INNOVATIVE WAY TO ESTIMATE GROUNDWATER RESIDENCE TIMES IN THE GLACIATED U.S. USING NEW MODELING METHODS AND WALL-TO-WALL DATA SETS (Invited Presentation)

Glacial aquifers present challenges to regional modeling approaches because they are spatially discontinuous and have highly variable properties. Groundwater residence-time distributions (RTDs) are critical for understanding lag times between recharge at the water table and base flow in streams; however, RTDs cannot be measured directly—they must be inferred from an analysis of data using models.

Numerical models often are underutilized for estimating RTDs because they can be expensive to create. In this study, we developed automated procedures to rapidly construct general models based on regional-scale hydrogeographic digital data. To demonstrate the approach, these procedures were used to create finite-difference groundwater flow models in 30 watersheds across the northeastern glaciated U.S. RTDs were calculated from these models using particle tracking.

The foundation of model construction is publicly available wall-to-wall digital data of the glaciated U.S. These data sets include NHDPlus (land-surface altitude and the location and altitudes of stream reaches); quaternary geologic atlas of the United States (extent and type of glacial deposits); quaternary sediment thickness and bedrock topography (altitude of bedrock); potential groundwater recharge (recharge).

The first step is to create a finite-difference model grid and populate it with the aforementioned geospatial data. Adjustable parameters of the models are K_h of coarse deposits, K_h of fine deposits, K_h of bedrock, and K_h/K_v. Geologic units at the surface are assumed to extend into the subsurface.

Recharge is not an adjustable parameter in the models. Spatially distributed recharge is calculated with a modified Thornthwaite-Mather method (Soil Water Balance; SWB). To achieve consistency and to facilitate automation of general model calibration, a novel calibration strategy is used. The hydraulic parameters are optimized to minimize the number of dry perennial stream reaches and the number of model cells where the water table is above land surface. RTDs calculated with general models exhibit predictable relations to regional recharge, aquifer thickness, and geologic heterogeneity.