Organic-Based Permeable Reactive Barriers for the Treatment of Heavy Metals, Arsenic, and Acidity - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 138

was estimated at 2.12 × 10 −3 cm/s based on measurements at 11 locations.
Hydraulic conductivity measurements within the PRB varied over three
orders of magnitude with a mean of 1.41 × 10 −3 cm/s. Hydraulic gradients in
the aquifer immediately upgradient of the PRB were estimated using data
from eight monitoring events between 2004 and 2009. The temporal head
data for eight monitoring wells were evaluated using a “three point prob-
lem” approach, similar to that described by Delvin (2003). These data were
used to determine the direction and magnitude of the hydraulic gradient
for the centroid of six triangular elements described by the eight monitor-
ing points. Using the available site-wide groundwater elevation data from
the six monitoring events, the average magnitude of the hydraulic gradient
toward the PRB was calculated at 0.0914 cm/cm. Based on these data and
assuming an average effective porosity in the PRB between 0.1 and 0.2, the
average seepage velocity of water moving through the PRB was 3-6 cm/d.
However, there was significant uncertainty in the potential range of repre-
sentative values for flow velocity through the PRB, due largely to the degree
of heterogeneity observed.
8.1.2.2 Chemical Monitoring
Chemical monitoring data confirmed groundwater flow through the PRB, as
evidenced by significant PRB-induced geochemical changes on the down-
gradient side of the PRB. The rate of advance of the ORP and alkalinity
fronts on the downgradient side of the PRB systems (based on data collected
6 months following installation) indicated a groundwater flow rate of at least
2.5 cm/d. Selected chemical monitoring data are presented in Tables 8.1 and
8.2. The data show cumulative averages (over several years of monitoring)
as well as the most recent 2010 results from the 33/33/33 cow manure/wood
chip/limestone pilot PRB (TEPA-1), the 67/33 cow manure/limestone pilot
PRB (TEPA-2), and the 67/33 cow manure/limestone full-scale PRB (TEPA-5
a n d T E PA- 6).
8.1.2.3 pH
Data collected over a 7-year period following the installation of the pilot PRBs
and full-scale PRB indicated the PRBs raised the pH of the groundwater
above 6 (Table 8.1). The increase in pH within the PRBs is attributed primar-
ily to dissolution of the limestone and production of bicarbonate associated
with microbially mediated sulfate reduction reactions. The average pH values
of groundwater entering the PRBs (average of upgradient [A + B] locations)
ranged from 2.99 at TEPA-6 to 3.18 at TEPA-1, while pH values within the
PRBs (average of [C + D] locations) ranged from 6.04 at TEPA-1 to 6.45 at TEPA-5.
The most recent data (April 2010) showed pH values remained above 6 at all
locations within the PRB, with the exception of location TEPA-1C in the cow
manure/wood chip/limestone pilot PRB where a pH of 4.64 was measured.