2
based on existing data to target areas with variable uranium
and ammonia concentrations (Figure 1). Freshwater in the
unconfined alluvial aquifer is underlain by a brine zone that
varies seasonally in elevation but is often encountered at
approximately 55 feet below ground surface (bgs) (DOE
2003). Soil and groundwater collection targeted the vadose
and freshwater saturated zones. At the Site, total dissolved
solids (TDS) concentrations below 35,000 mg/L are con-
sidered to be in the freshwater zone.
Soil Collection and Analysis
Soil was inspected, logged, and sampled as boreholes were
advanced to a maximum of 60 feet bgs. Soil samples were
collected at four depths within each boring to target the
top and bottom of the vadose and freshwater saturated
zones. Soil samples were shipped to GEL Laboratories of
Charleston, South Carolina, for analytical characterization
and Hazen Research of Golden, Colorado, for solid-phase
extraction analysis and column testing.
Solid-phase extractions were performed on vadose
and saturated zone soil samples to quantify the fraction
of solid-phase uranium that has the potential to mobilize
to groundwater. The solid-phase extractions consisted of
sequentially exposing soil samples to four extraction solu-
tions, each of which targeted a specific fraction of solid-
associated uranium. The four-step sequential extraction
procedure was based on the Tessier extraction method for
trace metals (Tessier et al. 1979) and modified to target
solid-associated uranium species based on the geochemical
behavior of uranium (Salome et al. 2017). Step 1 targeted
weakly sorbed/exchanged uranium Step 2 targeted strongly
sorbed/weak acid extractable uranium associated with car-
bonate minerals Step 3 targeted uranium associated with
iron and manganese (oxy)hydroxide minerals and Step 4
targeted uranium phosphate minerals and recalcitrant frac-
tions. Reduced uranium minerals were likely extracted dur-
ing Step 1 because the solution was oxic.
Groundwater Collection and Analysis
One grab groundwater sample was collected from the
upper freshwater saturated zone at each borehole for ana-
lytical characterization. Additional bulk groundwater sam-
ples were collected from three existing monitoring wells for
column and batch reactor testing (Figure 1). Bulk ground-
water collected from ATP-3, located upgradient of the tail-
ings pile, was used as influent for the column tests. Bulk
groundwater samples from UPD-18 and MW-3, located
downgradient of the tailings pile, were used for batch reac-
tor testing.
Groundwater samples were collected using a bladder
pump or disposable bailer and shipped to GEL Laboratories
of Charleston, South Carolina, for analytical characteriza-
tion and Hazen Research of Golden, Colorado, for batch
reactor testing.
SOIL ANALYTICAL RESULTS
Soil uranium concentrations ranged from 0.580 milligrams
per kilogram (mg/kg) to 9.80 mg/kg and were moderately
to strongly correlated with iron and manganese at some
locations (r2=0.57 to 0.93 and 0.52 to 0.79, respectively)
when soil results were analyzed at individual borings, con-
sistent with sequestration of uranium via adsorption to, or
co-precipitation within, iron and manganese (oxy)hydrox-
ide minerals. Uranium concentrations in soil samples were
also strongly correlated to soil phosphorus concentrations
at some borings (r2=0.66 to 0.98). Geochemical modeling
results indicate that uranyl phosphate minerals are under-
saturated in Site groundwater (discussed below), and there-
fore, strong correlations between uranium and phosphorus
are more likely to reflect the affinity of the phosphate and
uranyl ions for adsorption onto iron (oxy)hydroxide min-
erals rather than evidence of precipitation of uranyl phos-
phate minerals.
Solid-phase sequential extraction results indicate that
approximately 30% of solid-phase uranium was weakly
Figure 1. Site layout and soil and groundwater sampling
locations
based on existing data to target areas with variable uranium
and ammonia concentrations (Figure 1). Freshwater in the
unconfined alluvial aquifer is underlain by a brine zone that
varies seasonally in elevation but is often encountered at
approximately 55 feet below ground surface (bgs) (DOE
2003). Soil and groundwater collection targeted the vadose
and freshwater saturated zones. At the Site, total dissolved
solids (TDS) concentrations below 35,000 mg/L are con-
sidered to be in the freshwater zone.
Soil Collection and Analysis
Soil was inspected, logged, and sampled as boreholes were
advanced to a maximum of 60 feet bgs. Soil samples were
collected at four depths within each boring to target the
top and bottom of the vadose and freshwater saturated
zones. Soil samples were shipped to GEL Laboratories of
Charleston, South Carolina, for analytical characterization
and Hazen Research of Golden, Colorado, for solid-phase
extraction analysis and column testing.
Solid-phase extractions were performed on vadose
and saturated zone soil samples to quantify the fraction
of solid-phase uranium that has the potential to mobilize
to groundwater. The solid-phase extractions consisted of
sequentially exposing soil samples to four extraction solu-
tions, each of which targeted a specific fraction of solid-
associated uranium. The four-step sequential extraction
procedure was based on the Tessier extraction method for
trace metals (Tessier et al. 1979) and modified to target
solid-associated uranium species based on the geochemical
behavior of uranium (Salome et al. 2017). Step 1 targeted
weakly sorbed/exchanged uranium Step 2 targeted strongly
sorbed/weak acid extractable uranium associated with car-
bonate minerals Step 3 targeted uranium associated with
iron and manganese (oxy)hydroxide minerals and Step 4
targeted uranium phosphate minerals and recalcitrant frac-
tions. Reduced uranium minerals were likely extracted dur-
ing Step 1 because the solution was oxic.
Groundwater Collection and Analysis
One grab groundwater sample was collected from the
upper freshwater saturated zone at each borehole for ana-
lytical characterization. Additional bulk groundwater sam-
ples were collected from three existing monitoring wells for
column and batch reactor testing (Figure 1). Bulk ground-
water collected from ATP-3, located upgradient of the tail-
ings pile, was used as influent for the column tests. Bulk
groundwater samples from UPD-18 and MW-3, located
downgradient of the tailings pile, were used for batch reac-
tor testing.
Groundwater samples were collected using a bladder
pump or disposable bailer and shipped to GEL Laboratories
of Charleston, South Carolina, for analytical characteriza-
tion and Hazen Research of Golden, Colorado, for batch
reactor testing.
SOIL ANALYTICAL RESULTS
Soil uranium concentrations ranged from 0.580 milligrams
per kilogram (mg/kg) to 9.80 mg/kg and were moderately
to strongly correlated with iron and manganese at some
locations (r2=0.57 to 0.93 and 0.52 to 0.79, respectively)
when soil results were analyzed at individual borings, con-
sistent with sequestration of uranium via adsorption to, or
co-precipitation within, iron and manganese (oxy)hydrox-
ide minerals. Uranium concentrations in soil samples were
also strongly correlated to soil phosphorus concentrations
at some borings (r2=0.66 to 0.98). Geochemical modeling
results indicate that uranyl phosphate minerals are under-
saturated in Site groundwater (discussed below), and there-
fore, strong correlations between uranium and phosphorus
are more likely to reflect the affinity of the phosphate and
uranyl ions for adsorption onto iron (oxy)hydroxide min-
erals rather than evidence of precipitation of uranyl phos-
phate minerals.
Solid-phase sequential extraction results indicate that
approximately 30% of solid-phase uranium was weakly
Figure 1. Site layout and soil and groundwater sampling
locations