3
adsorbed or reduced (Step 1) 30% of uranium was strongly
adsorbed (Step 2) 25% of uranium was associated with
iron and manganese (oxy)hydroxides (Step 3) and 15%
of uranium was recalcitrant (Step 4). The percentage of
uranium in the adsorbed fraction (approximately 60% in
Steps 1 and 2 combined) was similar between the vadose
and saturated zones. This fraction of uranium could mobi-
lize via desorption during recharge through the vadose zone
or inflow of background groundwater through the satu-
rated zone. Iron- and manganese-associated uranium (Step
3) is less likely to mobilize to groundwater than adsorbed
uranium because it would require dissolution of iron and
manganese (oxy)hydroxides, which typically occurs under
reducing conditions. Uranium is typically less mobile
under reducing conditions because it tends to precipitate as
insoluble reduced uranium minerals, such as uraninite. The
fraction of uranium extracted during Step 4 (recalcitrant)
is unlikely to mobilize to groundwater under current and
future site conditions. The average percentage of potentially
mobile uranium (Steps 1 through 3) across all samples was
86%, with averages for individual boreholes ranging from
44% to 100%.
Soil ammonia concentrations at the Site ranged from
approximately 1 mg/kg to 300 mg/kg. On average, the
highest soil ammonia concentrations were beneath and
downgradient of the tailings pile (BH-03, -04, and -05),
consistent with the tailings pile as the primary source of
ammonia to soil and groundwater.
GROUNDWATER ANALYTICAL RESULTS
TDS concentrations of groundwater samples ranged from
approximately 1,000 to 31,200 mg/L and were less than
35,000 mg/L, confirming that samples were collected from
the freshwater zone. Sodium and chloride were the most
abundant cation and anion, respectively. Groundwater pH
was circumneutral, ranging from 7.03 to 7.96. Dissolved
oxygen (DO), oxidation-reduction potential (ORP), and
redox indicator species, including nitrate/nitrite, dissolved
iron, dissolved manganese, and sulfate, indicated that redox
conditions varied across the Site from oxidizing to strongly
reducing. Oxidizing conditions were observed at BH-04,
ATP-3, UPD-18, and MW-3. Reducing conditions were
observed in the northeastern plume area of the Site (BH-01
and BH-06) and at BH-03 and BH-05. Low concentra-
tions of dissolved iron and sulfate at BH-01 and BH-06,
combined with the other indicators of reducing conditions,
suggest that sulfate reduction and formation of iron sulfide
minerals were likely occurring at these locations. Phosphate
was measured in groundwater samples due to its impor-
tance for sequestering uranium as uranium-phosphate
minerals. Phosphate concentrations were not detected in
groundwater samples above the reporting limits (0.400 to
10.0 mg/L). Total organic carbon concentrations ranged
from 1.85 to 17.9 mg/L, suggesting that sufficient organic
carbon is available to support microbial transformations of
uranium and ammonia (e.g., nitrification of ammonia, bio-
reduction of uranium) where redox conditions are suitable
for those processes to occur. Groundwater uranium concen-
trations ranged from 3.57 to 3,480 µg/L and were not pro-
portional to soil uranium concentrations, consistent with
results of past site investigations (DOE 2012) and indicat-
ing that geochemical processes besides adsorption are likely
contributing to uranium sequestration in soil. Boring loca-
tions with the highest groundwater uranium concentrations
also had the most oxidizing conditions. In contrast, borings
with the lowest groundwater uranium concentrations had
the most reducing geochemical conditions.
PRECIPITATION OF URANIUM AND
AMMONIA MINERALS
The Geochemist’s Workbench ® (GWB ®) software pack-
age with the Minteq thermodynamic database (thermo_
minteq.dat) was used to evaluate the likelihood of uranium
and ammonia mineral precipitation in the freshwater satu-
rated zone. Uranium and ammonia minerals were under-
saturated in the groundwater samples, suggesting their
formation is unlikely. To further evaluate the precipitation
of uranium and ammonia minerals, solubility diagrams
were generated in GWB ®. Groundwater uranium con-
centrations were less than the solubility of rutherfordine,
UO2OH2 (beta), and schoepite, the least soluble uranyl
minerals under Site conditions, indicating that formation
of these phases is unlikely. The high ionic strength of Site
groundwater increases mineral solubility and likely pre-
vents precipitation of these phases. Although phosphate
was not detected above the laboratory reporting limit in the
groundwater samples, the impact of phosphate on uranium
mineral saturation was evaluated. Groundwater uranium
concentrations were less than the solubility of uramphite,
Na-autunite, and K-autunite, the most insoluble uranyl
phosphate minerals under site conditions, when phosphate
concentrations were equal to the laboratory reporting limits
(0.4 to 10 mg/L), indicating the formation of these phases
in Site soil is unlikely.
Groundwater samples were plotted on a Pourbaix dia-
gram (redox potential versus pH) of uranium speciation
generated in GWB ® using the groundwater composition at
BH-02 to evaluate the potential for precipitation of reduced
uranium minerals (Figure 2). Locations with strongly
reducing conditions (BH-01, BH-03, BH-05, and BH-06)
adsorbed or reduced (Step 1) 30% of uranium was strongly
adsorbed (Step 2) 25% of uranium was associated with
iron and manganese (oxy)hydroxides (Step 3) and 15%
of uranium was recalcitrant (Step 4). The percentage of
uranium in the adsorbed fraction (approximately 60% in
Steps 1 and 2 combined) was similar between the vadose
and saturated zones. This fraction of uranium could mobi-
lize via desorption during recharge through the vadose zone
or inflow of background groundwater through the satu-
rated zone. Iron- and manganese-associated uranium (Step
3) is less likely to mobilize to groundwater than adsorbed
uranium because it would require dissolution of iron and
manganese (oxy)hydroxides, which typically occurs under
reducing conditions. Uranium is typically less mobile
under reducing conditions because it tends to precipitate as
insoluble reduced uranium minerals, such as uraninite. The
fraction of uranium extracted during Step 4 (recalcitrant)
is unlikely to mobilize to groundwater under current and
future site conditions. The average percentage of potentially
mobile uranium (Steps 1 through 3) across all samples was
86%, with averages for individual boreholes ranging from
44% to 100%.
Soil ammonia concentrations at the Site ranged from
approximately 1 mg/kg to 300 mg/kg. On average, the
highest soil ammonia concentrations were beneath and
downgradient of the tailings pile (BH-03, -04, and -05),
consistent with the tailings pile as the primary source of
ammonia to soil and groundwater.
GROUNDWATER ANALYTICAL RESULTS
TDS concentrations of groundwater samples ranged from
approximately 1,000 to 31,200 mg/L and were less than
35,000 mg/L, confirming that samples were collected from
the freshwater zone. Sodium and chloride were the most
abundant cation and anion, respectively. Groundwater pH
was circumneutral, ranging from 7.03 to 7.96. Dissolved
oxygen (DO), oxidation-reduction potential (ORP), and
redox indicator species, including nitrate/nitrite, dissolved
iron, dissolved manganese, and sulfate, indicated that redox
conditions varied across the Site from oxidizing to strongly
reducing. Oxidizing conditions were observed at BH-04,
ATP-3, UPD-18, and MW-3. Reducing conditions were
observed in the northeastern plume area of the Site (BH-01
and BH-06) and at BH-03 and BH-05. Low concentra-
tions of dissolved iron and sulfate at BH-01 and BH-06,
combined with the other indicators of reducing conditions,
suggest that sulfate reduction and formation of iron sulfide
minerals were likely occurring at these locations. Phosphate
was measured in groundwater samples due to its impor-
tance for sequestering uranium as uranium-phosphate
minerals. Phosphate concentrations were not detected in
groundwater samples above the reporting limits (0.400 to
10.0 mg/L). Total organic carbon concentrations ranged
from 1.85 to 17.9 mg/L, suggesting that sufficient organic
carbon is available to support microbial transformations of
uranium and ammonia (e.g., nitrification of ammonia, bio-
reduction of uranium) where redox conditions are suitable
for those processes to occur. Groundwater uranium concen-
trations ranged from 3.57 to 3,480 µg/L and were not pro-
portional to soil uranium concentrations, consistent with
results of past site investigations (DOE 2012) and indicat-
ing that geochemical processes besides adsorption are likely
contributing to uranium sequestration in soil. Boring loca-
tions with the highest groundwater uranium concentrations
also had the most oxidizing conditions. In contrast, borings
with the lowest groundwater uranium concentrations had
the most reducing geochemical conditions.
PRECIPITATION OF URANIUM AND
AMMONIA MINERALS
The Geochemist’s Workbench ® (GWB ®) software pack-
age with the Minteq thermodynamic database (thermo_
minteq.dat) was used to evaluate the likelihood of uranium
and ammonia mineral precipitation in the freshwater satu-
rated zone. Uranium and ammonia minerals were under-
saturated in the groundwater samples, suggesting their
formation is unlikely. To further evaluate the precipitation
of uranium and ammonia minerals, solubility diagrams
were generated in GWB ®. Groundwater uranium con-
centrations were less than the solubility of rutherfordine,
UO2OH2 (beta), and schoepite, the least soluble uranyl
minerals under Site conditions, indicating that formation
of these phases is unlikely. The high ionic strength of Site
groundwater increases mineral solubility and likely pre-
vents precipitation of these phases. Although phosphate
was not detected above the laboratory reporting limit in the
groundwater samples, the impact of phosphate on uranium
mineral saturation was evaluated. Groundwater uranium
concentrations were less than the solubility of uramphite,
Na-autunite, and K-autunite, the most insoluble uranyl
phosphate minerals under site conditions, when phosphate
concentrations were equal to the laboratory reporting limits
(0.4 to 10 mg/L), indicating the formation of these phases
in Site soil is unlikely.
Groundwater samples were plotted on a Pourbaix dia-
gram (redox potential versus pH) of uranium speciation
generated in GWB ® using the groundwater composition at
BH-02 to evaluate the potential for precipitation of reduced
uranium minerals (Figure 2). Locations with strongly
reducing conditions (BH-01, BH-03, BH-05, and BH-06)