Magnitude and extent of arsenic and thallium concentrations in ground water and sediments at the Charleston Naval Complex, North Charleston, South Carolina, 1994-99

Water-quality samples were collected quarterly
during 1994-99 from 604 wells screened in the
surficial aquifer system beneath the Charleston Naval
Complex, North Charleston, South Carolina. Arsenic
and thallium were selected for analysis because
concentrations of these metals in some wells
consistently exceeded the established (2001) drinking water
maximum contaminant levels of 10 and
2 micrograms per liter, respectively. The analysis was
conducted to determine the magnitude and spatial
distribution of arsenic and thallium in ground water at
the Charleston Naval Complex and to quantify arsenic
and thallium concentrations in a dated sediment core
from Shipyard Creek marsh near the southern
boundary of the Naval Complex.
The surficial aquifer system beneath the
Charleston Naval Complex consists of an unconfined
upper surficial aquifer and a confined lower surficial
aquifer. Hydraulic connection between the two aquifers
is limited or nonexistent throughout the system at the
Naval Complex. The Charleston Naval Complex is
divided into nine operational units designated as zones
A through I. Arsenic and thallium concentration data
were compiled and interpreted for the two surficial
aquifers within each zone.
Mean arsenic (n=603) and thallium (n=604)
concentrations were calculated for water samples from
each well screened in the upper and lower surficial
aquifers. In the upper surficial aquifer, mean arsenic
concentrations ranged from 0.9 to 339 micrograms
per liter and exceeded 10 micrograms per liter in
29 percent of the wells. In the lower surficial aquifer,
mean arsenic concentrations ranged from 1.0 to
97.4 micrograms per liter and exceeded 10 micrograms
per liter in 23 percent of the wells. The greatest number
of water samples with mean arsenic concentrations
exceeding 10 micrograms per liter were collected from
wells in the upper surficial aquifer at zone E in the
northwestern part of the study area.
Well clusters, defined as three or more wells in
a solid-waste management unit or area of concern,
where the mean arsenic concentration exceeded
10 micrograms per liter, were identified in association
with 12 sites in the upper surficial aquifer-solid-waste
management unit 039 (a drum-storage area) in zone A;
solid-waste management units 044 (coal-storage area)
and 047 (burning dump) in zone C; solid-waste
management unit 065 (lead-storage area) and area of
concern 556 (dry docks 3 and 4) in zone E; areas of
concern 609 (building 1346 gas station) and 613
(locomotive shop) in zone F; solid-waste management
units 006 (public works storage yard) and 008 (oil
sludge pit), and area of concern 709 (fuel-delivery
system wells 12, 13, and 14) in zone G; and solid-waste
management units 009 (closed landfill) and 196 (south
landfill) in zone H. One well cluster was identified in
the lower surficial aquifer in association with solidwaste
management unit 009 (closed landfill) in zone H.
Mean thallium concentrations in water from all
wells ranged from less than 1.6 to 32.6 micrograms per
liter in water samples from the upper surficial aquifer,
and from less than 1.6 to 67.7 micrograms per liter in
water samples from the lower surficial aquifer. Mean
thallium concentrations equal to or greater than
10 micrograms per liter were present in water samples
from 21 of 604 wells (3.5 percent). Of the 21 wells,
14 wells were located at solid-waste management unit
009 (closed landfill) in zone H near Shipyard Creek,
8 wells in the upper aquifer, and 6 wells in the lower aquifer. One well cluster where thallium exceeded
10 micrograms per liter was identified in association
with solid-waste management unit 009 (closed landfill)
in the upper surficial aquifer.
Mean arsenic and thallium concentrations in
water were calculated for all wells screened in one
aquifer and located in a single zone, and are referred to
as zone mean concentration in this report. Zone mean
arsenic concentrations in all nine zones ranged from
3.2 to 18 micrograms per liter in water samples from
the upper surficial aquifer and from 2.7 to 22 micrograms
per liter in water samples from the lower
surficial aquifer. Zone mean thallium concentrations in
all nine zones ranged from 3.2 to 13 micrograms per
liter in water samples from the upper surficial aquifer
and from 3.2 to 14 micrograms per liter in water
samples from the lower surficial aquifer.
Ground-water samples rarely had elevated
(equal to or greater than 10 micrograms per liter)
concentrations of both arsenic and thallium. Water
samples had coincident elevated arsenic and thallium
concentrations in 10 wells in zone H, 1 well in zone A,
and 1 well in zone B.
Sediment quality at Shipyard Creek marsh was
investigated by collecting an 11.8-foot -long sediment
core (SYC-1) adjacent to zone I. The mean arsenic
concentration in sediment samples from SYC-1
(n= 160) was 3.05 milligrams per kilogram plus or
minus 0.92. The mean arsenic concentration and
standard deviation calculated for SYC-1 sediment
samples fall within the standard error for the
background mean arsenic concentration reported for
South Carolina sediments (1.5 milligrams per kilogram
plus or minus 2. 7). All but one sample (core depth
=50 inches) was less than the threshold-effects level of
7.24 milligrams per kilogram. Acid extracts of the
sediment samples were analyzed for thallium
concentration, but none were detected. These data
indicate no obvious change in arsenic or thallium
concentrations with depth in the core.