EIS-0113_Section_11 - Hanford Site

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3.5.2.15
non-conservative in both the drier and wetter climate scenarios, as
discussed below.
Because of soil variability and the difficulty of measuring moisture
flux in undisturbed conditions, great uncertainty exists in pro ject/ng
recharge rates from the areal ly restricted and generally artificial
(lysimeters with reconstituted soils) studies conducted to date. The DEIS
(Vol. 2, page xxvifi) indicates that existing quantitative predictions of
water recharge rates are goad only to within 2 or 3 cm/year. As noted in
volume 1 of the DEIS (tap of page 4,20), the value of recharge under
existing, relatively dry climatic conditions is expected to be resolved
through more sophisticated investigation between 0.5 and 5 cm/yr. The same
range is tentatively proposed by Gee and Heller (1985, page 11 . reference
cited in DEIS appendix M) based an methodology being developed in current
research. Kukla (1979), cited in Appendix M, indicates that present
conditions represent the dry extreme of potential climatic variation.
Therefore, It Is non-conservativefor USDOE to select the low end of this
0.5. to 5.0 cm/yr range in the DEIS as representative of dry climate
conditions.
No actual data exist on recharge under a wetter climate; however,
simulation of wet-climate recharge through a coarse soil was described In
appendix M. Test cases 2 and 7 (Table M.7, page M.20) indicated about 15 to
20 cm infiltration, depending on plant cover, after two years with 30 cm
annual precipitation. The DEIS also states (page M.9, first paragraph),
"The majority of soils and sediments in the vadose zone at Hanford consist
of coarse-textured materials which tend to dram readily.- I view of this
simulation, and the fact that recharge under present dry conditions could be
as much as 5 cm/yr, the assumption of 5 cm/yr average recharge appears
non-conservative for the wetter-climate scenario.
Groundwater Transport of Contaminants. The DEIS (pa?e 0.1) expresses
an intent to incorporate conservatism throughout its model ing analysis.
Allowing for the general uncertainty in soil and transport characteristics,
assumptions appear to be non-conservative in two main areas of the
conceptual model of the basic transport framework. First, the assumption
that hydraulic conductivities can be vertically averaged is non-conservative
with respect to contaminant travel times in the unconfined aquifer. Second,
and potentiallymore significant, assumptions regarding contaminant
retardation, which the authors of the DEIS state (page 0.15) '...cannot be
stated as necessarily conservative,' are in fact made no-conservatively
(see review of Appendix P).
Section 0.4.2 discusses assumptions made for numerical analysis of flow
in the unconfined aquifer. While the assumptions listed on page 0.26
represent great simplification of actual physical conditions, one In
particular appears significantly non-conservative: Vertical averaging of
hydraulic conductivities could result in horizontal travel times that are
too long by an order of magnitude or mere, if large variations in hydraulic
conductivity are present. This averaging in effect ignores aquifer-scale
I Ong I tudinal dispersion, as is indicated at the bottom of page 0.26. A
conservative approach for travel time calculation would use the largest
values of hydraulic conductivity observed. The effect of this assumption is
not large in the final analysis, however.
NumericalModel-Unconfined ppuite, There is uncertainty as to what
type of TRANSS model wasused to the transport modeling. Section 0.4.3.2
states that a stochastic formulation was used which according to
Section 0.4.3.3and its references (Simmons, 1981, 1982) eliminates the
dispersion term by setting . the dispersion coefficient to zero and in its
place, uses a random function for velocity to simulate dispersion. However,
Section 0.4.3.5 states that the transport was determined using the
convective-dispersive equation with a local-scale dispersion coefficient.
These statements are contradictory.
Because hydraulic flow velocities in the saturated zone are so high,
they are relatively unimportant in the overall analysts of contaminant
travel time. We are therefore not overly concerned with the process used in
calibrating the numerical model of the unconfined aquifer.
Unsaturated Flow Model. Transport in the vadose zone can be very slow
so that assumptions made for calculations of unsaturated travel time
(presented elsewhere In the DEIS) are important. Section 0.4.11 describes
the unit hydraulic gradient model used for hand calculating vertical
groundwater travel times in the vadose zone. Use of this model requires
estimating or determining three soil parameters; the saturated moisture
content, saturated hydraulic conductivity, and 'b' value, the tatter
depending in turn on the precise relationship between soil moisture content
and capillary water potential_. These soil parameters would appear from
references cited in the DEIS not to have been characterized with much
precision, especially considering hysteresis and spatial variation among
natural soil sat Hanford.
Under the assumptions used in this model, travel velocity could have
been obtained by simply dividing the assumed infiltration rate by the
estimate average moisture content. It is not clear whether this Is, in
effect, what has been done later in the DEIS to obtain travel times in the
vices. zone. as is suggested by the moisture content assumption at the
bottom of page P.6.
The diffusion controlled transport in the unsaturated zonebeneath the
protective barrier is discussed in Section 0.4.1.3. An assumption is made
that there will be a linear concentration profile throughout the diffusion
zone. Diffusion controlled profiles will be concave and not linear in this
region. It is uncertain whether this assumption is ultimately conservative.
The approach to modeling diffusion in this section is questioned since there
are analytical solutions to.the one-dimensional diffusion equation which
include source decay and contaminant decay which would be more appropriate.
Finally, the diffusion coefficients used (Appendix P) are in some cases not
conservative (see Appendix P review).
3.5.2.9
3.5.2.16
3.5.2.16
3.5.2.17
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