Delineating a recharge area for a spring using numerical modeling, Monte Carlo techniques, and geochemical investigation

Recharge areas of spring systems can be hard to identify, but they can be c ritically important for protection of a spring resource. A recharge area fo r a spring complex in southern Wisconsin was delineated using a variety of complementary techniques. A telescopic mesh refinement (TMR) model was cons tructed from an existing regional-scale ground water flow model. This TMR m odel was formally optimized using parameter estimation techniques; the opti mized "best fit" to measured heads and fluxes was obtained by using a horiz ontal hydraulic conductivity 200% larger than the original regional model f or the upper bedrock aquifer and 80% smaller for the lower bedrock aquifer. The uncertainty in hydraulic conductivity was formally considered using a stochastic Monte Carlo approach. Two-hundred model runs used uniformly dist ributed, randomly sampled, horizontal hydraulic conductivity values within the range given by the TMR optimized values and the previously constructed regional model. A probability distribution of particles captured by the spr ing, or a "probabilistic capture zone," was calculated from the realistic M onte Carlo results (136 runs of 200). In addition to portions of the local surface watershed, the capture zone encompassed areas outside of the waters hed-demonstrating that the ground watershed and surface watershed do not co incide. Analysis of water collected from the site identified relatively lar ge contrasts in chemistry, even for springs within 15 in of one another. Th e differences showed a distinct gradation from Ordovician-carbonate-dominat ed water in western spring vents to Cambrian-sandstone-influenced water in eastern spring vents. The difference in chemistry was attributed to distinc tive bedrock geology as demonstrated by overlaying the capture zone derived from numerical modeling over a bedrock geology map for the area. This find ing gives additional confidence to the capture zone calculated by modeling.