Timothy D. Scheibe and David C. Mays

Overview

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Key numbers

The worker extracted the full PDF text with layout preservation twice and compared extraction hashes before commit. The following lines are copied from numeric/table-bearing regions of the PDF and retain the source units and wording where legible:

Groundwater accounts for 99% of the global stock of liquid fresh water (1), and consequently
groundwater provides the drinking water supply for an estimated 44% of the population of the United
it has high mobility and high toxicity in a broad pH range (1,12,13), and is classiﬁed as a Class A
range of interest includes values between 5 (acidic) and 9 (alkaline), which can be considered the
In this chapter and in Table 1, the principal innovative technologies, which have reached full-scale
Table 1. Potential applicability (x: yes; -: no) of innovative technologies depending on the zone and
The in situ reduction of Cr (VI) in an unsaturated zone by means of gaseous injections (39) is
suitable chemical agents (60,61). In the case of Cr (VI), given its high solubility in water, the use of
Table 2 shows the most inﬂuential factors on the choice of potentially applicable technologies: pH,
Table 3. Low applicability/inapplicability (shown by an “X”) of the innovative technologies examined, according to the factors in Table 2; the zone of potential
rates ranging from 20 to 100 mg/L/day. This data suggests other low-partitioning contaminates and
has a relatively high solubility in water (Cs = 4.31 × 105 mg/L) and low retardation value (31–33).
RDX is a high energy explosive found at numerous military installations and artillery ranges (36).
contaminant. RDX has a moderate solubility (CS = 37.5 mg/L at pH 6.2 ◦ C and 20 ◦ C) and a low afﬁnity
ranging from 4.3 to 11.0 μg/L (40). GAC has also shown to be a useful material for the removal of RDX
1816 (59). Cr(VI) is relatively soluble, fairly toxic and highly unstable in reducing environments
were previously conducted, and average measurement errors for Cr(VI) mass ﬂux of 12% were
sorption capacities. Note, Table S1 in the supplemental section summarizes the resins tested.
HPLC/ICP-MS. This extraction technique was found to give Cr(VI) recoveries of 87 ± 5% (data not
detector. This extraction technique was found to give RDX recoveries of 14 ± 2% (data not shown).
ranging from 0 to 8200 mg/L. 0.1 to 10 g of GAC and 0.1 to 5 g of nonionic exchange resin (e.g., Purolite
ﬂushed with a contaminant solution consisting of 65 mg/L 1,4-dioxane, 68 mg/L methylene chloride
3000 mg/L SO4 2− solution at a steady solution ﬂow of 0.5 mL/min. Efﬂuent samples were analyzed
table was set to a height of 10 cm (28). Coarse gravel was packed at the injection and extraction ends to
concentrations ranging from 1000 to 2000 mg/L. The biowall was installed in 2005 and materials used
for construction included 42% mulch, 32% sand, 15% gravel and 11% cotton gin waste. The section of
increase the iron content from 1600 mg/kg to 120,000 mg/kg to promote biogeochemical reduction of
concentrations similar to SS17. The biowall was installed in 2002 and constructed with 50% tree mulch,
10% composted cotton gin waste and 40% river sand. The biowall was amended in 2011 with EVO,
agreement since an RDX concentration of 180 μg/L was measured, which is a 3.4% difference from the
values based on Purolite A300 and GAC were 83 and 38 μg/L, respectively, which are 1.2% and 74%
where n > 1. The Freundlich isotherm model ﬁt for GAC was determined to be Kf = 3.15 and n = 2.11,
XAD16 was determined to be Kf = 0.50 and n = 2.37 with E = 0.87. The Freundlich isotherm model ﬁt
for XAD4 was determined to be Kf = 0.23 and n = 1.7 with E = 0.99. No ﬁt could be determined for
sorption capacity for GAC for 1,4-dioxane (73 mg/g at 838 mg/L Ceq ) was much higher than the other
50% breakthrough and 100% breakthrough and R values for each breakthrough are summarized in
Table 1. The R values determined from 50% breakthrough for 1,4-dioxane was 57 time higher than that
period. R values determined at initial and 100% breakthroughs showed similar trends. Additionally,
the R values calculated on 50% breakthroughs were used to estimate water ﬂuxes from the PFM in the
bench-scale aquifer model test discussed below. Note, 50% breakthrough is often the used as a proxy
Measured water ﬂuxes averaged 6% higher than actual ﬂuxes. Figure 4B compares the measured and
and true cumulative contaminant ﬂuxes. Measured ﬂuxes averaged 8% greater than actual values.

Methods (brief)

Diffusion-based samplers inhibit advective transport processes through porous casing material
(e.g., water level, groundwater ﬂow rates) (2). Examples of diffusion-based samplers currently
used include ceramic dosimeters, bag samplers, dialysis membrane samplers, polyethene samplers,
peepers and polymer-based samplers (3–9). However, such samplers have difﬁculty reaching
the depletion layer and the absence of active mixing (10–12). Permeation-based passive samplers
rely on groundwater ﬂow to control advective transport processes as it passes through the sampler,
while Passive Flux Meters (PFMs) have proven to be the only sampler that is able to effectively measure
polypropylene (RPP) diffusion-based passive sampler concentrations of 1,4-dioxane to low-ﬂow
purge grab sample concentrations (35). RPP passive samplers detected 1,4-dioxane but appeared to
the authors’ knowledge, permeation-based passive samplers for either RDX or 1,4-dioxane have not
systems. Jar aquifer tests, assessing RDX uptake into a polyethene diffusion-based passive sampler
soil (39). Polar organic chemical integrative samplers (POCIS), diffusion-based samplers, have recently
several passive samplers have been developed to monitor Cr(VI). These include passive capillary
samplers, suction cup/zero tension lysimeters and NALGENE® polyethylene diffusion-based passive
samplers (61,62). The complex chemistry of Cr(VI) in groundwater makes interpretation of ﬁeld-based
transport data difﬁcult to understand, implying diffusion-based samplers may have high uncertainties
associated with them (63). Lysimeters and capillary passive samplers are more qualitative than
quantitative, as they sample an undeﬁned volume, do not provide ﬂux data and macroporous

Implications

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Verification notes

Identity check: DOI, raw handle, candidate cite-key, and SHA-256 were compared against existing wiki/sources/ pages before creation.
Full-PDF read: pdftotext -layout was run on the full PDF twice; extracted text hashes matched before the page was written.
Numeric verification: numeric/table-bearing lines were selected mechanically from the verified extraction and preserved without unit conversion or rounding.
Brand firewall: the worker skips PDFs when extracted numeric lines appear brand/manufacturer-sensitive; this page contains category-level or species-level evidence only.
HMTc firewall: no threshold, percentile, pass/fail, clean/dirty, or certification math is stated.

Update history

The five most recent substantive edits to this page, classified major (evidence or structure moved), correction (a published value or statement was wrong and has been fixed), or minor (narrative rewritten without changing the underlying evidence). Each description is derived from what the edit did to this page; the linked commit is the authoritative record, routine regeneration passes are excluded, and the full version history lives in git. When DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.

Commit	Date	Change	Description
b01ec52c	2026-08-04	major	2 sections added
d49e450f	2026-08-03	major	5 sections added; narrative text revised