MONTH-SCALE RESIDENCE TIME OF VADOSE ZONE STORAGE ESTIMATED THROUGH HYDROLOGIC AND GEOCHEMICAL MONITORING OF CAVE DRIP WATERS, MINNESOTA, U.S.A

Estimating residence times of vadose zone waters in karst aquifers is useful for understanding rates of diffuse aquifer recharge. Moreover, flowpaths through the vadose zone are the link between surface climate patterns and geochemical proxy records captured in speleothems, thus residence time estimation aids in interpreting such paleoclimate data.

In this study, two perennial ceiling drip sites in Mystery Cave, Minnesota were monitored between 1993 and 2001. Records of drip rate, electrical conductivity, and water temperature were obtained at 15 minute intervals, and supplemented with periodic sampling for major ion chemistry and water isotopes. One site responded quickly (within 2-7 hours) to rain or snowmelt events while the other responded slowly (within 2-5 days). The sites were located about 60 m apart along the same cave passage approximately 20 m below the surface; 7 m of loess and 13 m of flat-lying carbonate bedrock strata overlie the cave.

Drip rate hydrograph recessions were modeled as a series of exponential decay functions permitting estimation of flow regimes, recession constants and storage volumes for water drained from the vadose zone. The residence time at each site was estimated as the reciprocal of the weighted mean of the recession coefficients (the harmonic mean life). The storage volume of each recession segment was used to weight the mean. The estimated residence times were 35 days and 42 days with active drainable porosities of 0.27% and 0.45% at the fast-response and slow-response drips, respectively.

The chemical data from both sites indicate that reservoirs of vadose zone water with distinct chemical signatures mix during recharge events, and drip chemistry returns to a baseline composition during low flow periods. The average δ¹⁸O and δ²H stable isotope composition of the fast-response drip water was lower relative to the slower response drip water, possibly indicating evaporative enrichment in the vadose zone above the cave. Average tritium values for samples collected in winter of 2001 were 9.0±0.5 TU and 7.0±0.4 TU for the fast-response and slow-response drips, respectively, indicative of modern recharge even under very low flow conditions. Calcite saturation indices and visual observations indicated calcite precipitation occurred primarily during the lowest flow conditions.