MODELING GROUNDWATER AND SURFACE WATER DISCHARGE FROM THE GREAT DISMAL SWAMP TO MID-ATLANTIC ESTUARIES

The Great Dismal Swamp, in Virginia and N. Carolina, USA, is a 45,325 hectare forested wetland discharging to ditches which then drain through adjacent streams to estuaries of the Chesapeake Bay and Albemarle Sound. More than 90% of discharge from the swamp is derived from precipitation falling directly on the swamp. Groundwater discharge, as determined by baseflow separation, is about 70% percent of all discharge from the swamp, excluding evapotranspiration. Therefore, a detailed picture of groundwater flow paths and surface and shallow subsurface chemistry is needed to understand the geochemical budget and contributions to receiving estuaries. Because ditch management in the swamp is changing to restore high water levels and reduce historical impacts of ditching, rates of nutrient and contaminant delivery from the swamp to receiving estuaries will likely also change.

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.