Organic-Based Permeable Reactive Barriers for the Treatment of Heavy Metals, Arsenic, and Acidity - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 137

PRB transects (TEPA-1 and TEPA-2) and two transects through the full-scale
PRB (TEPA-5 and TEPA-6).
8.1.1 Methods
Each transect consists of 0.025-cm diameter Schedule 40 PVC screened wells
over 1.5 m with two wells located within the PRB, two upgradient of the
PRB, and four wells downgradient of the PRB. Wells were installed with a
Geoprobe 6600 direct push unit with formation sediments being allowed
to collapse around the wells following installation. The depth to the water
table at the location of the PRB was measured at approximately 1.5 m.
Groundwater samples were collected from the monitoring wells under low
flow conditions using a peristaltic pump. Samples that were taken for cat-
ion and anion analyses were 0.45 μm filtered in the field and refrigerated
until analyses. Cation samples were acidified with 12 M nitric acid following
collection to pH < 2 and analyzed by ICP-OES and/or ICP-MS. Analyses for
SO 2− and Cl − were conducted using capillary electrophoresis. Both pH and
oxidation-redox potentials (ORP) were measured in the field on unfiltered
samples using combination electrodes. ORP data are reported as values rela-
tive to the standard hydrogen electrode (E H ). Alkalinity was measured in
the field on unfiltered samples using a field titration kit (Hach ® method 8221
equivalent to Standard Methods 2320 B). Sulfide measurements were made
in the field on 0.45 μm filtered samples using Hach method 8131 (equivalent
to Standard Methods 4500-S −2 D). Dissolved organic carbon (DOC) samples
were 0.45 μm filtered in the field, collected in 40 mL glass VOA vials with no
headspace, and kept refrigerated until analyzed with a Dohrmann DC-80
carbon analyzer.
Hydrological studies at the Louisiana site included the measurement of
groundwater elevations in wells within and surrounding the PRB, and the
estimation of the hydraulic properties of the PRB and surrounding aquifer
materials. The hydraulic conductivity structure within and adjacent to the
PRB was estimated using physical slug testing techniques in approximately
70 wells. The tests were conducted based on methods proposed by Butler
(1997). The methodology utilized slugs produced from PVC stock to initiate
instantaneous changes in the head within the well, combined with high-fre-
quency monitoring of the aquifer response using data loggers and pressure
transducers. The aquifer response data were analyzed using the methods of
Bouwer and Rice (1976) and Bouwer (1989).
8.1.2 Results
8.1.2.1 Hydrologic Evaluation
The hydrologic data indicated that hydraulic conductivity in both the PRB
and the native materials upgradient of the PRB was highly heterogeneous.
The mean hydraulic conductivity of aquifer materials upgradient of the PRBs