Project Overview

Full Title
Development and Application of a Groundwater Flow and Salt-Transport Model of the Onslow County Region in North Carolina

Location
 In and around Onslow County, NC

Partner Agencies
Partner Agencies

Project Chief
Jason M. Fine
jmfine@usgs.gov

Period of Project
2007 - present

Team Members
Kristen B. Mcswain

Science Topic
Groundwater resources

Onslow County Region Groundwater Flow Model

Project Summary

Background

For the past three decades, groundwater levels in the central Coastal Plain of North Carolina have declined as much as 200 feet (ft) from overpumping and depletion of storage in the Black Creek and Upper Cape Fear aquifers of Cretaceous age. Because of these declines and the resulting increased potential for lateral saltwater migration along the coast and upward leakage of brackish water from deeper aquifers, the North Carolina Division of Water Resources instituted the Central Coastal Plain Capacity Use Area (CCPCUA) rules for 15 counties, effective August 1, 2002. Under the CCPCUA rules, groundwater withdrawal of more than 100,000 gallons per day from any of the Cretaceous aquifer systems is subject to a water-use reduction of as much as 75 percent over a 16-year period. Groundwater users in Onslow County that withdraw from the Cretaceous aquifers are subject to the CCPCUA rules.

The effects of the increased development of the Castle Hayne aquifer and the decreased use of the Black Creek aquifer are unknown. In 2007, the U.S. Geological Survey, in cooperation with the City of Jacksonville, Onslow Water and Sewer Authority, and the Marine Corps Base Camp Lejeune, began an investigation to delineate and describe the groundwater flow system in the Onslow County area. A groundwater flow model will be created for the aquifers which underlie the Onslow County Region.

Objectives

The objective of the proposed study is to develop and apply a groundwater flow and salt-transport model that can be used to analyze the effects of various scenarios that will enable water-resource managers to effectively manage groundwater withdrawals, ensure that the groundwater resource is sustainable, and saltwater intrusion is minimized. The variable-density groundwater flow model will be developed for the surficial, Castle Hayne, Beaufort, and Cretaceous aquifer systems in Onslow County.

Approach

The project will be completed in 3 phases:

• Phase 1: Assemble pertinent hydrogeologic data.
• Phase 2: MODFLOW Model Development and Calibration.
• Phase 3: Simulate Management Scenarios, Develop Dual Density Model, and publish model documentation and findings.

Progress

The project is currently in Phase 2. Hydrogeologic data have been collected and assembled to create the flow model of the Onslow County region. The model is currently being calibrated and will be completed by October 1, 2009, when phase 3 will begin. During Phases 1 and 2 , two groundwater-level data-collection efforts were completed. Approximately 350 water levels were collected in May 2008 and February 2009 to map the poteniometric surfaces of the Castle Hayne, Peedee, and Black Creek aquifers. These maps will be published in a future map report. Water-quality samples were collected in June 2009 to supply data to the dual density model and help locate the saltwater interface. These data were published in the 2009 annual data report.