5
uranium in the vadose zone that can mobilize during
groundwater table fluctuations or precipitation events.
Once the maximum effluent uranium concentration was
achieved, concentrations generally declined to a steady
state or below the reporting limit (1 µg/L), consistent with
an initial flushing of uranium from the column soil and
attenuation of uranium over time. Effluent uranium con-
centrations did not exceed the groundwater standard at
the deeper BH-01, BH-03, and BH-06 columns and the
shallow BH-04 column. Effluent uranium concentrations
exceeded the 40 CFR 192 groundwater standard of 44 µg/L
in the remaining 8 columns, and effluent concentrations
in 7 of those columns were below the groundwater stan-
dard by the end of the test. The number of pore volumes
pumped through the columns to achieve the groundwater
standard ranged from 6.3 to 34.5.
The percentage of total uranium leached from the
soil within each column ranged from 4.6% to 51%, and
the percentage of adsorbed uranium leached from the soil
within each column (sum of Steps 1 and 2 of the sequential
extraction test), ranged from 7.6% to 71%. These results
are consistent with the solid-phase extraction results which
demonstrated that some solid-phase uranium is unlikely to
mobilize to groundwater during flushing of background
groundwater through the saturated zone.
Effluent ammonia concentrations were relatively stable
during the column test however, the majority of effluent
ammonia concentrations were not detected above the lab-
oratory reporting limit (1 to 10 mg/L). Column effluent
ammonia concentrations were far below recent concentra-
tions in Site groundwater (up to around 450 mg/L, DOE
2023), indicating that tailing fluids likely contribute to
elevated ammonia concentrations recently observed in Site
groundwater.
BATCH REACTOR TESTING SETUP AND
RESULTS
The primary objective of batch reactor testing was to evalu-
ate whether biological degradation of ammonia (nitrifica-
tion) is occurring in groundwater at the Site. The batch
reactor test used groundwater collected from two monitor-
ing wells (MW-3 and UPD-18) and included operating
duplicate live reactors and one control reactor per location.
Glass bottles served as the batch reactors, and Site ground-
water served as the batch reactor medium. Batch reactors
were periodically sparged with air during operation so the
reactors remained oxic, a requirement for nitrification.
Prior to beginning the batch test, the control reactors were
sterilized in a 90 °C hot water bath for 2 hours. The reactors
operated for 3 weeks and were agitated throughout the test
using a shaker table. Batch reactors were sampled weekly
and analyzed for pH, ORP, DO, conductivity, ammonia,
nitrate, and nitrite.
Strong evidence of nitrification was observed in the
UPD-18 live reactors. Over the course of the test, ammo-
nia concentrations decreased, while nitrate concentrations
simultaneously increased (Figure 3). Concentrations of
nitrite, a transient nitrification intermediate, fluctuated
and ultimately decreased by the end of the test. Ammonia
concentrations in the control reactors remained relatively
stable at the initial concentration. Changes in the average
nitrogen species concentrations over the course of the test
confirmed stoichiometric consumption of ammonia and
nitrite and production of nitrate, indicative of nitrification.
Evidence of nitrification was not observed in the MW-3
live reactors ammonia concentrations decreased slightly
during the test, but nitrate concentrations remained rela-
tively constant at the initial concentration.
Results of the batch reactor test are consistent with
higher concentrations of oxidized nitrogen species (nitrate
and nitrite) in the groundwater sample collected at UPD-
18 (57.5 mg/L) compared to MW-3 (21.5 mg/L). These
results also indicate that the degree of nitrification may
vary across the ammonia plume. Although groundwater
at UPD-18 and MW-3 is geochemically oxidizing with
circumneutral pH and detectable concentrations of DO,
nitrification was only observed at UPD-18 during the
batch reactor test. Variable rates of nitrification across the
Site could be due to the distribution of nitrifying bacteria
in groundwater, which was not evaluated in this study.
BATCH PORE FLUSHING MODELING
The batch pore flushing model is an analytical model used
to estimate the remediation timeframe for flushing clean
Figure 3. Time series of nitrogen species in duplicate live
reactors (solid lines) and abiotic control reactors (dashed
lines) containing groundwater from UPD-18
uranium in the vadose zone that can mobilize during
groundwater table fluctuations or precipitation events.
Once the maximum effluent uranium concentration was
achieved, concentrations generally declined to a steady
state or below the reporting limit (1 µg/L), consistent with
an initial flushing of uranium from the column soil and
attenuation of uranium over time. Effluent uranium con-
centrations did not exceed the groundwater standard at
the deeper BH-01, BH-03, and BH-06 columns and the
shallow BH-04 column. Effluent uranium concentrations
exceeded the 40 CFR 192 groundwater standard of 44 µg/L
in the remaining 8 columns, and effluent concentrations
in 7 of those columns were below the groundwater stan-
dard by the end of the test. The number of pore volumes
pumped through the columns to achieve the groundwater
standard ranged from 6.3 to 34.5.
The percentage of total uranium leached from the
soil within each column ranged from 4.6% to 51%, and
the percentage of adsorbed uranium leached from the soil
within each column (sum of Steps 1 and 2 of the sequential
extraction test), ranged from 7.6% to 71%. These results
are consistent with the solid-phase extraction results which
demonstrated that some solid-phase uranium is unlikely to
mobilize to groundwater during flushing of background
groundwater through the saturated zone.
Effluent ammonia concentrations were relatively stable
during the column test however, the majority of effluent
ammonia concentrations were not detected above the lab-
oratory reporting limit (1 to 10 mg/L). Column effluent
ammonia concentrations were far below recent concentra-
tions in Site groundwater (up to around 450 mg/L, DOE
2023), indicating that tailing fluids likely contribute to
elevated ammonia concentrations recently observed in Site
groundwater.
BATCH REACTOR TESTING SETUP AND
RESULTS
The primary objective of batch reactor testing was to evalu-
ate whether biological degradation of ammonia (nitrifica-
tion) is occurring in groundwater at the Site. The batch
reactor test used groundwater collected from two monitor-
ing wells (MW-3 and UPD-18) and included operating
duplicate live reactors and one control reactor per location.
Glass bottles served as the batch reactors, and Site ground-
water served as the batch reactor medium. Batch reactors
were periodically sparged with air during operation so the
reactors remained oxic, a requirement for nitrification.
Prior to beginning the batch test, the control reactors were
sterilized in a 90 °C hot water bath for 2 hours. The reactors
operated for 3 weeks and were agitated throughout the test
using a shaker table. Batch reactors were sampled weekly
and analyzed for pH, ORP, DO, conductivity, ammonia,
nitrate, and nitrite.
Strong evidence of nitrification was observed in the
UPD-18 live reactors. Over the course of the test, ammo-
nia concentrations decreased, while nitrate concentrations
simultaneously increased (Figure 3). Concentrations of
nitrite, a transient nitrification intermediate, fluctuated
and ultimately decreased by the end of the test. Ammonia
concentrations in the control reactors remained relatively
stable at the initial concentration. Changes in the average
nitrogen species concentrations over the course of the test
confirmed stoichiometric consumption of ammonia and
nitrite and production of nitrate, indicative of nitrification.
Evidence of nitrification was not observed in the MW-3
live reactors ammonia concentrations decreased slightly
during the test, but nitrate concentrations remained rela-
tively constant at the initial concentration.
Results of the batch reactor test are consistent with
higher concentrations of oxidized nitrogen species (nitrate
and nitrite) in the groundwater sample collected at UPD-
18 (57.5 mg/L) compared to MW-3 (21.5 mg/L). These
results also indicate that the degree of nitrification may
vary across the ammonia plume. Although groundwater
at UPD-18 and MW-3 is geochemically oxidizing with
circumneutral pH and detectable concentrations of DO,
nitrification was only observed at UPD-18 during the
batch reactor test. Variable rates of nitrification across the
Site could be due to the distribution of nitrifying bacteria
in groundwater, which was not evaluated in this study.
BATCH PORE FLUSHING MODELING
The batch pore flushing model is an analytical model used
to estimate the remediation timeframe for flushing clean
Figure 3. Time series of nitrogen species in duplicate live
reactors (solid lines) and abiotic control reactors (dashed
lines) containing groundwater from UPD-18