Re-Purposing Groundwater Flow Models For Age Assessments: Important Characteristics

Groundwater flow model construction is often time-consuming and costly, with development ideally focused on a specific purpose, such as quantifying well capture from water bodies or providing flow fields for simulating advective transport. As environmental challenges evolve, the incentive to re-purpose existing groundwater flow models may increase. However, few studies have evaluated which characteristics of groundwater flow models deserve greatest consideration when re-purposing models for groundwater age and advective transport simulations. In this paper, we compare simulated age metrics produced by three MODFLOW-MODPATH models of the same area but with differing levels of complexity (layering and heterogeneity). Comparisons are made at three watershed scales (HUC 8 to HUC 12). Groundwater age metrics, specifically the young fraction and median age of the young and old fractions, are used for evaluation because they relate to intrinsic susceptibility of aquifers and are simpler to interpret than full age distributions used for advective transport. Results indicate that: (1) the young fraction is less sensitive to model layering than the median age of young and old fractions, suggesting that simple models may suffice for basic intrinsic susceptibility assessments; (2) water table mounding and associated discharge into partially penetrating boundaries, such as head-water streams, is important for simulating both the young fraction and the median age of the young fraction; and (3) the influence of partially penetrating head-water streams is maintained regardless of the porosity distribution. Results of this work should aid modelers with evaluating the appropriateness of re-purposing existing groundwater flow models for age simulations.