Project Overview

Full Title
Coastal Plain Ground-Water Recharge

Location
Coastal Plain surficial aquifer

Cooperating Agencies
· USGS Ground Water Resources Program
· NCDENR Ground Water Section

Project Chief
Alissa Coes

Period of Project
2003-2005

Team Members

Tim Spruill

Melinda Chapman

Jeanne Robbins

Mary Giorgino

USGS IN YOUR STATE

USGS Water Science Centers are located in each state.

Coastal Plain Ground-Water Recharge

Project Summary

Ground-water recharge estimates for different geomorphic settings, North Carolina (Mew and Spruill, 2000)
Click on image for larger map

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
A new surficial well was installed at the Rose Hill site. At each of the Comfort, Rose Hill, and Marston well sites, 1-, 3-, and 6-foot tensiometers; 1- and 6-foot thermocouples; and a precipitation gage were installed.

June 2003
Twelve well sites were canvassed to determine suitablitity for instrumentation and to confirm the DENR-defined geomorphic settings.

References cited

Heath, R.C., 1993, Principal ground water discharge areas of North Carolina: North Carolina Department of Environment, Health, and Natural Resources, Division of Environmental Management, Groundwater Section, 52 p.

Hseih, P.A., Wingle, W., and Healy, R.W., 2000, VS2DI-A graphical software package for simulating fluid flow and solute or energy transport in variably saturated porous media, U.S. Geological Survey Water-Resources Investigations Report 9-4139, 16 p.

Mew, T., and Spruill, T., 2000, Determination of aquifer recharge, ground-water flow and basin discharge - Methods and examples [abs.]: Proceedings of the Cross-Discipline Ecosystem Modeling and Analysis Workshop, August 15-17, 2000, Research Triangle Park, North Carolina.

Mew, H.E., Hirth, D.K., Lewis, D.K., Daniels, R.B., and Keyworth, A.J., 2002, Methodology for compiling ground water recharge maps in the Piedmont and Coastal Plain provinces of North Carolina: North Carolina Department of Environment and Natural Resources, Ground Water Bulletin Number 25, 68 p.

Rutledge, A.T., 1993, Computer programs for describing the recession of ground-water discharge and for estimating mean ground-water recharge and discharge from streamflow records - update: U.S. Geological Survey Water-Resources Investigations Report 93-4121, 45 p.