Saturated Zone Plumes in Volcanic Rock: Implications for Yucca Mountain Page: 2 of 8
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pulses of radionuclides that might be released,
providing a limit to the degree to which such
pulses can he transmitted to the compliance
boundary.
The processes of advectton, diffusion into
dead-end pore space. sorption onto rock surfaces
and colloid fiacilitated transport are expected to
significantly affect the RTD. and thus have a
first-order effect on SZ performance. The other
processes of potential significance are
longitudinal and transverse dispersion.
Longitudinal dispersion (dispersion in the
direction of fluid flow) spreads radionuclides in
space, and therefore acts to broaden the
distribution of arrival times at a location along
the flow path. however, in comparison to other
processes that push the entire RTD to longer or
shorter travel times, longitudinal dispersion has a
small to moderate effect on SZ performance
Transverse dispersion is the process by which
local heterogeneities cause pathways spread in a
direction normal to the main flow direction. In
contaminant transport studies transverse
dispersion is known to have a significant impact
on the local concentration within a plume
However, because the mass flux at the 1 km
compliance boundary is the relevant metric for
determining the impact of the natural system on
dose, transverse dispersion is expected to have a
small effect on SZ performance. An exception to
this could arise if the lateral spreading of the
plume causes the radionuclides to enter portions
of the aquifer with more strongly retarding
conditions. If this were to occur, transverse
dispersion coupled with retarding conditions
could have a first order effect on the travel time
on a portion of the plume.
The expected flow paths for the radionuclide
transport through the SZ at Yucca Mountain start
in fractured volcanic units near the repository
footprint and transition into alluvium further
downstream. The focus of this study is the
influence of dispersion within the saturated
volcanic units on the breakthrough curves
(BTCs) at the compliance boundary as well as
the effect ofdispersion on plume shapes.
IMPACT OF HIGHER TRANSVERSE
DISPERSIVITY VALUES ON Si
TRANSPORT MODEL
The current Site-Scale St Transport model
[l was used to assess the impact of higher
values of transverse dispersivitics in the volcanic
units on transport pathways and the breakihiaouglh
c! C" obihlincd at the I1 km tiomlit1tancehuanOl I Ie hi cakilII 0ui I I ill i, s IbCLni Cot i
a high value of horizontal transverse dispersn' i
in volcanics (10 m, taken equal to the base cay
value of longitudinal dispersivity) compared i'
the 0.05 m for base case, and for a high value ,!
vertical transverse dispersivity (10 in compared
to the 0.0005 in for base case) are shown in
Figure I along with the breakthrough curve hr
the base case Site Scale SZ Transport model. \
expected. the breakthrough curves are not ver
sensitive to the values of the transverse
dispersivities in the volcanics, and the time tir
breakthrough of 50% of the inlet mass is abno
unchanged. This is due to the Cact that th,
breakthrough curves include the entire man
crossing the boundary that stretches along IlL
entire model domain in a direction
approximately normal to flow. The some" il
earlier breakthrough seen for the case of hil l
vertical transverse dispersivity is due tti olnv
solute dispersing into a region of hll cr Il %k
velocities.
The path lines for nine particle, If a
distributed source with initial locations spiEl1
out over the repository footprint are shown il
Figure 2. Those for a high value of vertical
transverse dispersivity (10 m) are in red, alt,
with the path lines with the same starting
locations for the base case show n in black. Thbe
plumes remain relatively narrow, largely tloe to
the converging nature of the flow field.100
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ase aeI
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Trn Qeant since rudonucides sneed stn,.Figure 1 BTC for high values of transvero
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Kelkar, S.; Roback, R.; Robinson, B.; Srinivasan, G.; Jones, C. & Reimus, P. Saturated Zone Plumes in Volcanic Rock: Implications for Yucca Mountain, report, February 14, 2006; Las Vegas, Nevada. (https://digital.library.unt.edu/ark:/67531/metadc891127/m1/2/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.