Assessment of the Groundwater Flows at Juniper Bay and their Impact on the Surrounding Area

The North Carolina Department of Transportation purchased a 750-acre, roughly elliptical tract of agricultural land, known as Juniper Bay (a Carolina Bay), to convert to wetlands as part of their wetlands mitigation program. Preliminary water balance work suggested that there are significant flows of groundwater entering and leaving the tract. This study was initiated to examine the subsurface potentials and determine the degree to which a ditch around the perimeter of the tract controls the lateral fluxes of groundwater in the surficial aquifer. Five nests of piezometers were installed along each of four 492-ft (150-m) transects crossing the perimeter ditch at approximately the major and minor axes of the tract, which correspond to the suspected maxima of influx and efflux. Deep soil cores (up to 42 ft) were collected along the transects to guide placement of piezometers for monitoring hydraulic heads. Water levels in the piezometers were recorded at 15-minute intervals. Meteorological data were collected with a weather station near the middle of the bay. Models were developed for the four transects using Visual MODFLOW. The models were calibrated with observed hydraulic heads. The maximum absolute error in the calibration process was 1.6 ft (0.5 m). The modeling results suggested that the ditch drained water from the surficial system from both sides. In the deeper sand layers, there was an indication of groundwater flowing into the bay at the Northwest (NW) and Northeast (NE) transects. Modeling of the Southwest (SW) transect indicated outflows. The Southeast (SE) transect showed water draining into the ditch from both sides. The models were extended to 2600 ft (800 m) inside the bay to simulate conditions after the interior ditch system was blocked. Simulation results showed groundwater inflows at the NW, NE, and SE transects, and groundwater outflow at the SW transect. The lateral influence of the perimeter ditch had a maximum of approximately of 330 ft (100 m), observed at the SW transect, and a minimum of 100 ft (30 m), observed at the SE transect. The extent of influence of the perimeter ditch also depend on the weather conditions, showing more influence in summer months than in winter months. Influence of the perimeter ditch was entirely in the upper sands at the NE and SE transects, but some influence was seen in the middle sand layers at the NW and SW transects. Groundwater flow estimates from the transect models were extrapolated over the entire perimeter of Juniper Bay to obtain an estimate of net groundwater inflow. Net groundwater inflow was approximately 0.41 ft (125 mm) for the period of 1 January 2004 to 30 June 2004. To develop recommendations for maintenance and operation of the perimeter ditch, the models were run for various scenarios focused on water levels in the perimeter ditch. Control levels were imposed on the ditch and options were investigated. A water level of 117.8 ft (35.9 m) MSL was identified as a critical point of control of the perimeter ditch. Controlling the water level in the perimeter ditch at that level would, according to the model, minimize offsite impacts and result in maximum wetland area.

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  • Corporate Authors:

    North Carolina State University, Raleigh

    Biological and Agricultural Engineering Department
    Box 7637, 909 Capability Drive, Suite 3200
    Raleigh, NC  United States  27695

    North Carolina Department of Transportation

    P.O. Box 25201, 1 South Wilmington Street
    Raleigh, NC  United States  27611

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Authors:
    • Huffman, R L
    • Vepraskas, M J
    • Pati, S
  • Publication Date: 2007-2


  • English

Media Info

  • Media Type: Web
  • Edition: Final Report
  • Features: Figures; Maps; Photos; References; Tables;
  • Pagination: 87p

Subject/Index Terms

Filing Info

  • Accession Number: 01099529
  • Record Type: Publication
  • Report/Paper Numbers: FHWA/NC/2006-29, Research Project 2002-19
  • Created Date: May 5 2008 2:32PM