Environmental Engineering Reference
In-Depth Information
long-term PRB performance using site-specific parameters such as ground-
water chemistry and hydrologic conditions.
The granular iron PRB installed at the US Coast Support Center located
near Elizabeth City, North Carolina, (USA) is a well-documented full-scale
PRB designed and constructed for removing hexavalent chromium (Cr
VI
)
from groundwater. This chapter provides an update on the contaminant
removal efficiency of this PRB after 14 years of operation.
6.2 Site Background
The US Coast Guard Support Center is located about 100 km south of Norfolk,
Virginia, and 60 km inland from the Outer Banks region of North Carolina.
The base is situated on the southern bank of the Pasquotank River, about
5 km southeast of Elizabeth City, North Carolina. A metal plating shop oper-
ated for more than 30 years in Hangar 79, which is about 60 m south of the
river (Figure 6.1). Following its closure in 1984, soils beneath the shop were
found to contain chromium concentrations up to 14,500 mg/kg.
Subsequent investigations revealed a chromate plume extending from
beneath the shop to the river. At that time, the contaminant plume had high
(>10 mg/L) concentrations of chromate, elevated sulfate (150 mg/L), and
small amounts of volatile chlorinated organic compounds: trichloroethene
(TCE),
cis
-DCE, and vinyl chloride (VC).
The groundwater flow velocity at the site is extremely variable with depth,
with a highly conductive layer at roughly 4.5-6.5 m below ground surface.
This layer coincides with the highest aqueous concentrations of chromate.
The groundwater table ranges from about 1.5 to 2.0 m below ground surface
and the average horizontal hydraulic gradient varies from 0.0011 to 0.0033.
Slug tests conducted on monitoring wells with 1.5 m screened intervals
between 3 and 6 m below ground surface indicate hydraulic conductivity
values between 0.5 and 10 m/day. A multiple borehole tracer test showed
groundwater velocities between about 0.10 and 0.20 m/day.
In June 1996, a 46 m long, 7.3 m deep, and 0.6 m wide PRB (continuous
wall configuration) of zero-valent iron (Peerless Metal Powders, Inc., Detroit,
MI) was installed approximately 30 m from the Pasquotank River (Figure
6.1). The reactive wall was designed to remediate hexavalent chromium-
contaminated groundwater and portions of the larger overlapping plume
of volatile chlorinated organic compounds. In 1999, a pilot-scale injection
of sodium dithionite was conducted to evaluate the response of the source-
zone hexavalent chromium (Kahn and Puls, 2003). On the basis of the suc-
cess of this test, a full-scale treatment with sodium dithionite was carried out
in 2001 (Malone et al., 2004). The objective of the dithionite treatment was to
allow for the reduction of naturally occurring ferric iron-bearing minerals in
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