Re: tritium concentration in Hanford groundwater

["Dukelow, James S Jr" <jim.dukelow@pnl.gov>]

Water Cofer questioned some aspects of my back-of-the-envelope calculation of
dilution of Hanford groundwater by the flow of the Columbia River.  He wrote:

I question your use of 6" as the annual amount of rainfall for the Hanford
watershed.  I attempted to develop a water budget for the entire Columbia River
watershed ... I ended up using 29" (I forget the source) and my professor
commented that he thought that number was too low.

  <snip>

If you are looking at just the Hanford subbasin, 6" may be accurate, but I
suspect it may be too low, and if that's the case, there will be greater
groundwater movement than you have calculated.  So my question is, what was the
source for your annual rainfall number, and how confident are you that it is
accurate?  Also, a 95% evapotranspiration rate sounds high even for a desert
climate, were you able to obtain a solid source for that as well?

I'm not disputing the main point of your submittal, that river dilution may
lower the tritium levels to regulatory limits, but the rate that contamination
moves through the groundwater to the river may be higher than you calculated.

Walter Cofer
Walter_Cofer@doh.state.fl.us


Jim Dukelow responds:

Walter's number of 29" of moisture per year for the Columbia River basin
(excluding the Snake River and its tributaries) sounds pretty reasonable to me.
Most of that moisture is coming from mountain areas of Southwestern British
Columbia, Northern Idaho, Northwestern Montana, and Northeastern Washington,
where precipitation ranges from 40" to 100" per year.  The US Geological Survey
has some lovely multi-colored precipitation maps (available somewhere on the
web) for the Columbia/Snake basin, and the rest of the country too, for that
matter.  That 29" a year is what is producing all of the water in the Columbia
River that is diluting the Hanford groundwater flow.

The Hanford site recharge basin is a different critter.  All of eastern
Washington sits in the rain shadow of the Cascade mountains.  We are the Mostly
Brown part of the Evergreen State.  The Hanford site sits in the further rain
shadow of Rattlesnake Mountain, a five mile long ridge that rises some 2500 to
3000 feet above the rest of the Hanford site along the Western edge of the site.
6" is a pretty good number for rainfall on the central Hanford plateau, where
the processing areas and most of the contamination are.  Rainfall amounts in the
uplands, both on the side of Rattlesnake and to the northwest of the Hanford
site, are higher, with the result that 8" might be a better guess at an overall
site average.  On the other hand, the 500 square miles I used for the size of
the basin is probably too big, by about the same fraction.

Another factor makes my estimate of factor of 400 dilution for low water flow in
the Columbia a significant UNDERESTIMATE.  The areas of high tritium
concentration on site are quite small.  The factor of 400 dilution estimate had
the hidden assumption that all 500 square miles of Hanford groundwater were
contaminated to the same high level.  In fact, the tritium concentrations will
be significantly diluted by surrounding, less contaminated groundwater on its
way to the river.

My calculation did not assume anything or require any assumptions about the rate
of groundwater flow or the rate of contaminant movement in the groundwater.  The
equilibrium assumption I used is essentially "recharge flow into the groundwater
equals groundwater flow out to the river".

The first time I did this calculation, I assumed 50% evapotranspiration.
Subsequently, I saw data in one of the Hanford environmental monitoring reports
indicating that 95% was a better estimate.  The Final Environmental Impact
Statement for Safe Interim Storage of Hanford Tank Wastes uses a figure of 0.04
inches per year recharge for the Hanford 200 Areas.  That is equivalent to a
LOCAL evapotranspiration rate of more than 99%.  The local rate is of interest
because it provides the moisture for carrying contamination down through the 60
to 75 meter deep vadose zone to the water table.  The evapotranspiration rate
for the upland areas is probably lower, but I'm confident that 95%
evapotranspiration for the whole site is reasonable estimate.  At Hanford, you
have a combination of low rainfall, high temperatures and low humidity in the
summer (moisture captured by vegetation in the winter may be put back into the
air during the summer), a deep water table, and vegetation with deep roots that
is adapted to capture most of the available moisture.

Information on Hanford hydrogeology can be found, among other places, on the web
at

nepa.eh.doe.gov/eis/eis0212/eis0212_4.html

in Section 4.2.2 and 4.2.3 of the above-mentioned EIS.

Best regards.

Jim Dukelow
Pacific Northwest National Laboratory
Richland, WA
jim.dukelow@pnl.gov

These comments are mine and have not been reviewed by my management or by the
U.S. Department of Energy.
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