7
lowest estimated natural flushing flow rate (70 gpm). These
estimates assume that ammonia biodegradation is occur-
ring uniformly across the plume at a consistent average rate.
The plume-scale batch pore flushing model indicates that
achieving the target cleanup concentration for uranium may
require more than 100 years under natural flushing condi-
tions. For expected natural flushing flow rates (190 gpm)
through the high concentration portion of the plume, the
estimated predicted remediation timeframe is 180 years.
The estimated predicted remediation timeframe for the low
concentration portion of the plume is at least 110 years.
However, because of the complex processes associated with
uranium transport, there is high uncertainty in the plume-
scale batch pore flushing model estimates for uranium. The
batch pore flushing model does not account for other rele-
vant geochemical processes that have been identified in this
investigation, including variable redox conditions, mineral
precipitation, and nonlinear sorption. The model also does
not account for heterogeneous hydrogeological conditions
at the Site.
CONCLUSIONS
Groundwater redox conditions across the Site were het-
erogenous and ranged from oxidizing to strongly reduc-
ing. The primary mechanism of uranium sequestration at
sampling locations with oxidizing geochemical conditions
was adsorption to iron and manganese (oxy)hydroxides.
Co-precipitation of uranium with metal (oxy)hydroxides
was a secondary mechanism of sequestration. Precipitation
of oxidized uranium minerals (e.g., uranyl phosphates) at
the sampling locations is unlikely due to the high ionic
strength and low phosphate concentrations of Site ground-
water. At sampling locations with reducing geochemi-
cal conditions, the dominant mechanisms of uranium
sequestration included adsorption of uranium to iron and
manganese (oxy)hydroxides, co-precipitation of uranium
with metal (oxy)hydroxides, and precipitation of reduced
uranium minerals (e.g., uraninite). On average, 63% of
solid-phase uranium was adsorbed or reduced, 23% of
solid-phase uranium was associated with iron and manga-
nese (oxy)hydroxides, and 14% of solid-phase uranium was
recalcitrant and unlikely to mobilize to groundwater under
naturally occurring geochemical conditions. The primary
mechanism of ammonia sequestration at sampling loca-
tions with oxidizing and reducing geochemical conditions
was adsorption/cation exchange. Evidence of nitrification
was observed in one reactor, and therefore, nitrification
may be contributing to attenuation of ammonia in ground-
water but at variable rates within the groundwater ammo-
nia plume.
Based on the batch pore flushing model fits of the col-
umn testing results, the calculated number of pore volumes
to achieve the groundwater uranium standard was between
5 and 20 pore volumes (with an average of 14 pore vol-
umes). The updated plume-scale batch pore flushing model
results show that groundwater standards may be achieved
under natural flushing conditions within 100 years for the
ammonia plume, but longer than 100 years may be needed
for the uranium plume. The plume-scale batch pore flush-
ing model does not account for the complex uranium geo-
chemical processes identified during this investigation or
for heterogeneous hydrogeological conditions. Results of
this investigation support development of a three-dimen-
sional numerical groundwater fate and transport model
that accounts for these geochemical processes to improve
the accuracy of remedial timeframe estimates.
REFERENCES
[1] Antelmi, M., et al. 2020. “Analytical and Numerical
Methods for a Preliminary Assessment of the
Remediation Time of Pump and Treat Systems.”
Water 12: 2850. August 10.
[2] Domenico, P.A. 1987. “An analytical model for mul-
tidimensional transport of decaying contaminant spe-
cies.” Journal of Hydrology 91: 49–58.
[3] United States Department of Energy (DOE). 2003.
Site Observational Work Plan for the Moab, Utah,
Site. GJO-2003-424-TAC. December.
[4] DOE. 2012. Moab UMTRA Project Northeastern
Uranium Plume Investigation Report. DOE-EM/
GJTAC2020. January.
[5] DOE. 2023. Moab UMTRA Groundwater and
Surface Water Monitoring Report January through
June 2023, Revision 0. DOE-EM/GJRAC3106.
October.
[6] Salome, K.R., Beazley, M.J., Webb, S.M., Sobecky,
P.A., and Taillefert, M. 2017. “Biomineralization of
U(VI) phosphate promoted by microbially-mediated
phytate hydrolysis in contaminated soils.” Geochim.
et Cosmochim. Acta. 197, 27–42.
[7] Tessier, A., Campbell, P.G.C., and Bisson, M. 1979.
“Sequential extraction procedure for the speciation of
particulate trace metals.” Anal. Chem. 51: 844–851.
lowest estimated natural flushing flow rate (70 gpm). These
estimates assume that ammonia biodegradation is occur-
ring uniformly across the plume at a consistent average rate.
The plume-scale batch pore flushing model indicates that
achieving the target cleanup concentration for uranium may
require more than 100 years under natural flushing condi-
tions. For expected natural flushing flow rates (190 gpm)
through the high concentration portion of the plume, the
estimated predicted remediation timeframe is 180 years.
The estimated predicted remediation timeframe for the low
concentration portion of the plume is at least 110 years.
However, because of the complex processes associated with
uranium transport, there is high uncertainty in the plume-
scale batch pore flushing model estimates for uranium. The
batch pore flushing model does not account for other rele-
vant geochemical processes that have been identified in this
investigation, including variable redox conditions, mineral
precipitation, and nonlinear sorption. The model also does
not account for heterogeneous hydrogeological conditions
at the Site.
CONCLUSIONS
Groundwater redox conditions across the Site were het-
erogenous and ranged from oxidizing to strongly reduc-
ing. The primary mechanism of uranium sequestration at
sampling locations with oxidizing geochemical conditions
was adsorption to iron and manganese (oxy)hydroxides.
Co-precipitation of uranium with metal (oxy)hydroxides
was a secondary mechanism of sequestration. Precipitation
of oxidized uranium minerals (e.g., uranyl phosphates) at
the sampling locations is unlikely due to the high ionic
strength and low phosphate concentrations of Site ground-
water. At sampling locations with reducing geochemi-
cal conditions, the dominant mechanisms of uranium
sequestration included adsorption of uranium to iron and
manganese (oxy)hydroxides, co-precipitation of uranium
with metal (oxy)hydroxides, and precipitation of reduced
uranium minerals (e.g., uraninite). On average, 63% of
solid-phase uranium was adsorbed or reduced, 23% of
solid-phase uranium was associated with iron and manga-
nese (oxy)hydroxides, and 14% of solid-phase uranium was
recalcitrant and unlikely to mobilize to groundwater under
naturally occurring geochemical conditions. The primary
mechanism of ammonia sequestration at sampling loca-
tions with oxidizing and reducing geochemical conditions
was adsorption/cation exchange. Evidence of nitrification
was observed in one reactor, and therefore, nitrification
may be contributing to attenuation of ammonia in ground-
water but at variable rates within the groundwater ammo-
nia plume.
Based on the batch pore flushing model fits of the col-
umn testing results, the calculated number of pore volumes
to achieve the groundwater uranium standard was between
5 and 20 pore volumes (with an average of 14 pore vol-
umes). The updated plume-scale batch pore flushing model
results show that groundwater standards may be achieved
under natural flushing conditions within 100 years for the
ammonia plume, but longer than 100 years may be needed
for the uranium plume. The plume-scale batch pore flush-
ing model does not account for the complex uranium geo-
chemical processes identified during this investigation or
for heterogeneous hydrogeological conditions. Results of
this investigation support development of a three-dimen-
sional numerical groundwater fate and transport model
that accounts for these geochemical processes to improve
the accuracy of remedial timeframe estimates.
REFERENCES
[1] Antelmi, M., et al. 2020. “Analytical and Numerical
Methods for a Preliminary Assessment of the
Remediation Time of Pump and Treat Systems.”
Water 12: 2850. August 10.
[2] Domenico, P.A. 1987. “An analytical model for mul-
tidimensional transport of decaying contaminant spe-
cies.” Journal of Hydrology 91: 49–58.
[3] United States Department of Energy (DOE). 2003.
Site Observational Work Plan for the Moab, Utah,
Site. GJO-2003-424-TAC. December.
[4] DOE. 2012. Moab UMTRA Project Northeastern
Uranium Plume Investigation Report. DOE-EM/
GJTAC2020. January.
[5] DOE. 2023. Moab UMTRA Groundwater and
Surface Water Monitoring Report January through
June 2023, Revision 0. DOE-EM/GJRAC3106.
October.
[6] Salome, K.R., Beazley, M.J., Webb, S.M., Sobecky,
P.A., and Taillefert, M. 2017. “Biomineralization of
U(VI) phosphate promoted by microbially-mediated
phytate hydrolysis in contaminated soils.” Geochim.
et Cosmochim. Acta. 197, 27–42.
[7] Tessier, A., Campbell, P.G.C., and Bisson, M. 1979.
“Sequential extraction procedure for the speciation of
particulate trace metals.” Anal. Chem. 51: 844–851.