Delineating a Spring’s Recharge Area Using Numerical Groundwater Flow Modeling and Geochemical Investigation

Executive Summary: The Pheasant Branch watershed is in an area expected to undergo significant development. There are concerns that this development will adversely effect the water resources of the area, including a large spring complex. This work was conducted as part of a larger three-year study that included an integrated groundwater-surface water modeling and data collection effort.

Initially, a previously developed county-wide scale MODFLOW model (regional model) was used to assess the efficacy of possible refinements in the vicinity of Pheasant Branch. A telescopic mesh refinement (TMR) model was constructed using layering and boundary conditions from the regional MODFLOW model, and further refined by adding nearby surface water features, updating the recharge array to reflect results of the surface water model, and increasing the vertical connection between the deep Mt. Simon aquifer and the upper Paleozoic bedrock in the area of the Frederic Springs. These changes resulted in an improved overall calibration. This TMR model was then formally optimized using the parameter estimation code UCODE. Using the head and flux targets from this work and the original regional model, initial parameter sensitivity analyses showed that only the horizontal hydraulic conductivity of the upper Paleozoic bedrock (layer 2) and lower Mt. Simon Cambrian sandstone (layer 3) were sensitive. Therefore, optimization was performed using these two parameters. During the optimization, the most emphasis was placed on the Pheasant Branch flux target at Highway 12 because it had the longest and most complete data set of the calibration targets.