Cross-Sectional Models of Groundwater Flow: Review and Correction for Transverse Flow

Cross-sectional (2D) groundwater models are commonly applied to simulate complex processes that are challenging to capture using the coarse grids of 3D regional-scale models. 2D models are often extracted from 3D models for this purpose. However, translating groundwater properties from 3D to 2D models so that regional flow patterns are preserved poses several challenges. A methodology is presented here to maximize agreement between the heads of 2D and 3D groundwater models, considering MODFLOW models with rectilinear grids. This includes careful averaging of hydraulic properties and stresses from the 3D model to create commensurate properties and stresses in cross section. The approach was evaluated by examining the statistical match of transient heads within 10 cross sections extracted from a 3D model of the Limestone Coast (Australia). Concordance between 2D and 3D models was generally poor but was improved by incorporating lateral flow as inflows/outflows in 2D models. Lateral flows required inputs from the 3D model, which limits the application of 2D models as independent predictive tools. Pumping in the 3D model was redistributed to neighboring cells to reduce errors in the 2D model that arise from the limited capability to simulate pumping effects. Although pumping redistribution led to minimal improvement for the case study model, simpler modeling scenarios with more intense, localized pumping showed substantially better head matches between 2D and 3D models when pumping redistribution was applied. The methodology for creating cross-sectional models offered in this article provides relatively simple steps for creating 2D models that are consistent with 3D parent models, although further work is needed to develop a methodology for 2D models that are oblique to 3D model grids.