Minocqua and Kawaguesaga Lakes are 1,318- and 690-acre interconnected lakes in the popular recreation area of north-central Wisconsin. The lakes are the lower end of a complex chain of lakes in Oneida and Vilas Counties, Wis. There is concern that increased stormwater runoff from rapidly growing residential/commercial developments and impervious surfaces from the urbanized areas of the Town of Minocqua and Woodruff, as well as increased effluent from septic systems around their heavily developed shoreline has increased nutrient loading to the lakes. Maintaining the quality of the lakes to sustain the tourist-based economy of the towns and the area was a concern raised by the Minocqua/Kawaguesaga Lakes Protection Association. Following several small studies, a detailed study during 2006 and 2007 was done by the U.S. Geological Survey, in cooperation with the Minocqua/Kawaguesaga Lakes Protection Association through the Town of Minocqua to describe the hydrology and water quality of the lakes, quantify the sources of phosphorus including those associated with urban development and to better understand the present and future effects of phosphorus loading on the water quality of the lakes.

The water quality of Minocqua and Kawaguesaga Lakes appears to have improved since 1963, when a new sewage-treatment plant was constructed and its discharge was bypassed around the lakes, resulting in a decrease in phosphorus loading to the lakes. Since the mid-1980s, the water quality of the lakes has changed little in response to fluctuations in phosphorus loading from the watershed. From 1986 to 2009, summer average concentrations of near-surface total phosphorus in the main East Basin of Minocqua Lake fluctuated from 0.009 mg/L to 0.027 mg/L but generally remained less than 0.022 mg/L, indicating that the lake is mesotrophic. Phosphorus concentrations from 1988 through 1996, however, were lower than the long-term average, possibly the result of an extended drought in the area. Water‑quality data for Kawaguesaga Lake had a similar pattern to that of Minocqua Lake. Summer average chlorophyll a concentrations and Secchi depths also indicate that the lakes generally are mesotrophic but occasionally borderline eutrophic, with no long-term trends.

During the study, major water and phosphorus sources were measured directly, and minor sources were estimated to construct detailed water and phosphorus budgets for the lakes for monitoring years (MY) 2006 and 2007. During these years, the Minocqua Thoroughfare contributed about 38 percent of the total inflow to the lakes, and Tomahawk Thoroughfare contributed 34 percent; near-lake inflow, precipitation, and groundwater contributed about 1, 16, and 11 percent of the total inflow, respectively. Water leaves the lakes primarily through the Tomahawk River outlet (83 percent) or by evaporation (14 percent), with minor outflow to groundwater. Total input of phosphorus to both lakes was about 3,440 pounds in MY 2006 and 2,200 pounds in MY 2007. The largest sources of phosphorus entering the lakes were the Minocqua and Tomahawk Thoroughfares, which delivered about 39 and 26 percent of the total, respectively. The near-lake drainage area, containing most of the urban and residential developments, disproportionately accounted for about 12 percent of the total phosphorus input but only about 1 percent of the total water input (estimated with WinSLAMM). The next largest contributions were from septic systems and precipitation, each contributing about 10 percent, whereas groundwater delivered about 4 percent of the total phosphorus input.

Empirical lake water-quality models within BATHTUB were used to simulate the response of Minocqua and Kawaguesaga Lakes to 19 phosphorus-loading scenarios. These scenarios included the current base years (2006–07) for which lake water quality and loading were known, nine general increases or decreases in phosphorus loading from controllable external sources (inputs from the tributaries and nearshore areas around the lakes and input from septic systems), and nine scenarios corresponding to future changes in phosphorus loading from residential and urban development, referred to as “2030 buildout,” and removal of septic system inputs. The 2030 buildout scenario with existing stormwater controls resulted in a degradation in water quality: phosphorus concentrations increased by about 0.001 mg/L, chlorophyll a concentrations increased by 0.2–0.8 μg/L, and Secchi depths decreased slightly. The largest degradation in water quality was estimated to occur in Kawaguesaga Lake. If 2030 buildout occurred with implementation of best management practices to achieve a 50-percent reduction in loading from near-lake drainages, it is possible that water quality would change very little from existing conditions. Numerous noncontributing areas exist within the watershed that help minimize surface runoff and nutrient loading to the lakes; however, if future development included extending or connecting drainage from these areas into the lakes, loading to the lakes could greatly increase and cause a degradation in the water quality of the lakes. Simulations of removal of phosphorus loading from septic systems around Minocqua Lake improved the water quality of the lakes: in simulations for that scenario, phosphorus concentrations decreased by about 0.001 mg/L, chlorophyll a concentrations decreased by 0.5–0.7 μg/L, and Secchi depths increased by 0.3–0.7 ft. If all controllable external phosphorus loading could be reduced by 50 percent, the lakes would become oligotrophic with respect to phosphorus concentration but would still remain mesotrophic with respect to chlorophyll a concentration and Secchi depth. Improvements in the water quality of the lakes are likely only with a combination of management actions that decrease inputs from the developed near-lake drainage areas and from septic systems.