ESTIMATION OF RECHARGE AND EFFECTIVE RECHARGE AREA FOR BLUE LAKE, MYSTERY CAVE, MINNESOTA

Blue Lake (BL) is a fluctuating pool in Mystery Cave, Minnesota. BL has a maximum volume of about 280 m³, and a minimum volume < 1.5 m³. BL fills following major recharge events and slowly drains at its base through occluded joints over a period of 3-6 months, long after inputs cease. Recharge water is relatively muddy, transporting clay- and silt-sized particles, primarily loess, in amounts that vary and which affect exfiltration rates. Stage recessions follow smooth curves that can be approximated by polynomial functions, but then vary slightly in slope and duration from one hydrologic year to the next.

The shape and elevation of the floor of BL were mapped during dry periods and the data were used to construct a mathematical model of BL that allowed calculation of a relationship between stage and volume. Stage was monitored at 15-minute intervals with a pressure transducer and stored in a data logger. Changes in volume with time (storage) then represent a “net discharge” Qn, which equals the difference between recharge into the lake Qi and discharge via exfiltration Qo through the occluded joints. After inputs cease, Qi equals zero. The stage recession curves when Qi = 0 can be used to estimate and model Qo as a head-driven exfiltration function of water depth (or stage), which is assumed to operate throughout the hydrologic year. This allows calculation of recharge Qi using Qi = Qn ± Qo. Recharge for individual events can then be summed to determine cumulative or yearly recharges. Estimates of the effective surface area contributing recharge to Blue Lake can then be made using area A = V / P, where V is the volume recharged and P is the precipitation, for individual events or entire hydrologic years.