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Sea-level Rise Hazards and Decision Support

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Coastal Groundwater Systems

Changes in climate and sea level will drive changes to the coastal groundwater system that will impact both human populations and coastal ecosystems.  Increases in sea-level will raise the fresh water table in many coastal regions (figure GW1).  Impacts to humans may include an increase in the potential for basement or septic system failure.   Sea-level rise can also contaminate groundwater supplies due to landward and upward movement of sea-water in coastal aquifers.  The intrusion of saltwater into groundwater systems will also impact coastal ecosystems such as marshes by changing the elevation of the freshwater-saltwater interface.

A major concern for water managers is the potential adverse effect of sea-level rise on the depth to the freshwater-saltwater interface near public groundwater supply wells. Pumping from public-supply wells in coastal aquifers underlain by saltwater can lower the water table with respect to sea level, decreasing the depth to the freshwater-saltwater interface beneath the pumping well.  This increases the potential for saltwater intrusion, and potentially limits the amount of potable water available from the well.

Understanding how sea-level rise may impact the groundwater hydrology in shallow, unconfined coastal aquifers such as those occurring on barrier islands is important when assessing potential impacts on the sustainability of coastal habitats like salt marshes.

This project is currently focused  on developing a calibrated three-dimensional ground-water flow model capable of simulating both the fresh and saltwater flow systems on Assateague Island and collecting the hydrogeologic data necessary to calibrate  and evaluate the model (figure GW2).  These data will be used to conduct a sensitivity/uncertainty analysis for different climate change and sea-level rise scenarios.  These scenarios will be combined with the model uncertainty (estimated using the field calibration data set) to estimate probabilities of ecosystem health relevant to Piping Plovers and other assets located within the boundaries of our study (e.g., other habitat and park infrastructure elements).  Results from the hydrogeologic analyses will be integrated with related predictions of island erosion, overwash and inundation and marsh resilience developed by other parts of the overall project using a Bayesian Network.  The network will be used to evaluate information to inform the decision making process.  Decision makers should therefore be able to focus their limited resources where they are most likely to maximize effectiveness. The quality of the forecasts should improve as fundamental knowledge of coastal systems is incorporated into these assessments.

Diagram showing sea level rise effects
Figure GW1.  A rise in sea-level will affect ground-water flow in coastal aquifers (1). An increase in the elevation of the water table (dashed–blue line) may result in basement flooding and compromise septic systems (2).  A rise in sea level may also result in an upward and landward shift in the position of the freshwater-saltwater interface (3).  Where streams are present, an increase in the water-table elevation also may increase ground-water discharge to streams and result in local changes in the underlying freshwater-saltwater interface (4).
photo of a USGS Drill rig
Figure GW2.  Subsurface characterization of hydrogeologic conditions requires information on the elevation of the water table, the depth to boundary between freshwater and saltwater, and the hydraulic properties of the sediments that comprise the aquifer system.  The USGS drill rig shown here was used to install monitoring wells across Assateague Island to monitor changes in water levels and the position of the freshwater/saltwater interface in response to changes in sea level over time. (John Masterson, USGS).
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