Dynamic hydrologic and geochemical response in a perennial karst spring

Storms induce rapid variations in the discharge, specific conductivity, and temperature of a perennial karst spring in eastern Missouri that are followed by gradual return to normal conditions. These dynamics reflect the varying relative proportions of base flow'' and event water'' components that have different delta(18)O signatures, solute concentrations, flow paths, and transport timescales, which combine with other transport impedances to govern the temporal behavior of water quality parameters. A new Darcian model accurately reproduces the hydrograph and its separated components, defines the time constants that govern their physical and geochemical responses, and affords a quantitative method to investigate these linked behaviors. Analysis of 58 storm events reveals an average pulse time constant of 0.4 +/- 0.2 days that is much shorter than the similar to2 year residence time of water in the aquifer derived from long-term delta(18)O variations. For individual pulses this short time constant for total flow approximates that of the base flow component, but the time constant for the event water component is even shorter. The same model also approximates other storm-induced variations and indicates they are all triggered at the same time but respond according to different time constants of 1.6 +/- 0.2 days for oxygen isotopes, 1.6 +/- 0.9 days for temperature, and 3.4 +/- 1.0 days for specific conductivity and major ion concentrations. The time constant for discharge decreases somewhat with greater peak flows, while the geochemical time constants increase.