Topographic and Statistically Based Model of Water Table Position, Groundwater Storage, and Transit Times

The distribution of water transit times serves as a fundamental control over catchment hydrology, solute transport, and biogeochemistry. It integrates the many hydrologic processes and flow pathways that influence solute transport. This is particularly important for studying the movement of soluble pollutants such as nitrate and phosphate. Due to the long-tailed transit time distributions associated with groundwater, historical contaminants can be slowly transported to stream decades after they were introduced. However, traditional watershed-scale groundwater modeling approaches to estimate water tables and transit times require extensive user expertise and time-consuming model development and training. Here we introduce a simple GIS-driven model for estimating water table elevations and transit times across catchments. This model uses Dupuit's equation for one-dimensional horizontal flow in a phreatic aquifer between two parallel rivers to calculate water table elevations along each GIS-determined drainage pathway. The resulting set of water table profiles is then smoothed statistically. Our analysis focuses on three adjacent groundwater-driven streams, located in low-relief and permeable headwater catchments of unconsolidated sands and clays draining the Savannah River Site in the Upper Atlantic Coastal Plain in South Carolina, USA. Following a clearcut timber harvest and fertilization of young plantations at the sites, a pulse of nitrate was recorded at the top of the surficial aquifer, but this nitrate has not appeared in riparian groundwater or streamwater several years after reaching the groundwater, leading to questions regarding groundwater ages and transit times across the site. We used this water table model to estimate groundwater transit times across headwater catchments and validated these estimates with long-term groundwater records. This catchment-wide estimate of transit times will give insight into the lingering questions regarding nitrate transport from plantations and allow further research into watershed storage dynamics during times of intermittent streamflow.