Hydrologic Assessment of New Hanover County, NC Aquifers Completed

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Summary:

Four utilities supply the majority of drinking water to New Hanover County residents: The Cape Fear Public Utilities Authority (CFPUA) and the towns of Carolina Beach, Kure Beach, and Wrightsville Beach. The principal sources of freshwater supply in the County are withdrawals of surface water from the Cape Fear River and groundwater from the underlying Peedee and Castle Hayne aquifers.

Increasing challenges for fresh water availability and quality include:

• Industrial, mining, irrigation, and aquaculture withdrawals increasingly compete with public-supply utilities for freshwater resources;
• Shallow wells installed in areas adjacent to tidal creeks, prior to well construction regulations implemented in 1995, have the potential to enhance saltwater intrusion;
• The proposed deepening of the shipping channel in Wilmington Harbor has the potential to alter nearby groundwater flow in the surficial, Castle Hayne, and Peedee aquifers, lower aquifer water levels, and potentially facilitate a direct pathway for salt-water intrusion into fresh groundwater supplies if dredging exposes the top of the Castle Hayne aquifer;
• Production supply wells at Carolina Beach and Wrightsville Beach are experiencing an increase in chloride concentrations due to salt-water intrusion; and
• Seasonal and decadal groundwater level changes resulting from groundwater withdrawals as well as the potential for salt water intrusion from deeper, more brackish aquifers due to upconing, or lateral encroachment from the Atlantic Ocean.

Future growth in New Hanover County will require supplemental groundwater supplies. The CFPUA is interested in expanding its groundwater supply in northern part of the County and would like a better understanding of the hydraulic properties of the Castle Hayne and Peedee aquifers.

• The barrier island towns of Carolina Beach, Kure Beach, and Wrightsville Beach rely entirely on groundwater from these aquifers and have noticed a recent rise in chloride concentrations within the supply wells.
• Increased groundwater withdrawals from these aquifers in close proximity to the Atlantic Ocean enhance the potential for movement of saltwater into the aquifers.
• The Castle Hayne aquifer is unconfined in parts of the county and has large dissolution voids from its carbonate composition, making it susceptible to contamination from human activities that occur near the land surface.

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Current information on factors that influence the quantity and quality of groundwater within the surficial, Castle Hayne, and Peedee aquifers in this area is needed.

Approach:

The hydrologic evaluation will be conducted in three phases:

• Phase 1. COMPLETED - Published as Open-File Report 2011-1205
  • Document well pumping test results from northern New Hanover County. Compile existing geologic reports, maps, subsurface data, well drillers logs, and hydrogeologic information.
• Phase 2. COMPLETED - Published as Scientific Investigations Report 2014-5169
  • Compile and interpret groundwater availability and quality information and historical data for the surficial, Castle Hayne, and Peedee aquifers in New Hanover County.
  • Task A

    • Compile historic information on the hydrogeologic setting, groundwater flow system, and quality of groundwater in the surficial, Castle Hayne, and Peedee aquifers throughout New Hanover County.

      Historic information on the hydrogeologic setting, groundwater-flow system, and quality of groundwater throughout the entire County (including the barrier islands) for the surficial, Castle Hayne, and Peedee aquifers will be compiled. Existing geologic reports and maps, drilling and geophysical logs, water-quality analyses, pumping tests, and water-use data will be assembled to update the regional and county scale frameworks.

  • Task B

    • Expand and update the hydrogeologic framework and water-quality of the surficial, Castle Hayne and Peedee aquifers in all of New Hanover County and integrate with the Brunswick County hydrogeologic framework.

      

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      The hydrogeologic framework previously developed for the northern part of New Hanover County will be modified to incorporate data for the entire County including the barrier islands. The expanded framework will be based on an estimated 100 stratigraphic wells within New Hanover County, including previously published information, and will be integrated across the Cape Fear River with the Brunswick County hydrogeologic framework. (Water-Resources Investigations Report 03-4051)

      Available aquifer test data will be interpreted to fill in areas with little or no data.

  • Task C

    • Describe the occurrence and movement of groundwater within and interaction between the surficial, Castle Hayne and Peedee aquifers.

      Two synoptic surveys of groundwater and surface water levels will be conducted in the fall of 2012 and spring of 2013 to create potentiometric surface and water-level maps of the surficial, Castle Hayne, and Peedee aquifers in the New Hanover County area. Water levels will be measured in at least 75 wells, but as many wells as possible will be measured, especially ones with historic water-level measurements. Gradients determined from measured water levels and estimates of hydraulic conductivity will be used to make estimates of flow within and between the aquifers.

      

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  • Task D

    • Quantify chloride levels in the surficial, Castle Hayne and Peedee aquifers throughout New Hanover County to assess the location of the freshwater-saltwater interface.

      A synoptic survey of water-quality conditions in the surficial, Castle Hayne, and Peedee aquifers will be conducted in the fall of 2012 to assess the current location of the freshwater-saltwater interface in the New Hanover County area. Considerable effort will be made to sample wells located near the freshwater/ saltwater transition zone to obtain the best data coverage. Samples will be analyzed for common field parameters, water isotopes, and major ions, including chloride, bromide, iron, sulfate, total dissolved solids, specific conductance, pH, temperature, and dissolved oxygen.

      Additional electromagnetic-induction (EM) borehole logging will be conducted at 5 to 20 wells, depending on how many suitable wells are identified. These logs will provide insight into the horizontal extent of the freshwater-saltwater interface.

• Phase 3. Develop groundwater-flow and saltwater-intrusion models for the surficial, Castle Hayne, Peedee, and Black Creek aquifers in New Hanover County