Chapter 8 preprint from *Groundwater Remediation of Trace Metals,
Radionuclides, and Nutrients, with Permeable Reactive Barriers,* edited
by D.L. Naftz, S.J. Morrison, J.A. Davis, and C.C. Fuller, Academic
Press (2002) *in press.\
*

**Treatability Study of Reactive Materials to Remediate Ground Water
Contaminated with Radionuclides, Metals and Nitrates in a Four-Component
Permeable Reactive Barrier**

James Conca^1^, Elizabeth Strietelmeier^1^, Ningping Lu^1^, Stuart D.
Ware^1^, Tammy P. Taylor^1^, John Kaszuba^1^, and Judith Wright^2^

^1^Los Alamos National Laboratory, MS J 514, Los Alamos, NM 87545

^2^UFA Ventures, Inc., 2000 Logston Blvd, Richland, WA 99352

ABSTRACT

The treatment of a shallow multicontaminant plume of ^239,240^Pu,
^241^Am, ^90^Sr, nitrate and perchlorate in Mortandad Canyon, Los
Alamos, NM, was investigated in the laboratory using a multiple
permeable reactive barrier consisting of four sequential layers. These
layers include a polyelectrolyte-impregnated porous gravel for
flocculating colloids, an Apatite II layer for plutonium, americium, and
strontium immobilization, a layer of pecan shells as a biobarrier to
nitrate and perchlorate, and a limestone gravel layer for any anionic
species that may slip through the other layers, especially those of
americium-carbonate. These layers can perform multiple functions, e.g.,
the pecan shells also sorb strontium very well and the Apatite II also
remediates nitrate and perchlorate very well. Nitrate, perchlorate,
plutonium, americium, and ^90^Sr concentrations were reduced to below
their maximum concentration limits (MCL) and usually to below detection
limits in laboratory studies. The materials for this particular multiple
barrier are inexpensive and readily available.

I. INTRODUCTION

The purpose of this study was to investigate the performance of
alternative reactive barrier materials for possible use in a multiple
permeable reactive barrier (PRB) at Mortandad Canyon. A barrier refers
to anything that functions to prevent, bar, or retard the passage or
movement of anything else, e.g., water, chemicals, trade goods, or
ideas. In hydrology, traditional hydraulic barriers include impermeable
walls, liners or caps made of bentonite, grout, asphalt, other clay
minerals, precipitates of calcium carbonate or other chemicals,
geotextiles, gels, or any material that can be made relatively
impermeable. Containment of contaminant plumes can be accomplished by
preventing the flow of the entire system, fluid and contaminant, through
these conventional hydraulic barriers. Recently, more selective barriers
have been developed that are barriers to one or a few components of the
system, but not a barrier to the bulk of the system. Under unsaturated
conditions, a capillary barrier consisting of a sharp boundary between
two materials with greatly dissimilar pore sizes will act as an
advective barrier to water but will not be a barrier to vapor flow.
Likewise a saturated, compacted bentonite clay liner will act as an
advective barrier to water and vapor but will not be a barrier at all to
aqueous diffusion of molecular species for which sorption to bentonite
is not significant, e.g., most anionic species. Permeable reactive
barriers to particular chemical contaminants consist of a water
permeable material with specific chemical reactivities towards one or
more chemical constituents via mechanisms such as adsorption, exchange,
oxidation-reduction, or precipitation. These barriers can have varying
ranges of specificity, e.g., adsorption of specific species by a
modified zeolite (Sullivan *et al.*, 1994), precipitation of metals and
radionuclides by apatite (Conca *et al.*, 2000; Bostick *et al.*, 1999),
or overall reduction of the system by zero valent iron, other iron
phases, or microbial activity (Blowes *et al.*, 1997; Tratnyek *et al.*;
1997; Puls *et al.*, 1999).

PRBs can be emplaced physically by trenching and replacing the natural
substrate with reactive media to form a wall of new material, or can be
emplaced by mixing or injecting reactive components into the substrate.
Either way, the migrating contaminant plume encounters the reactive
material in the barrier, the contaminant is sequestered, altered or
degraded and the water moves on through the barrier and exits the system
successfully treated. PRBs require precise knowledge of both the
hydraulic and chemical nature of the system. Impermeable barriers and
PRBs can be used in combination to control both the hydrological and
chemical nature of the treatment zone. Funnel and gate systems, or other
systems that use impermeable barriers to channel ground water flow, are
well suited for these applications (Starr and Cherry, 1994).

Mixed waste plumes will often require PRBs consisting of multiple layers
of components configured specifically for a particular site and
contaminants. Materials used in the field must be effective, inexpensive
and readily available in multiple-ton quantities to be able to treat
large volumes of water, or soil. Possible materials include zero valent
iron, compost, apatites, MgO, carbonates, pecan shells, peat moss, lime,
and other materials that have been investigated by researchers to
varying degrees. In order to be cost-effective and produced in multiple
ton quantities, most materials will come from mining production,
agricultural waste production, or industrial waste production, with few
modifying steps. The materials used in this study fit these criteria.

Radionuclides, including ^90^Sr, ^238,239,240^Pu, and ^241^Am, have been
detected since 1963 (LANL, 1997) in the shallow alluvial ground water
system that exists in Mortandad Canyon at Los Alamos National Laboratory
(LANL) in Los Alamos, New Mexico. These radionuclides are stable as
dissolved species in the ground water and in the suspended phase where
they adsorb onto colloids consisting of calcium carbonate, silica,
ferric hydroxide, and solid organic carbon (LANL, 1997). Nitrogen
species (nitrate and total Kjeldahl nitrogen) also occur in the alluvial
ground water in Mortandad Canyon. Perchlorates have been discharged at
concentrations above 1 mg/L (ppm) and occur in the Mortandad Canyon
alluvial ground water at levels above 300 μg/L (ppb). Perchlorates were
not monitored until recently because there were no regulatory limits.
However, since the State of California recently instituted regulatory
limits of 18 μg/L, and the United States Environmental Protection Agency
(EPA) is considering an MCL of 32 μg/L for perchlorate, the
concentrations in Mortandad Canyon may become a concern.

As a group, the contaminants in Mortandad Canyon can occur at
concentrations up to several times the public dose limit (DCG) for the
radionuclides or MCL for the nitrate and perhaps the perchlorate.
Discharges into the canyon containing radionuclides, perchlorate, and
nitrate have varied dramatically since 1963, generally dropping off with
time. Most contaminants are assumed to come from the TA-50 water
treatment facility near the head of the canyon, which presently
discharges approximately 20,000 gallons per day over a short time
period. Table 1 gives both discharge water chemistry from TA-50 during
1997 and some alluvial ground water chemistry at one of the Mortandad
Canyon observation wells (MCO-3) in 1997 (LANL, 1997) just upstream of
the proposed emplacement site of a multiple PRB. The chemistry of
discharges and ground water vary greatly over time, e.g., nitrate varies
from a few mg/L to over 50 mg/L.

The stratigraphy of Mortandad Canyon at the proposed PRB site consists
of about 7 feet of alluvium composed of sand and gravel in a matrix of
silt and clay overlying about 10 to 12 feet of clay and silt sediments
that have resulted from weathering of tuff in place. A multiple PRB is
proposed to be emplaced across the canyon near this point and down to
the tuff bedrock to ensure maximum contact with ground water. The
hydraulic conductivity of the sediments ranges from less than 10^-5^
cm/s locally in the silts and clays to 10^-2^ cm/s in the sands.
Therefore, to avoid hydrologic complications in the overall flow in the
canyon, the barrier materials need to be chosen with conductivities well
above 0.1 cm/s to prevent plugging of the PRB. As will be discussed
below, the materials chosen are gravel size and have conductivities
greater than 1 cm/s, total porosities greater than 40%, and average pore
sizes greater than 0.5 cm. Therefore, flux of ground water through the
barrier will be determined by the hydraulic conductivity of the
undisturbed sediments on either side of the barrier, plus any boundary
properties that develop as a result of the barrier.

Numerous studies have been performed for zero valent iron, iron in other
forms, polysulfides, granulated activated charcoal (GAC), and compost
(Benner *et al.*, 1999; Blowes *et al.*, 1997; Fruchter, 1996; Tratnyek
*et al.*; 1997; Puls *et al.*, 1999; Williamson *et al.*, 2000). Results
of these studies are well known, having resulted in many field
deployments (Goldstein *et al.*, 2000; Naftz *et al.*, 2000; Hocking *et
al.*, 2000; Wickramanayake *et al.*, 2000). Sequential treatment has
been investigated by researchers, including Morkin *et al.* (2000), who
delineated the challenges facing treatment of multiple contaminants with
multiple materials.

The multiple PRB chosen for this study and this site consists of four
sequential layers: a polyelectrolyte-impregnated porous volcanic gravel
for flocculating colloids, an Apatite II^®^ layer for plutonium,
americium, and strontium stabilization, a layer of pecan shells as a
biobarrier substrate to nitrate and perchlorate, and a limestone gravel
layer for anionic species that may not be trapped by the other layers,
especially those of americium-carbonate. These layers can perform
multiple functions, e.g., the pecan shells also sorb strontium very well
and the Apatite II also remediates nitrate and perchlorate very well.
The two types of gravels are inexpensive and easily available in ton
quantities. The pecan shells are an agricultural waste that is easy to
obtain in ton quantities and fairly durable in the subsurface given its
high lignin content. The Apatite II is an inexpensive, primarily
amorphous form of a carbonated hydroxy-apatite that has random
nanocrystal of apatite embedded in it, resulting in efficient and rapid
precipitation of various phosphate phases of metals and radionuclides.
Apatite II is an efficient non-specific surface sorber and is readily
available in multiple-ton quantities.

Table 1. Discharge water and alluvial ground water chemistry in
Mortandad Canyon during 1997.

Concentration DCG (pCi/L) Concentration in Ground

in Discharge Water or Water from well MCO-3

[Constituent]{.underline} [(mg/L or pCi/L)]{.underline} [MCL
(mg/L)]{.underline} [(mg/L or pCi/L)]{.underline}

Perchlorate \>\> 1 mg/L 0.032 mg/L*^a^* 0.36 mg/L

^238^Pu 11.39 pCi/L 1.6 pCi/L 16.2 pCi/L (4.0x10^-15^ *M*)

^239,240^Pu 4.58 pCi/L 1.2 pCi/L 12.5 pCi/L (8.4x10^-13^ *M*)

^241^Am 58.0 pCi/L 1.2 pCi/L 14.1 pCi/L (1.7x10^-14^ *M*)

^90^Sr 28.5 pCi/L 8.0 pCi/L 20.2 pCi/L (1.6x10^-15^ *M*)

Nitrate 68 mg/L 10 mg/L 3.86 mg/L

Ammonium 2.15 \--

Calcium 102 22.1

Magnesium 0.5 2.8

Potassium 11.7 7.9

Sodium 140 40

Silica 50.7 46

Iron \<0.01 \--

Bicarbonate 251 105

Carbonate 24.4 \--

Sulfate 28 12.0

Chloride 24.4 8.0

Fluoride 1.01 1.08

Total Dissolved Solids 686 274

pH 8.47 7.5

specific conductance (μS/cm) 1,372 288

ionic strength 0.0137 *M* \--

*^a^*possible regulatory limit being proposed

II\. MATERIALS AND METHODS

Past investigations and feasibility test results were used to focus on a
design for the multiple PRB in Mortandad Canyon. Results are described
in the following sections. Batch tests and one dimensional (1--D) column
tests conducted using Mortandad Canyon ground waters and reactive media
are also described below. On the basis of these results, large-scale
columns and aquifer-box tests were conducted as the final feasibility
tests prior to emplacement, and the results were used for the final
emplacement design. Figure 1 gives a schematic of how batch and column
tests were used to reflect field conditions. Analytical techniques
included Ion Chromatography, Inductively Couple Plasma (ICP) Emission
and Mass Spectroscopy, Liquid Scintillation Counting, Transmission
Electron Microscopy plus standard methods for measuring pH and dissolved
oxygen.

Ground water collected directly from Mortandad Canyon was used as the
stock solution in all new feasibility studies (LANL, 1997). For
contaminated studies, plutonium, americium, strontium, nitrate, or
perchlorate were added in sufficient quantities for particular
experiments. A large volume of uncontaminated ground water was collected
from Mortandad Canyon well MCO-5 for subsequent laboratory studies.

![](media/image1.pct){width="6.497916666666667in"
height="6.413194444444445in"}

**Figure 1. Definition of the Retardation Factor and contaminant
breakthrough behavior. Batch and column studies are related to behavior
in the field by the above relationship. The S--shape of the breakthrough
curve reflects hydrodynamic dispersion. The number of inflection points,
the shape of the curve and the absolute position along the axis reflect
the number, distribution and type of sorption sites. C/C~o~ is the
concentration of the specific chemical species in the effluent solution
divided by the concentration in the starting solution.**

Batch sorption studies were performed as initial screening tools for
evaluating solid media. Batch studies generally consisted of a small
amount of substrate combined with 10 times the amount of contaminated
water by mass. The combination is shaken for 24 hours or longer
depending upon the estimate of kinetic reaction times. The concentration
of contaminant in the water was measured to determine how much
contaminant had been removed from solution by interaction with the
substrate.

Column studies were performed to reflect more relevant field conditions.
These studies monitor the contaminant concentrations and other solution
properties in the effluent exiting the column following contact with
reactive media. Results are often given as a breakthrough curve that
shows the normalized concentration of the contaminant as a function of
the number of pore volumes exiting the column (Figure 1). This
normalizes the tests to any size system. The 1-D column studies were
small-scale columns 4.8 cm in inside diameter and 6 to 10 cm in length.
These were loaded with small grain-size splits of the various materials
and run at a range of flow rates to provide a scientific basis and
design criteria for larger columns. Flow rates were up to five times the
natural velocity of ground water in Mortandad Canyon. Data from these
types of column studies were also used to evaluate rate-limited
immobilization of contaminants. Feasibility studies with larger columns,
in which flow rates, material sizes, and juxtaposition of materials were
scaled closer to actual field conditions, were performed as a test of
the multiple PRB. In addition, two-dimensional (2-D) aquifer cell
studies were performed to approach a realistic remediation scenario with
the media contacting each other *in situ*.

**A. BARRIER MATERIALS**

**1. Porous Gravel**

A porous gravel coated with a material to capture colloids was chosen as
the first layer in the PRB to retard or prevent the movement of colloids
in the Mortandad Canyon ground water. There is significant evidence
indicating that enhanced transport of actinides occurs in subsurface
geologic media through their association with colloid-sized particles in
ground water, resulting in colloid-facilitated transport (Kersting, *et
al*., 1999). The colloids have a high potential for adsorbing these
contaminants because of their large surface area and generally negative
charge. The resulting pseudo-colloids, i.e., contaminants attached to
the surface of silica, clay, iron oxides and other non-contaminant
colloidal materials, move greater distances faster than dissolved
species in subsurface aquifers. These colloid-sized contaminants include
radionuclides, metals, pesticides, organics, viruses, and bacteria, and
synthetic materials such as asbestos fibers. The transport of
radionuclides as pseudo-colloids has been described recently by many
authors at multiple DOE sites, including LANL, the Nevada Test Site and
others (Triay *et al.*, 1997). The rapid movement of colloidal
contaminants through the subsurface is also the subject of numerous
modeling studies as cited by Kale (1993).

Kale (1993) developed a concept of a colloid barrier using glass beads
and a cationic, highly soluble polyelectrolyte with the ability to
capture and agglomerate colloids. The polyelectrolyte used was a
commercially available material, polydialylldimethylammonium chloride,
named Catfloc^®^. This material is used in the wastewater treatment
industry to flocculate small particles, and was selected for its ability
to adsorb onto silica substrates, e.g., silica colloids, rock, or glass
beads. Catfloc also was chosen for this study because of the low
concentrations necessary for good performance, the relatively low cost,
and because it is not easily biodegraded. The laboratory-scale studies
used glass beads and cm-sized rock chips of the gravel layer and
Mortandad Canyon ground water with and without colloids, at
concentrations of 2 x 10^8^ particles/mL. The biodegradability of
Catfloc was evaluated in studies where Catfloc acted as the sole carbon
source in a nutrient culture medium inoculated with Mortandad Canyon
ground water. The culture medium also contained potassium nitrate and
sodium phosphate as a supply of nitrogen and phosphorous nutrients to
ensure that nutrient deficiency was not a growth-limiting factor.
Aliquots of the culture medium were taken and stained for total
microbial counts.

**2. Apatite II**

A special form of the mineral apatite, Apatite II, was chosen as the
second layer in the PRB to treat metals, particularly Pu. Feasibility
studies were performed to determine the metal-stabilization potential of
various materials with respect to acid mine drainage in northern Idaho
and actinide-contaminated waters at DOE sites (Wright *et al.*, 1995;
Conca, 1997; Bostick *et al.*, 1999; Conca *et al.*, 2000; Runde, 2000).
Materials investigated included different sources of apatite (various
mineral sources, synthetic, commercial, cowbone charred or uncharred,
and Apatite II), zeolites (clinoptilolite and chabazite), various
polymers including Dowex II, C-sorb, Cabsorb, Mersorb, peat moss, zero
valent iron, hematite, goethite, and activated charcoal. The Apatite II
performed the best with respect to all metals studied, i.e., lead,
cadmium, zinc, uranium, strontium, and plutonium, but with different
mechanisms depending upon the metal and the conditions. Because of this
generally good performance for many metals under various conditions,
Apatite II was chosen for this Mortandad Canyon study.

The Apatite II can be obtained in almost any grain size from powdered to
gravel size. The gravel size has hydraulic conductivities exceeding 10
cm/s and a bulk density between 0.5 and 1.0 g/cm^3^. Apatite is ideal
for stabilizing many metals because it instigates heterogeneous
nucleation of metal-apatite phases under environmental conditions (Lower
*et al.*, 1998). The excellent stabilization efficiency comes from the
extremely low solubility products of the resultant metal-apatites, e.g.,
for uranium--phosphate (autunite) K~sp~ = 10^--49^, and for
lead--apatite (pyromorphite) K~sp~ = 10^-76^. Table 2 lists some
solubility products as examples (the reactions are given in MINTEQ-A2;
Geochem Software Inc. 1994). There are several apatite sources with
widely varying reactivities and properties, but most are not appropriate
for metal remediation. For best results, the apatite should: 1) have as
much carbonate ion substituted as possible, 2) have no fluorine
substitution, 3) have no trace metals initially in the structure, 4) be
poorly crystalline or even amorphous, and 5) have high internal porosity
(Conca *et al.,* 2000). For these reasons, traditional phosphate ores
and cow bone, charred or not, are not as effective as Apatite II.

::: Definition-Term
Table 2. Stabilities of Some Relevant Phases (from MINTEQ-A2; Geochem
Software Inc. 1994)
:::

::: Definition-Term
Solubility Solubility
:::

Mineral Phase Product (log K~sp~) Mineral Phase Product (log K~sp~)

Pb~5~(PO~4~)~3~(OH,Cl) -76.5 Am(PO~4~) -24.8

Ca(UO~2~)~2~(PO~4~)~2~ • 10H~2~O -49.0 Pu(PO~4~) -24.4

Sr~5~(PO~4~)~3~(OH) -51.3 UO~2~HPO~4~ -10.7

Zn~3~(PO~4~)~2~ -35.3 Cd~3~(PO~4~)~2~ -32.6

The nominal composition of Apatite II is
Ca~5-x~Na~x~(PO~4~)~3-x~(OH)(CO~3~)~x~ where x \< 0.2. The high
reactivity of Apatite II relative to other apatites comes from the
unusual structure of the solid. Most of the material is amorphous
hydroxy calcium phosphate with respect to both XRD and to TEM. However,
there are random nanocrystals of well-crystallized apatite embedded
throughout the amorphous phase (Figure 2). Also, there is substantial
carbonate substitution which makes the structure less stable and more
reactive (Wright *et al.*, 1990). However, once metals are in the
apatite structure, it is much more stable than the initial Apatite II.
Because of this arrangement, the amorphous structure is relatively
reactive and provides a sufficient excess of phosphate ion to solution,
locally exceeding the solubility limit of many metal-phosphate phases
and resulting in precipitation, particularly for lead, uranium and
plutonium. At the same time, the nanocrystals provide the apatite
structure for nucleating the precipitates. Other apatites, such as
phosphate rock, bone char, or reagent grade tricalcium phosphate, are
crystalline (Figures 3 and 4), have little or no carbonate substitution,
have substantial fluorine substitution, have significant trace metal
substitution (strontium, barium, uranium and others), or have less
microporosity, all of which reduce the reactivity and make them less
optimal for metal stabilization. The concentration of phosphate ion in
solution in equilibrium with Apatite II is in the 10 to 100 μg/L (ppb)
range, and depends upon solution chemistry, particularly pH. It is
important, however, because these low values avoid the phosphate loading
issues common to organophosphates and soluble inorganic phosphates such
as most fertilizers.

> **Figure 2. HR-TEM image of Apatite II showing general amorphous
> nature with random nanocrystal inclusions of crystalline apatite
> (low--resolution image is inserted in upper left corner).**
>
> ![](media/image2.jpeg){width="5.929861111111111in"
> height="6.299305555555556in"}
>
> **Figure 3. HR-TEM image of mineral apatite (NC phosphate rock)
> showing the crystalline nature of the apatite.**

![](media/image3.png){width="5.972916666666666in"
height="5.972916666666666in"}

> **Figure 4. HR-TEM image of bone char showing the crystalline nature
> of the apatite.**

![](media/image4.png){width="6.013888888888889in" height="6.375in"}

The Apatite II was found to leach ^87^Sr at a constant concentration,
regardless of the strontium concentration in the influent water. For
batch tests with radioactive ^85^Sr, this was not a problem as the two
isotopes are easily distinguished. However, during the column studies
that could used only ^87^Sr because of laboratory activity limits, there
was no way to distinguish the difference between ^87^Sr leached from, or
sorbed onto, the Apatite II. It was found that ^87^Sr leached from the
Apatite II in columns at a constant concentration of 0.85 mg/L,
regardless of influent concentration, e.g., columns with influent ^87^Sr
concentrations of 2 mg/L or 0 mg/L both showed effluent concentrations
of about 0.85 mg/L. The column study described in Section III.B.3 used
^85^Sr and so avoided this problem. Further studies will use only
^85^Sr. It is not known whether the^87^Sr leaching from the Apatite II
was coming from the amorphous phosphate phase or from the organics.

**3. Pecan Shells**

Pecan shells were chosen as the third layer in the PRB to treat nitrate
and perchlorate. In the case of nitrate, biodegradation has been widely
investigated, and the microbial processes are well understood (Zumft,
1999). Nitrate can be removed from contaminated water by either
assimilatory or dissimilatory processes. Assimilatory processes result
in ammonia production and uptake, with subsequent conversion to cellular
constituents such as protein and nucleic acids. Dissimilatory nitrate
reduction processes result in gaseous products, such as nitrogen gas,
nitric oxide and nitrous oxide. The microbial reduction of perchlorate
has only recently been demonstrated (Attaway and Smith, 1993; Herman and
Frankenberger, 1998; Wallace *et al.*, 1998), but field-scale studies
for bioremediation of this contaminant have not been reported.

In a PRB system where one of the barriers is a biobarrier, the catalyst
for the destruction of nitrate is the indigenous microbial population,
which uses nitrate as both a nitrogen source, and as an electron
acceptor under denitrifying conditions. The catalyst for perchlorate
destruction is also microbial in nature, where the perchlorate is
reduced to non-toxic chloride products, but less is known about the
reduction pathways, and the physiological requirements and identities of
the organisms involved. Frequently, the only missing element for
destruction of both of these anionic species is carbon, which can be
supplied by an appropriate material added to the biobarrier, e.g,
residual organics on the Apatite II.

Pecan shells were the carbon-based support material chosen for the
biobarrier section of the multiple PRB. It does not serve as an
efficient carbon source but as an efficient substrate upon which to form
biofilms. This agricultural waste product fits the needs of this system
very well insofar as it is inexpensive, can be obtained in
ton-quantities as gravel-sized particles with large average pore sizes,
has great porosity and surface area for attachment of biofilms, persists
in the subsurface because of its high lignin content, and has no
disadvantageous reactions with the other barrier materials. This study
has taken an empirical approach to development of this biobarrier. The
ability of the biofilms that form on the pecan shells to reduce the
nitrate present in ground water has been determined, as have the
nitrogen-based reduction products that are present and in what amounts.
The populations of bacteria have been determined using a Most Probable
Number (MPN) technique that measures viable bacterial cells that will
grow in nitrate-amended media under denitrifying conditions.

**4. Limestone Gravel**

Actinides form strong carbonate complexes in aqueous solutions,
especially U and Am (Triay at al., 1991; Runde *et al.*, 1992). In
Mortandad Canyon, Am appears to form carbonate complexes and may pass
through the other layers (LANL, 1997). However, in many laboratory
studies for the Yucca Mountain Project and for other nuclear waste
programs, it has been observed that actinide carbonate complexes will
sorb strongly to carbonate rocks (Triay *et al.*, 1991; Triay *et al.*,
1997). Therefore, the final layer in the PRB was chosen to be limestone
gravel.

B. BATCH TEST METHODS

Batch sorption studies were performed on a set of aggregate materials to
determine the reactive sorption coefficient (K~d~) of radionuclides in
MCO-5 water. Ten potentially reactive materials and mixtures of
materials were analyzed in this study, as summarized in Table 3. Apatite
refers to Apatite II except when designated mineral apatite. Various
treatments of Apatite II were performed to observe any change in
reactivity.

The aggregate materials were mainly used as received; no crushing or
size separation was performed. However, the coagulant treated gravel was
prepared by first cleaning the gravel in weak (\~0.15 M) hydrochloric
acid by overnight soaking. Next, the gravel was rinsed in MCO-5 water
and immersed in a colloidal coagulant polymer solution (Catfloc \~2 g/L)
overnight. Finally, the gravel was given a final rinse in MCO-5 water
and kept immersed in MCO-5 water to guard against unknown effects of
allowing the coagulant-treated gravel to dry.

Apatite II samples that were treated with phosphoric acid were prepared
by overnight soaking in 0.5 *M* phosphoric acid followed by rinsing in
MCO-5 water and air drying. As will be seen below, this treatment did
not increase performance sufficiently to warrant its use in the field.
Any field application will use the Apatite II as received in the
specified grain size, and not treated in any way.

Three radionuclides were utilized in these batch tests: ^85^Sr^2+^,
^239^Pu^5+^ and ^241^Am^3+^. ^85^Sr concentrations were quantified using
a Packard Cobra Gamma Counter, whereas both ^239^Pu and ^241^Am were
analyzed on a Packard Model 2500 liquid scintillation counter (LSC).
Separate batch sorption studies were repeated for each radionuclide. The
^85^Sr batch sorption study used a liquid to solid ratio of
approximately 10:1 (150 mL solution and 15 g solid). The studies with
^239^Pu and ^241^Am used a liquid to solid ratio of approximately 15:1
(225 mL solution and 15 g solid). Apatite/limestone mixtures were formed
by adding 2 additional grams of limestone, an amount that only slightly
modified the liquid-solid ratios. The starting concentration of each
radionuclide solution was 7.6x10^-9^ *M* ^85^Sr, 2.4x10^--8^ *M*
^239^Pu, and 8.5x10^-12^ *M* ^241^Am.

> Table 3. Summary of Reactive Materials and their Properties.
>
> Material Acronym Size distribution Source
>
> Gravel GRAV 1 -- 3 cm Los Alamos Transit
>
> Coagulate treated gravel CGRAV 1 -- 3 cm Los Alamos Transit
>
> Mineral apatite AP 0.1 -- 0.5 cm PCS, Inc.
>
> Apatite II UF 0.5 -- 3 cm UFA Ventures
>
> Apatite II treated FWP 0.5 -- 3 cm UFA Ventures
>
> with phosphoric acid
>
> Apatite II mixed UFWL 0.5 -- 3 cm UFA Ventures & La Farge
>
> with limestone Mining
>
> Apatite II treated FWPL 0.5 -- 3 cm UFA Ventures & La Farge
>
> with phosphoric acid and Mining
>
> mixed with limestone
>
> Limestone LIM 1 --2 cm La Farge Mining
>
> Clinoptilolite ZEO 0.5 -- 1 cm St. Cloud Mining
>
> Pecan Shells PS 1 -- 2 cm San Saba Pecans

Aggregate materials were equilibrated with water from well MCO-5 before
batch sorption procedures began. Dry aggregates were weighed out and
placed in 250 mL round Teflon^®^ bottles with sterilized water from well
MCO-5 and agitated for 2 weeks prior to beginning batch studies.
Agitation was accomplished by placing the bottles horizontally onto
laboratory shakers. At the completion of the 2-week equilibration phase,
the solids were drained of water from well MCO-5, water from well MCO-5
containing a spike of known radionuclide concentration was added, and
agitation continued.

Water from well MCO-5 was sampled and analyzed over time to determine
sorption behavior and kinetics. Sampling of the container solutions was
performed using pipettes. To remove particulates, each sample was
centrifuged at 16,500 rotations per minute for an hour in Teflon vials.
Samples were then split for radionuclide and water chemistry analysis.
Since the ratio of liquid to solid changed for each sample interval,
solution volumes were carefully measured and accounted for in
calculating the sorption coefficient isotherms.

For the biobarrier studies, pecan shells (2 g), or pecan shells and dog
food (2 g/0.2 g) were loaded into Oak Ridge tubes and 20 mL of water
from well MCO-5 was added for a solution/solid ratio of 10:1 (and a
pecan shell/dog food ratio of 10:1). Reaction tubes were incubated at
room temperature on a shaker. Successive samples were taken on the day
after the tubes were loaded, Day 1, and on Day 2, Day 7, Day 14, and Day
21 in most cases. Nitrate, nitrite, and ammonia concentrations were
determined using EPA method 300.0, an ion chromatographic method. MPN
analysis involved the use of a nitrate-reducing MPN method developed
from methods found in Microbiological Methods 7^th^ edition (Collins *et
al.* 1995).

C. COLUMN TEST METHODS

**1. Single Column Tests with ^87^Sr, Nitrate, and Colloids**

A single column study to evaluate pecan shells with respect to nitrate,
strontium and colloids in water from well MCO-5 used single 6-cm long,
4.8-cm inside diameter columns. The column was packed with pecan shells
with no food source, and was infused with water at a flow rate of 0.13
mL/min. The ground water was spiked with 2 mg/L ^87^Sr, 50 mg/L NaNO~3~
(for a total nitrate concentration of 100 mg/L) and 2x10^8^ fluorescent
colloids per L of water. Fluorescent polymer (polystyrene) microsphere
colloids (1μm) were obtained from Duke Scientific Corp., Palo Alto, CA.
Sieve analysis of the pecan shells indicated that 75.9% of the shells
were 2.00-4.75 mm in size while 24.1% were 1.00-2.00 mm in size.
Approximately 6.5-mL aliquots were retrieved automatically by an Isco^®^
Foxy Junior fraction collector. ICP analysis of ^87^Sr and nitrate-ion
chromatography were performed on the effluent following collection from
the column, while the fluorescent colloids were counted using a
microscopic technique (Abdel-Fattah and Reimus, 2000).

**2. Sequential Column Tests with ^87^Sr and Nitrate**

Four 1--D columns were arranged in sequence to evaluate the ability of
various reactive media to remove contaminants from flowing ground water.
These columns were packed with materials selected based on the results
of the batch sorption treatability studies, and run at a range of flow
rates up to 5 times the natural velocity of ground water in Mortandad
Canyon. Material included gravel coated with Catfloc, volcanic rock
(basalt) coated with Catfloc, Apatite II, pecan shells, pecan shells
mixed with dog food (10:1, respectively), and limestone.

MCO-5 water was spiked with various contaminants to desired
concentrations. Columns were packed with various reactive media through
which contaminated ground water was flushed at a constant flowrate. The
columns were constructed of borosilicate glass with a 4.8 cm inside
diameter and are equipped with an adjustable endplate for bedlength
variation from 1-13 cm.

All media used in the four sequential columns were sieved to less than
4.75 mm size fraction but greater than 1.00 mm. Catfloc-coated gravel or
basalt was prepared by contacting 2 g/L Catfloc solution with the gravel
or basalt overnight on a rotator (60 rpm; approximately 2:1,
liquid-solid ratio). Apatite II was used as obtained or was soaked in
90^o^C hot water to further remove residual organics. Pecan shells were
used as received or crushed to the desired size fractions. In the first
column study only, dog food was added to the pecan shells (1:10, dog
food:pecan shells) to serve as a carbon source stimulant for microbial
growth. All column studies were designed to run until breakthrough,
however, the Los Alamos Cerro Grande fire resulted in complete shut down
of the study before breakthrough was achieved.

::: Definition-Term
**3. Sequential Large-Scale Column Tests with ^239^Pu, ^241^Am, ^85^Sr,
Nitrate, and Perchlorate**
:::

Four columns were manufactured and machined from transparent acrylic
sheets and tubing. The columns measured 5.5 inches (\~14 cm) in internal
diameter and 12 inches (\~30 cm) in length. Two insteps were milled into
each end of the acrylic tubing to allow the emplacement of perforated
Teflon rings. The perforated rings created an area of dead volume and
slight backpressure to allow dispersion of the tracer before contacting
the column media. Flow ports were established on each endcap of the
columns using 1/4 X 28 thread peek flow fittings.

Four aggregates were chosen for use in the columns. The selection of
materials was based on the batch sorption studies. The columns were
packed with these materials and filled with MCO-5 water. The columns
were filled with de-aerated water and purged of large air pockets. No
attempts were made to eliminate small air bubbles. The columns were
connected inline to each other via 1/16 inch ID Teflon tubing. De-aired
MCO-5 water was pumped through the columns so that flow introduced into
the first column flowed through the second, third, and out the fourth,
respectively. Column media was equilibrated with MCO-5 water by flushing
the water through the columns at approximately 20 mL/hr for two weeks
prior to injection of contaminants. Prior to injecting the column
network with spiked MCO-5 water, the columns were equilibrated to the
injection flow rate by adjusting the pump's flow rate to the planned
injection rate 72 hours in advance of the injection. The pore volume of
the column network was slightly greater then 11 liters, distributed
approximately equally within the 4 columns.

A 100 mL pulse of MCO-5 water spiked with 7x10^--9^ *M* ^85^Sr, 5x10^-7^
*M* ^239^Pu (750,000 pCi) and 1x10^-9^ *M* ^241^Am was injected into the
columns after the two-week equilibration period. This plutonium
concentration represents about 600,000 times the field concentrations,
about 60,000 liters at the field concentrations, or about 60 years of
flow at present plutonium concentrations. This strontium concentration
represents about 440,000 times the field concentrations, about 44,000
liters at the field concentrations, or about 44 years of flow at present
^90^Sr concentrations. This americium concentration represents about
100,000 times the field concentrations, about 10,000 liters at the field
concentrations, or about 10 years of flow at present ^241^Am
concentrations. Nitrate and perchlorate were at the field concentrations
of 3.9 mg/L and 0.4 mg/L, respectively. No colloids were injected as
part of this particular column study. The purpose of the pulse was to
see if rapid contaminant loading would be stable against future flow, or
if sorbed radionuclides would be remobilized. The spiked water was
injected via a KD Scientific^®^ syringe pump. After a one-hour
injection, a Scilog Chemtec^®^ piston pump then maintained the
approximate 100 mL/hr flow of uncontaminated MCO-5 water. Initially, an
Isco Foxy^®^ funnel fraction collector was used to collect the elutants
hourly into Teflon collection bottles. After the first 3 days, this was
changed to a large collection bottle sampled approximately every 72
hours. The elutants were analyzed for radionuclide breakthrough and
solution chemistry. Radionuclide breakthrough was analyzed using the
same instruments as described in the batch sorption studies. However,
because the plutonium and americium have similar radioactive energy
emissions, differentiating between these two radionuclides was not done.
Only an analysis of gross alpha decay was performed. Direct analysis of
^85^Sr was possible because ^85^Sr is the only gamma emitter in the
study.

::: Definition-Term
**4. 2-D Aquifer Box Tests with ^87^Sr, Nitrate, and Perchlorate**
:::

A 2-D aquifer cell was manufactured from aluminum. The inside dimensions
of the rectangular box were 32 cm (height) x 62 cm (length) x 5 cm
(width). Rectangular end chambers, constructed of aluminum alloy, were
screened over the entire vertical height of the box. The bottom and back
of the box were aluminum and a glass plate served as the front (see
Figure 12 in Section III.B.4). The pore volume of the cell was
approximately 2 L. ^87^Sr and perchlorate in water from well MCO-5 were
at concentrations of 2 mg/L and 300 μg/L, respectively. During the first
60 days, the influent nitrate concentrations were held constant at
approximately 125 mg/L. Thereafter, the nitrate concentrations were
increased to 250 mg/L in order to evaluate the ability of microorganisms
in the PRB materials to reduce irregular nitrate concentrations. The
cell was constructed to allow discrete sampling between each layer
interface.

III\. RESULTS

A.  BATCH TESTS

1\. Colloid Tests in Mortandad Canyon Ground Water

The laboratory-scale studies used glass beads and cm-sized rock chips of
the gravel layer and Mortandad Canyon ground water with and without
colloids at concentrations of 2 x 10^8^ particles/mL. These studies were
performed to determine the effect of a number of different parameters on
agglomeration of microspheres on the cm-sized rock surfaces. There was a
strong dependence on Catfloc concentration and colloid concentration,
but the presence of Catfloc generally increased colloid removal from
solution by at least three orders of magnitude. The Catfloc was not
significantly degraded after 1 year.

2\. Batch Tests for ^239^Pu, ^241^Am, and ^90^Sr in Mortandad Canyon
Ground Water

Results of the batch sorption studies are presented as K~d~ versus time
for ^239^Pu, ^241^Am, and ^85^Sr in Figures 5, 6, and 7, respectively.
The results indicate that untreated Apatite II has a large affinity for
all three radionuclides. All combinations of Apatite II displayed large
affinities for plutonium. Limestone/untreated apatite mixtures and
treated Apatite II also display large affinities for americium. Apatite
II has greater affinities for all three radionuclides compared to
mineral apatite.

![](media/image5.pct){width="7.6in" height="4.6618055555555555in"}

Materials that will function as components of the multiple PRB, such as
pecan shells in the biobarrier, were evaluated in batch sorption studies
to determine radionuclide capture properties. These materials displayed
much less affinity for radionuclides than apatite.

Although the amounts of plutonium used were too small for phase
identification, the extremely high K~d~ values are consistent with the
extremely low solubilities of plutonium-phosphates and consistent with
studies of apatites showing precipitation with respect to lead and
uranium (Ma *et al.,* 1993; Moody and Wright, 1995; Bostick *et al.,*
1999; Runde, 2000). For plutonium, the various samples of apatite
exhibited the highest sorption values. The mineral apatite was slower to
react than the Apatite II, but eventually reached the same levels. The
other materials still showed reasonably high K~d~ values for plutonium,
up to 1,000. For americium, the apatites exhibited K~d~ values between
1,000 and 10,000, and the other materials had K~d~ values below 1,000.
^85^Sr showed the least sorption affinity for any of these materials.
While Apatite II performed the best with K~d~ values of about 500, all
were less than 1,000. The large amount of ^87^Sr that leached from the
Apatite II during column tests makes it difficult to evaluate these
batch tests with ^85^Sr. There was probably significant isotopic
exchange which may account for most of the sorption. Geochemically,
strontium has a strong affinity for apatite phases and readily
precipitates in phosphate phases in nature, including co-precipitation
with calcium in bone materials (Wright, 1990). The poor showing in
laboratory tests may also reflect kinetic effects.

**3. Biobarrier Batch Tests for Nitrate**

The biobarrier support material selected was pecan shells. Batch tests
were performed to determine the effects of initial sterilization of
materials, effects of readily available food sources, the efficiency of
the medium, and the time necessary for inducing microbial population
growth to sufficient levels to reduce nitrate concentrations in the
groundwater. The effect of a food source on the microbial activity was
investigated using dry dog food as a readily accessible food source. Two
studies were used to determine the amount of bacterial growth and
contaminant degradation in nitrate (NO~3~)-contaminated water. Each
study contained 5 different sets of samples: 1) the water and the pecan
shells were both sterilized (sterile control), 2) the pecan shells were
sterilized, the water was non-sterile thereby providing the bacteria
(sterile PS, unsterile H~2~O), 3) the water was sterilized, the pecan
shells were not (unsterile PS, sterile H~2~O), 4) both the water and the
pecan shells were non-sterile (unsterile control), and 5) filtered
nitrate-contaminated water without any substrate. Duplicate samples were
run under each set of conditions in each study. Another set of
experiments were run with dog food as an additive along with pecan
shells. Experiments were run both with field concentrations of nitrate
(\~20 mg/L) and with high nitrate (\~600 mg/L).

Results for the pecan shells alone with the 600 mg/L
nitrate-contaminated groundwater are shown in Figure 8. These results
are similar for all runs. All filtered water samples with no substrate
showed no reduction in nitrate concentrations. Sterilizing the
substrates (pecan shells or the pecan shells and the dog food)
diminished the degree of nitrate reduction more than sterilizing the
groundwater. Experiments with non-sterilized media resulted in greater
reduction of nitrate and shorter induction periods needed to form a
sufficiently active microbial population to significantly reduce nitrate
concentrations especially with both components providing bacteria.
Non-sterile substrates did contain denitrifying bacteria. In 2 out of 3
experiments, the presence of dog food reduced the induction periods
needed to form an active microbial population by about half,
particularly at high nitrate concentrations. With pecan shells alone,
the experiments with high nitrate concentrations required about twice as
long as those with low nitrate concentrations to reduce nitrate levels
to half the initial concentrations.

The results demonstrate that a healthy microbial population with
denitrifying activity was established that can reduce even high nitrate
concentrations to undetectable levels within about 1 week with the pecan
shell/dog food combination, and in about twice that time without the dog
food. As will be seen below, the Apatite II has sufficient residual
organics to serve as a readily accessible food source to the adjacent
biobarrier so that no additional food source is needed. The rate of
denitrification will be adequate to reduce nitrate in the proposed
multiple PRB for Mortandad Canyon given the flow rates and proposed
dimensions.

**B. COLUMN STUDIES**

**1. Pecan Shells with ^87^Sr, NO~3~^-^, and Polystyrene Colloids**

A single column study was undertaken to determine if pecan shells are a
feasible material for the biobarrier part of the multiple PRB for
degradation of nitrate. ^87^Sr and colloids were also included to
observe their behavior. The ground water was spiked with 2 mg/L ^87^Sr,
50 mg/L NaNO~3~ (for a total nitrate concentration of 100 mg/L) and

![](media/image7.pct){width="6.497916666666667in" height="4.46875in"}

2x10^8^ fluorescent colloids per L of water. ^87^Sr was removed to below
detection limits throughout the course of the experiment, an unexpected
advantageous result. Nitrate concentrations initially appeared to be
reduced, however, they reached a maximum concentration of 60 mg/L and
decreased with time to approximately 30 mg/L. This incomplete removal of
nitrate probably resulted from insufficient time being provided to
establish an active microbial community, and no food source being
available. Only eight pore volumes were put through the column, which at
the 0.13 mL/min flow-rate took just over two days. As was seen later in
column tests with Apatite II upstream of the pecan shells, the presence
of an available food source from the Apatite II lead to rapid
degradation of nitrate in the Apatite II. The pecan shells did not
result in complete removal of colloids from the influent solution,
however they did reduce the colloid concentration by at least an
order-of-magnitude, providing evidence that pecan shells could serve as
a secondary trap for retention of colloids that may escape the primary
colloid barrier material. The average pH of collected effluents was 6.92
± 0.10 and the average dissolved oxygen (DO) concentrations were 4.91 ±
0.12 mg/L.

It is difficult to compare the results for the pecan shells between the
batch and column tests because the conditions are so different. The
water to substrate ratio is much greater in the batch tests, and the
residence time in the batch tests were much longer (21 day incubation
times in the batch tests and only 2 days in the column tests). In the
subsequent sequential column tests with all layers, the nitrate was
unexpectedly removed by the Apatite II layer and so the performance of
the pecan shells with respect to the nitrate could not be evaluated in a
long-term column study.

::: Definition-Term
**2. Sequential Column Tests with ^87^Sr and Nitrate**
:::

Mortandad Canyon ground water retrieved from well MCO-5 was spiked with
2 mg/L ^87^Sr and introduced to the columns at a constant flow rate of
0.2 mL/min. Nitrate concentrations in the water from well MCO-5 were
measured to be approximately 50 mg/L, therefore no additional nitrate
was added to the ground water at first, but nitrate concentrations were
increased to 205 mg/L in the influent on 4/14/00 (see Figure 10). Four
columns, approximately 10 cm long with 4.8 cm inside diameter and loaded
with the reactive media, were arranged in series as depicted in Figure
9. Column 1 was loaded with Catfloc coated gravel (0.5 L of 2 g/L
Catfloc solution contacted with 280 g of gravel for 12 hours), column 2
with Apatite II pre-washed with hot water, column 3 with crushed pecan
shells mixed with dog food to enhance microbial proliferation, and
column 4 with limestone. Pore volumes were about 70 ml for each column
and water residence times were about six hours for each of the columns.
Three-way valves were placed between columns so that samples could be
collected following contact with each reactive medium. Approximately 5.5
mL aliquots were retrieved manually at regular time periods between the
columns while an Isco Foxy Junior fraction collector collected effluent
from column 4 following complete treatment through the column sequence.
ICP analysis of ^87^Sr and nitrate ion chromatography ensued following
collection of effluent from the columns.

Figure 10 illustrates ^87^Sr and nitrate concentrations detected in the
effluent following treatment through all four columns. Nitrate was
removed to below detection limits throughout the course of the study
except on the day that the concentration was increased to 205 mg/L,
after which it dropped to below detection limits for the rest of the
experiment. ^87^Sr concentrations rarely exceeded the 0.85 mg/L leached
baseline value of the Apatite II. The pH and dissolved oxygen (DO)
concentration were monitored daily for each of the four columns. A
comparison of average pH and DO concentration values, along with the
influent water from well MCO-5, is presented in Table 4.

> Table 4. MCO-5 water pH and DO concentration values, and daily average
> pH and DO concentration values individual columns.
>
> Source Description pH DO (mg/L)
>
> MCO-5 well water 8.03 ± 0.27 6.34 ± 0.20
>
> Column 1 (Catfloc coated gravel) 7.82 ± 0.36 5.34 ± 0.15
>
> Column 2 (Apatite II) 6.96 ± 0.11 1.89 ± 0.17
>
> Column 3 (pecan shells + dog food) 6.47 ± 0.30 2.35 ± 0.20
>
> Column 4 (limestone) 6.94 ± 0.19 2.00 ± 0.13
>
> ![](media/image9.png){width="6.6in" height="5.036111111111111in"}
>
> **CATFLOC Apatite II Pecan Shells Limestone**
>
> **Coated gravel gravel**
>
> **Figure 9. Sequential column tests of multiple PRB for Mortandad
> Canyon.**

It appears from the DO concentrations that the Apatite II and its
inherent residual organics was supporting a sufficient denitrifying
microbial population to explain the removal of nitrate from solutions
exiting the Apatite II. The pH results in Table 4 are consistent with
the observation that apatite minerals buffer the pH to about neutral.
The pecan shells and limestone do not have much effect in this column
study because of the removal of nitrate by the Apatite II and the
buffering effect of the Apatite II. There was concern that the limestone
would raise the pH back towards 8, but the water residence time in the
coarse limestone gravel is probably not long enough. Again the
experiment was interrupted before breakthrough occurred.

::: Definition-Term
**3. Sequential Column Tests with ^239^Pu, ^241^Am, ^85^Sr, Nitrate, and
Perchlorate**
:::

Four aggregates were chosen for use in the 4 sequential columns in this
test (Figure 11, Table 5). A 100 mL pulse of MCO-5 water spiked with
7x10^--9^ *M* ^85^Sr, 5x10^-7^ *M* ^239^Pu (750,000 pCi) and 1x10^-9^
*M* ^241^Am was injected into the columns after the two-week
equilibration period. The use of ^85^Sr avoided the leaching problem of
^87^Sr in the determination of total strontium sorption in this column
test. As stated earlier, these radionuclide concentrations

> ![](media/image10.pct)**Figure 10. ^87^Sr and nitrate concentrations
> in the final effluent from the sequential column study.**

represent about 60 years of flow at present plutonium concentrations, 44
years of flow at present ^90^Sr concentrations, and 10 years of flow at
present ^241^Am concentrations. Nitrate and perchlorate were at the
field concentrations of 3.9 mg/L and 0.4 mg/L, respectively. No colloids
were injected as part of this test. Relating a high-concentration pulse
to long-term flow at lower concentrations is valid only if there is no
degradation of removal efficiency of the solid over this time period.

Analysis of column elutants was conducted for three months until the
experiment was interrupted. After injection of the contaminant pulse,
approximately 300 pore volumes exited the columns with no detectable
nitrate, perchlorate, plutonium, americium, or strontium (neither ^85^Sr
nor ^87^Sr; no ^85^Sr exited the Apatite II and no ^87^Sr leached from
the Apatite II exited the pecan shells), in the effluent. The
immobilized plutonium, americium, and strontium, therefore, appear
stable against future subsequent flow, at least for this number of pore
volumes, a condition that needs to be addressed if the spent barrier
materials need to be left in place after the plume is treated.

> Table 5. Column Volumes and Masses.
>
> Media column 1 column 2 column 3 column 4
>
> Catfloc treated Apatite II Pecan Shells Limestone
>
> volcanic rock
>
> Aggregate Dry 1910 2722 1510 8663
>
> Mass (g)
>
> Pore Volume (mL) 3217 2705 3125 2304

**Figure 11. Feasibility column network**

![](media/image13.pct){width="6.2in" height="3.7131944444444445in"}

> **Figure 12. 2-D aquifer cell for treatment demonstration of
> contaminated groundwater.**

::: Definition-Term
**4. 2-D Aquifer Box Tests with ^87^Sr, Nitrate, and Perchlorate**
:::

A 2-dimensional (2-D) aquifer cell was assembled and loaded with basalt
coated with Catfloc, Apatite II, pecan shells and limestone arranged in
sequential vertical layers between sand similar to that in the field
design (Figure 12). ^87^Sr and perchlorate in water from well MCO-5 were
injected in a constant concentration of 2 mg/L and 300 μg/L,
respectively. Nitrate was injected at a constant concentration of 125
mg/L until day 60 when it was increased to 250 mg/L to evaluate the
ability of microorganisms in the PRB materials to reduce irregular
nitrate concentrations. With the flow rate of about 85 mL/hour and a
total cell pore volume of about 2 L, the water residence time was about
1 day, the same as that observed in Mortandad Canyon shallow groundwater
(1 pore volume per day).

![](media/image14.pct)Results were obtained for 100 days of continuous
contaminant injection. Figure 13 shows the results exiting the aquifer
box test after passing through all layers. The cell was constructed to
allow discrete sampling between each layer interface. Samples taken
between the first sand layer and the volcanic gravel (layer 1) and the
volcanic rock and Apatite II (layer 2) showed no measurable change in a
concentration for any of the contaminants. Effluents

> **Figure 13. Effluent concentrations for strontium, nitrate and
> perchlorate exiting the multiple PRB in the aquifer test.**

from all other layers showed the same results as in Figure 13, i.e., the
Apatite II determined the contaminant concentration for all effluent. As
was seen in previous tests, ^87^Sr was leached from Apatite II at
constant concentration of 0.85 mg/L and was not significantly reduced by
passage through the remaining layers after the first week. All effluents
following the Apatite II showed a strontium concentration of about 0.85
mg/L. The Apatite II layer reduced the nitrate and the perchlorate
before entering the pecan shells to below detection limits in almost all
samples. Detection limits for perchlorate were 3 μg/L (ppb).

**IV. CONCLUSION**

The feasibility studies performed, thus far, indicate that a multiple
PRB composed of four layers in sequence**,** Catfloc-coated volcanic
gravel, Apatite II, pecan shells, and limestone gravel, could remediate
shallow ground water at Mortandad Canyon, contaminated with
^238,239,240^Pu, ^241^Am, ^90^Sr, nitrate and perchlorate, sufficiently
to meet regulatory concerns. However, the performance period cannot be
determined from the existing results because the feasibility studies
were not able to result in breakthrough over the time periods of the
study. Feasibility column tests for all contaminants, using ^85^Sr,
should be run to completion, or breakthrough, prior to installation in
the field. Because the materials used are inexpensive, a large amount
can be used to ensure conservative performance. If the barrier performs
as well as desired in the field, the total expected inventory of
radionuclides in the shallow aquifer is low enough that complete removal
by so many tons of barrier material will not result in activities high
enough for the barrier to become a hazardous or radioactive waste. The
nitrate and perchlorate are reduced to harmless components and also will
not cause the barrier to become a hazardous waste. Therefore, the
materials could be left in place as long as regulatory approval could be
obtained.

**V. REFERENCES**

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detachment kinetics from direct visualization experiments" Report to the
U.S. Department of Energy, Nevada Operation Office/Underground Test Area
(UGTA) Program, April 2000.

Abdelouas, A., Lu, Y.M., Lutze, W. and Nuttall, H.E. 1998a. Reduction of
U(VI) to U(IV) by Indigenous Bacteria in Contaminated Ground Water.
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Abdelouas, A., Lutze, W. and Nuttall, H.E. 1998b. Reduction of Nitrate
and Uranium by Indigenous Bacteria. *Journal of the French Academy of
Sciences* (*Comptes Rendus de L'Academie des Sciences, Serie II
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Attaway, H. and Smith, M. 1993 Reduction of Perchlorate by an Anaerobic
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Benner, S.G., Blowes, D.W., Gould, W.D., Herbert, R.B.Jr. and Ptacek,
C.J. 1999. Geochemistry of a permeable reactive barrier for metals and
acid mine drainage. *Environmental Science and Technology*, 33,
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Benner, S.G., Blowes, D.W. and Ptacek, C.J. 1997. A Full-Scale Porous
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Bowman, R.S. and Sullivan, E.J. 1995. Surfactant-modified zeolites as
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Blowes, D.W., Ptacek, C.J. and Jambor, J.L. 1997. In-situ remediation of
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Bostick, W.D., Jarabek, R.J., Bostick, D.A. and Conca, J. 1999.
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the Removal of Soluble Radionuclides in Authentic and Simulated DOE
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Chen, X.-B., Wright, J.V., Conca, J.L. and Peurrung, L.M. 1997.
Evaluation of Heavy Metal Remediation Using Mineral Apatite. *Water, Air
and Soil Pollution*, 98, 57-78.

Collins, C.H., Lyne, P.M., and Grange, J.M. 1995. Microbiological
Methods, 7^th^ edition, Butterworth-Heinemann Ltd., publishers, Oxford,
UK.

Conca, J.L. 1997. Phosphate-Induced Metal Stabilization (PIMS), Final
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