Groundwater Modeling

To evaluate groundwater flow patterns within the Garber-Wellington Aquifer, a simplified two-dimensional model was created using Excel. The model is based on the conceptual understanding presented by Mashburn et al. (2019), which characterizes the aquifer as a hydraulically heterogeneous system composed of interbedded sandstones, siltstones, and mudstones. These lithologic variations result in a wide range of hydraulic conductivities, which influence both the direction and rate of groundwater movement.

The aquifer was modeled as confined, consistent with conditions reported in the deeper, saturated portions of the system. A 15 × 15 cell grid was used to represent the aquifer domain, with constant-head boundaries set to 30.5 meters (100 feet) on the west side and 27.4 meters (90 feet) on the east. This configuration establishes a horizontal hydraulic gradient, approximating regional flow directions observed in the field (Mashburn et al., 2019; OWRB, 2019). Interior head values were solved using a steady-state finite-difference method, in which each cell is updated based on the weighted average of neighboring heads and the hydraulic conductivity of the flow paths.

To simulate aquifer heterogeneity, the model included zones with high conductivity (2.16 m/day) representing sandstone units and low conductivity (0.091 m/day) corresponding to mudstone or shale layers. These values were converted from aquifer test results reported in feet/day (Mashburn et al., 2019). The distribution of conductivity values produced clear distortions in the head contours, with flow lines bending around lower-permeability regions and moving more directly through high-permeability pathways.

To build on this initial simulation, a second model was developed to explore how groundwater flow responds to stress conditions, specifically distributed recharge and a centrally located pumping well. These additions reflect common hydrologic stresses on the Garber-Wellington system and allow for examination of the interactions between natural inputs, anthropogenic withdrawals, and aquifer structure.

Recharge was applied across the upper portion of the model domain using a representative average value of 0.000125 m/day, based on long-term infiltration estimates for the region (Mashburn et al., 2019). A single pumping well was introduced near the center of the domain, extracting water at a rate of Q = –20 m³/day. This value represents a localized withdrawal roughly consistent with small-scale municipal or agricultural use.

The resulting head distribution displays a cone of depression centered on the pumping cell, along with a subtle mounding effect in the recharge zone. Flow lines converge sharply toward the well and are deflected by surrounding low-conductivity zones, highlighting the continued influence of aquifer heterogeneity on flow response. This simulation demonstrates how hydraulic stresses modify flow direction and gradient magnitude, and it underscores the value of simplified modeling in assessing groundwater system behavior.