A HYDROLOGIC MODEL FOR BLUE LAKE, MYSTERY CAVE, MINNESOTA

Blue Lake (BL) is a fluctuating pool noted for its prominent raft cones. Measurements of stage, water temperature, and conductivity at 15-minute intervals at BL during the hydrologic year 2000 support a modified five-stage hydrologic model postulated on the basis of hydrologic and chemical data collected 1992-1995. Water enters via a vadose passage. It exits occluded fractures in the lake bottom. In Stage 5 in the winter, under a frozen landscape, BL exists as isolated pools with a total volume of < 1.5 m³. No active inputs exist, water is at a deep-cave temperature of 8.7°C, and conductivity is 450-500 µS/cm at 25°C. Primary fill events (Stage 1) begin with a fall or winter response (delayed <1-6 days) to rain or snowmelt. Recharge infiltrates a dolomite karst mantled with calcareous loess and displaces stored water. Stored water arrives 0.2-0.4°C cooler. Conductivity rises 50-70 µS/cm at 25°C during initial input pulses, which raise water depth from 10 cm to 2 m over 1-5 days and add 5-10 m³ to the volume. Several larger fill events are usually necessary to fill BL to its maximum volume of about 280 m³. These initiate the high, fluctuating water levels of Stage 2. Smaller temperature drops and conductivity rises are associated with these succeeding fill events, as increasing amounts of event water mix with stored water. The relatively long contact time of the event water with calcareous loess during transit, combined with a short (15-20 m) vertical transit through a rudimentary epikarst in the Dubuque and Stewartville formations, are sufficient to allow an approach toward thermal equilibrium and chemical equilibria for calcite and dolomite saturation. Stage 2 lasts through wet springs and summers until recharge wanes. Stage 3 begins when recharge declines, resulting in a falling stage. Stage 4 begins when recharge ceases. Stage 4 is characterized by an exponentially falling water level. Stage 5 begins as water level drops so low that BL breaks into isolated shallow pools. Late during Stage 4 and during Stage 5, in the late summer to early winter, water temperature gradually rises to 8.7°C. Conductivity also drops rapidly 50-70 µS/cm at 25°C during Stage 5, suggesting active deposition of calcite as pool linings, coatings on raft cones, and calcite rafts, which occasionally are observed floating.