THE EFFECTS OF SEA LEVEL RISE ON SALTWATER INTRUSION AND SOLUTE TRANSPORT IN GROUNDWATER FLOW SYSTEMS OF THE COASTAL MID-ATLANTIC: RESULTS OF A FRAMEWORK NUMERICAL MODELING STUDY

A number of coastal cities located at the mouth of the Chesapeake Bay, such as Norfolk and Virginia Beach, are currently experiencing the effects of sea level rise (SLR) and are projected to continue to do so in years to come (e.g., Ezer and Corlett, 2012). Many factors affect relative SLR in the Mid-Atlantic coastal region, including eustatic sea level changes, postglacial rebound, groundwater withdrawal, and stratigraphic subsidence. The composite effect of these factors result in SLR rates of approximately 3.4 mm/yr - 5.8 mm/yr (e.g., Boon et al., 2010), placing the Hampton Roads area on a list of the cities most affected by rising coastal waters. The focus of this study is how submarine groundwater discharge (SGD) changes in response to SLR. SGD, or groundwater flowing from the seabed, is important to quantify in hydrologic and geochemical budgets because it serves as a significant pathway for contaminants and nutrients. This investigation considers three different settings often found in coastal regions, which are modeled after the relatively high-relief southern Eastern Shore, low-relief Elizabeth River, and wave-dominated Ocean View beach. Each of these three settings is discretized into a two-dimensional model domain as a shore-perpendicular transect approximately 5 km in the horizontal direction and 10 m - 100 m in vertical extent. SGD is calculated using SUTRA, a USGS hybrid finite-element and finite-difference numerical model that solves the coupled equations for flow and solute transport assuming that density varies with space throughout the model. For each of the three locations, a series of models was run with sea level boundary conditions reflecting different SLR scenarios extending to the year 2100. Results simulated the locations of the freshwater/saltwater interface at various future times. The effects of topographic slope and heterogeneous, anisotropic hydrogeologic structure were quantified. This forward modeling approach provides a general framework to better understand how SLR scenarios could impact a range of idealized, present-day groundwater flow systems.