COMSOL Modeling of Groundwater Flow and Contaminant Transport in Two-Dimensional Geometries With Heterogeneities

The Delmarva Peninsula is located on the East Coast of the United States, between the Chesapeake Bay and Atlantic Ocean. Industrial farming in the Delmarva Peninsula leads to levels of nutrients, in particular nitrogen, which grossly exceed natural levels. Excess nitrogen reaches the freshwater streams of the peninsula, which then flows to the Chesapeake Bay. The presence of extreme levels of nitrogen greatly impairs the health of the bay, 48% of a streams nitrogen load has been discharged from groundwater. The surficial aquifer geometry in this area is marked by significant geological structures: a general sloping confining layer, angled toward the ocean with unconfined surface strata of sand and clay. The aquifer contains groundwater that flows to the streams of the peninsula. The clay strata are sloping banks three-to-four meters thick, through which groundwater flows much more slowly than the sand strata. We use the software package COMSOL Multiphysics 4.1 to quantify how water residence times change due to heterogeneities within two-dimensional cross-sections by creating a model representative of the region. We find that variation in clay strata affects flow paths. A phreatic divide is the point on the surface that delineates the output river for recharge. The presence of clay banks within the aquifer shifts the location of the phreatic divide. Furthermore, the hydraulic pressure head increases with the presence of clay strata shortening the length of time water and nutrients spend within the aquifer.