USGS North Carolina Water Science Center
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Coastal Plain Ground-Water Recharge![]()
Background In 1992, a project was initiated by the North Carolina Department of Environment and Natural Resources (DENR) to characterize ground-water recharge rates in the surficial aquifers of North Carolina. A 1:500,000-scale map was produced to delineate discharge areas along major stream networks in the State (Heath, 1993). Recharge maps (1:24,000-scale) for the Neuse and Tar-Pamlico River basins were compiled by characterizing drainage-basin landforms as upland flats, valley slopes, or riverine bottomland and with further refinements from county soils maps. Recharge rates for the full mapped area were estimated with Monte Carlo model simulations using calculated recharge for 11 U.S. Geological Survey (USGS) gaged drainage basins in the North Carolina Coastal Plain. The Rorabaugh-Daniel hydrograph separation model (Rutledge, 1993) was used to estimate ground-water recharge in each gaged basin (Mew and others, 2002). Recharge maps were developed from most of the state by weighting initial recharge estimates by mean rainfall rates (Mew and Spruill, 2000). Although the DENR method provides a useful framework for estimating ground-water recharge rates for different geomorphic settings over large areas of the North Atlantic Coastal Plain, the estimated recharge rates have not been verified with direct field measurements. Therefore, in 2003, the USGS began a project to evaluate recharge estimates for different geomorphic settings in the North Carolina Coastal Plain. Recharge estimates derived from the DENR method will be compared with field measurements of recharge for each geomorphic setting. The DENR calculated recharge estimates are based on long-term averages for the period of record at each USGS gaging station and represent average annual recharge. To better define seasonal variability, seasonal recharge rates will be estimated for different geomorphic settings by using direct field measurements and hydrograph separation techniques. Seasonal recharge estimates are needed to more accurately evaluate seasonal ground-water availability and the potential for contaminant infiltration, transport, and movement. Objectives (1) Compare the DENR recharge estimates for different geomorphic settings with direct field measurements. (2) If a relation is found between the direct field measurements and the DENR estimates for each geomorphic setting, develop recharge estimates and seasonal recharge estimates for specific geomorphic settings in the North Atlantic Coastal Plain. (3) Determine if reasonable seasonal estimates of recharge can be made by using hydrograph separation techniques and recharge allocation methods, and develop predictive equations for estimating seasonal recharge for each geomorphic setting. Approach Objective 1 will be accomplished by measuring recharge in each of the three major geomorphic settings and comparing the measured recharge rates to the estimated recharge rates. A minimum of two well sites will be chosen in each setting. Water-level recorders and satellite equipment will be installed and operated by the USGS at each well. The approximate age of the water at the top of the water table at each site will be determined by sampling for chlorofluorocarbons, sulfur hexafluoride, and dissolved gasses. Unsaturated zone soil moisture and temperature will be monitored at each site with tensiometers and thermocouples, respectively. Soil cores will be collected at each site and analyzed for saturated hydraulic conductivity, bulk density, thermal properties, and particle-size analysis. Vertical fluxes to the water table will be determined by modeling the measured pressure and temperature data with VS2DH (Hsieh and others, 2000). A precipitation gage also will be installed at each site. Monitoring at each site will take place over 1 year. Field measurements of recharge will be correlated to the annual recharge potential defined by DENR. Objective 2 will focus on deriving equations to describe recharge and seasonal recharge estimates for each geomorphic setting. The equations will describe recharge as a percentage of seasonal precipitation and as a function of variables or indices that can be used to predict seasonal recharge. Objective 3 will be accomplished by selecting USGS gaging stations in the Chowan River and Roanoke Sound drainage areas (Pasquotank Drainage Basin) in the North Atlantic Coastal Plain for analysis. Only stations that have adequate streamflow record to perform seasonal hydrograph separation using techniques presented in Rutledge (1993) will be used. Predictive equations for estimating seasonal recharge for each geomorphic setting will be developed. Progress October 2003 June 2003 |
Real-time Ground-Water Data (NWIS Web)
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Recharge study site name | Site ID | NCDENR landscape unit |
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Rose Hill | 345051078012109 | Upland flat, wet, mineral |
Roxobel | 361420077111407 | Upland flat, dry, mineral |
Great Sandy Run | 343442077292301 | Upland flat, wet, organic |
Marston | 345812079313401 | Valley slope, gentle |
Comfort | 345809077301408 | Valley slope, gentle |
Hadnot Point | 344139077211207 | Valley bottom, stream terrace |
Ballard's Crossroads | 353143077303501 | Valley bottom, stream terrace |
References cited