MODELING GROUNDWATER AND SURFACE WATER DISCHARGE FROM THE GREAT DISMAL SWAMP TO MID-ATLANTIC ESTUARIES
A detailed 3-D groundwater and surface-water model is being constructed to improve understanding of swamp hydrology, calculate current nutrient and carbon budgets associated with the hydrologic system, and predict future budgets under various ditch management scenarios. Results from the modeling study will be combined with results from ongoing water-chemistry studies to estimate export rates of nutrients, carbon, and other contaminants from the swamp to receiving estuaries. The flow simulation model is based on MODFLOW-NWT, using the SWR package to simulate ditches and inundated surfaces. Ditches generally act as groundwater discharge sinks while roads generally act as barriers to both surface water and groundwater flow. Complicating the modeling effort, ditches and roads run closely parallel to each other, causing steep hydraulic gradients within an otherwise very flat hydraulic surface. A 2-D cross-sectional model was first constructed to determine key model features and calibrate hydraulic parameters within an area of dense observations. Specified heads representing ditches at each end of the cross-sectional model were changed to simulate management actions to raise water levels in ditches. The cross-sectional model results show that small changes to ditch water levels can cause large changes to the length and direction of groundwater flow paths, which will likely affect geochemistry of water discharging from the swamp.