Abstract
Chesterfield County is located in the northeastern part of South Carolina along the southern border of
North Carolina and is primarily underlain by unconsolidated sediments of Late Cretaceous age and younger
of the Atlantic Coastal Plain. Approximately 20 percent of Chesterfield County is in the Piedmont
Physiographic Province, and this area of the county is not included in this study. These Atlantic
Coastal Plain sediments compose two productive aquifers: the Crouch Branch
aquifer that is present at land surface across most of the county and the deeper, semi-confined
McQueen Branch aquifer. Most of the potable water supplied to residents of Chesterfield County
is produced from the Crouch Branch and McQueen Branch aquifers by a well field located near McBee,
South Carolina, in the southwestern part of the county. Overall, groundwater availability is good
to very good in most of Chesterfield County, especially the area around and to the south of McBee,
South Carolina. The eastern part of Chesterfield County does not have as abundant groundwater
resources but resources are generally adequate for domestic purposes.
The primary purpose of this study was to determine groundwater-flow rates, flow directions, and
changes in water budgets over time for the Crouch Branch and McQueen Branch aquifers in the Chesterfield
County area. This goal was accomplished by using the U.S. Geological Survey finite-difference MODFLOW
groundwater-flow code to construct and calibrate a groundwater-flow model of the Atlantic Coastal Plain of
Chesterfield County. The model was created with a uniform grid size of 300 by 300 feet to facilitate a
more accurate simulation of groundwater-surface-water interactions. The model consists of 617 rows from north
to south extending about 35 miles and 884 columns from west to east extending about 50 miles, yielding
a total area of about 1,750 square miles. However, the active part of the modeled area, or the part
where groundwater flow is simulated, totaled about 1,117 square miles.
Major types of data used as input to the model included groundwater levels, groundwater-use data, and
hydrostratigraphic data, along with estimates and measurements of stream base flows made specifically for
this study. The groundwater-flow model was calibrated to groundwater-level and stream base-flow conditions
from 1900 to 2012 using 39 stress periods. The model was calibrated with an automated parameter-estimation
approach using the computer program PEST, and the model used regularized inversion and pilot points. The
groundwater-flow model was calibrated using field data that included groundwater levels that had been
collected between 1940 and 2012 from 239 wells and base-flow measurements from 44 locations distributed
within the study area. To better understand recharge and inter-aquifer interactions, seven wells were
equipped with continuous groundwater-level recording equipment during the course of the study, between
2008 and 2012. These water levels were included in the model calibration process. The observed groundwater
levels were compared to the simulated ones, and acceptable calibration fits were achieved. Root mean square
error for the simulated groundwater levels compared to all observed groundwater levels was 9.3 feet for the
Crouch Branch aquifer and 8.6 feet for the McQueen Branch aquifer.
The calibrated groundwater-flow model was then used to calculate groundwater budgets for the entire
study area and for two sub-areas. The sub-areas are the Alligator Rural Water and Sewer Company well
field near McBee, South Carolina, and the Carolina Sandhills National Wildlife Refuge acquisition boundary
area. For the overall model area, recharge rates vary from 56 to 1,679 million gallons per day (Mgal/d)
with a mean of 737 Mgal/d over the simulation period (1900–2012). The simulated water budget for the
streams and rivers varies from 653 to 1,127 Mgal/d with a mean of 944 Mgal/d. The simulated “storage-in
term” ranges from 0 to 565 Mgal/d with a mean of 276 Mgal/d. The simulated “storage-out term” has a range
of 0 to 552 Mgal/d with a mean of 77 Mgal/d. Groundwater budgets for the McBee, South Carolina, area and
the Carolina Sandhills National Wildlife Refuge acquisition area had similar results.
An analysis of the effects of past and current groundwater withdrawals on base flows in the McBee area
indicated a negligible effect of pumping from the Alligator Rural Water and Sewer well field on local
stream base flows. Simulate base flows for 2012 for selected streams in and around the McBee area were
similar with and without simulated groundwater withdrawals from the well field. Removing all pumping
from the model for the entire simulation period (1900–2012) produces a negligible difference in
increased base flow for the selected streams. The 2012 flow for Lower Alligator Creek was 5.04 Mgal/d
with the wells pumping and 5.08 Mgal/d without the wells pumping; this represents the largest
difference in simulated flows for the six streams.
U.S. Department of the Interior |
U.S. Geological Survey