4
plotted within or near the stability field of uraninite, indi-
cating that geochemical conditions at these locations are
sufficiently reducing to support uraninite formation.
COLUMN TESTING SETUP AND RESULTS
The primary objectives of column testing were to (1) esti-
mate the number of pore volumes of groundwater to be
flushed through Site soil to reach target groundwater con-
centrations of uranium and ammonia and (2) quantify
cation exchange/absorption processes for uranium and
ammonia for use in a future groundwater fate and trans-
port model. The column test included operation of 12
columns—one column for each saturated zone soil sample
collected from each boring. The columns were constructed
from polyvinylchloride (PVC) and measured 3 inches in
diameter and 20 inches in length. PVC endcaps held the
soil in place within the column. Tubing was connected to
each end of the column through a hole drilled in each end
cap. Tubing delivered influent to the bottom of the column
and collected effluent from the top end of the column.
Bulk homogenized saturated zone soil samples col-
lected from the Site served as the media within the col-
umns. Approximately one inch of clean sand was emplaced
below and above the soil within the column to ensure even
distribution of the influent across the diameter of the col-
umn and to prevent soil particles from exiting the column.
Columns were tapped and swirled during loading to mini-
mize formation of void spaces and preferential pathways.
The columns were operated for at least 28 days, rep-
resenting approximately 165 years of groundwater flow
through the Site based on the groundwater flow rate
(approximately 0.17 pore volumes per year) and the flow
rate through the columns (approximately 1 pore volume per
day [PV/day]). Background groundwater from ATP-3 with
a low uranium concentration (3.57 µg/L uranium) served
as the column influent. Influent was pumped through each
column in an up-flow direction with an average flow rate of
1.4 PV/day. Column influent and effluent were periodically
sampled and analyzed for pH, DO, ORP, conductivity,
and dissolved (0.45 μm filtered) uranium, ammonia, and
alkalinity. For five days during the test, column influent
was amended with bromide (41 mg/L), and bromide was
measured in the column effluent. Bromide breakthrough
curves were fitted with the Domenico analytical model
(Domenico 1987) to calculate the flow rate and effective
pore volume of each column.
The column influent composition remained stable
throughout the duration of the column tests and was rep-
resentative of background Site groundwater. Column influ-
ent pH was circumneutral, with values ranging from 7.76
to 7.94. Influent groundwater was oxidizing, indicated by
high ORP (200 mV) and DO concentrations (9 mg/L).
Effluent pH generally ranged from 7.6 to 8.6, and efflu-
ent DO concentrations were lower than the influent con-
centration. Effluent ORP values were initially lower than
the influent value, then increased to the influent value over
the course of the column test. These results indicate that
the soil within the columns was more reducing than the
influent groundwater, consistent with reducing conditions
observed in some areas of Site groundwater. Effluent ura-
nium concentrations exceeded the influent concentrations
at all 12 columns, consistent with leaching of solid-phase
uranium from Site soil as background groundwater flushes
through uranium-impacted soil. Maximum uranium efflu-
ent concentrations ranged from 13 to 340 µg/L and were
strongly correlated with concentrations of total uranium (r2
=0.84) and adsorbed uranium (sum of Steps 1 and 2 of the
sequential extraction, r2 =0.73) in column soil.
Maximum column effluent uranium concentrations
were less than recent groundwater uranium concentrations
measured at the site (up to 7 mg/L, DOE 2023), indicat-
ing that solid-associated uranium is not the only source of
uranium to groundwater. Additional sources of uranium
to groundwater include tailing fluids and solid-associated
Figure 2. Uranium Pourbaix diagram calculated in GWB®
using the groundwater composition at BH-02. Symbols
represent groundwater samples collected during this
investigation
plotted within or near the stability field of uraninite, indi-
cating that geochemical conditions at these locations are
sufficiently reducing to support uraninite formation.
COLUMN TESTING SETUP AND RESULTS
The primary objectives of column testing were to (1) esti-
mate the number of pore volumes of groundwater to be
flushed through Site soil to reach target groundwater con-
centrations of uranium and ammonia and (2) quantify
cation exchange/absorption processes for uranium and
ammonia for use in a future groundwater fate and trans-
port model. The column test included operation of 12
columns—one column for each saturated zone soil sample
collected from each boring. The columns were constructed
from polyvinylchloride (PVC) and measured 3 inches in
diameter and 20 inches in length. PVC endcaps held the
soil in place within the column. Tubing was connected to
each end of the column through a hole drilled in each end
cap. Tubing delivered influent to the bottom of the column
and collected effluent from the top end of the column.
Bulk homogenized saturated zone soil samples col-
lected from the Site served as the media within the col-
umns. Approximately one inch of clean sand was emplaced
below and above the soil within the column to ensure even
distribution of the influent across the diameter of the col-
umn and to prevent soil particles from exiting the column.
Columns were tapped and swirled during loading to mini-
mize formation of void spaces and preferential pathways.
The columns were operated for at least 28 days, rep-
resenting approximately 165 years of groundwater flow
through the Site based on the groundwater flow rate
(approximately 0.17 pore volumes per year) and the flow
rate through the columns (approximately 1 pore volume per
day [PV/day]). Background groundwater from ATP-3 with
a low uranium concentration (3.57 µg/L uranium) served
as the column influent. Influent was pumped through each
column in an up-flow direction with an average flow rate of
1.4 PV/day. Column influent and effluent were periodically
sampled and analyzed for pH, DO, ORP, conductivity,
and dissolved (0.45 μm filtered) uranium, ammonia, and
alkalinity. For five days during the test, column influent
was amended with bromide (41 mg/L), and bromide was
measured in the column effluent. Bromide breakthrough
curves were fitted with the Domenico analytical model
(Domenico 1987) to calculate the flow rate and effective
pore volume of each column.
The column influent composition remained stable
throughout the duration of the column tests and was rep-
resentative of background Site groundwater. Column influ-
ent pH was circumneutral, with values ranging from 7.76
to 7.94. Influent groundwater was oxidizing, indicated by
high ORP (200 mV) and DO concentrations (9 mg/L).
Effluent pH generally ranged from 7.6 to 8.6, and efflu-
ent DO concentrations were lower than the influent con-
centration. Effluent ORP values were initially lower than
the influent value, then increased to the influent value over
the course of the column test. These results indicate that
the soil within the columns was more reducing than the
influent groundwater, consistent with reducing conditions
observed in some areas of Site groundwater. Effluent ura-
nium concentrations exceeded the influent concentrations
at all 12 columns, consistent with leaching of solid-phase
uranium from Site soil as background groundwater flushes
through uranium-impacted soil. Maximum uranium efflu-
ent concentrations ranged from 13 to 340 µg/L and were
strongly correlated with concentrations of total uranium (r2
=0.84) and adsorbed uranium (sum of Steps 1 and 2 of the
sequential extraction, r2 =0.73) in column soil.
Maximum column effluent uranium concentrations
were less than recent groundwater uranium concentrations
measured at the site (up to 7 mg/L, DOE 2023), indicat-
ing that solid-associated uranium is not the only source of
uranium to groundwater. Additional sources of uranium
to groundwater include tailing fluids and solid-associated
Figure 2. Uranium Pourbaix diagram calculated in GWB®
using the groundwater composition at BH-02. Symbols
represent groundwater samples collected during this
investigation