on the quality of lakes in Eagan, Minnesota