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 last fall for a grad school assignment and had difficulty in obtaining consensus on the basin's annual precipitation levels. I ended up using 29" (I forget the source) and my professor commented that he thought that number was too low. The area is very heterogenous and many factors influence rainfall (topography, wind currents, etc.). 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 number 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 groundwater to the river may be higher that you calculate.
Walter Cofer
Walter_Cofer@doh.state.fl.us
-----Original Message-----
From: Dukelow, James S Jr [SMTP:jim.dukelow@pnl.gov]
Sent: Friday, February 04, 2000 8:39 PM
To: Multiple recipients of list
Subject: RE: tritium contamination of Hanford groundwater
Sandy Perle forwarded to RADSAFE a news article on tritium contamination of
Hanford groundwater:
<snip>
Thursday February 3 10:43 PM ET
High Level of Tritium Found
RICHLAND, Wash. (AP) - A groundwater sample shows the presence of
tritium at 400 times the federal drinking water standard in a
monitoring well 3.6 miles from the Columbia River at the Hanford
nuclear reservation.
An internal Hanford memo, obtained by the Tri-City Herald, says it
could take the underground plume as little as three years or as many
as 30 years to reach the river.
The U.S. Department of Energy's Pacific Northwest National Laboratory
and the state officials will begin conducting additional tests on
Monday at the site.
``We're hopping on this,'' Mike Thompson, DOE's official in charge of
groundwater monitoring, said Thursday.
He hopes to know better in a couple of weeks the concentration of the
tritium and how serious a threat it poses to the river.
A tritium concentration of 20,000 picocuries of radiation in one
liter of water is the federal drinking water limit. The reading at
the monitoring well was 8 million picocuries.
<snip>
JSD comment:
A few years ago I did a back-of-the-envelope calculation to estimate the rate of
groundwater flow off the Hanford site into the Columbia River and the dilution
factor as the groundwater enters the river.
Assuming
a 500 square mile recharge basin,
6" of rainfall per year,
95% evapo-transpiration (i.e., only 5% of the rainfall stays in the ground --
it's a desert here),
and equilibrium conditions (i.e., GW to river = rainfall minus
evapotranspirtation),
gives a flow rate of 11 cubic feet per second groundwater flow to river.
That compares with 117 cubic meters per second low river flow, 3400 cubic
meters per second normal flow, and 19000 cubic meters per second maximum flow.
These three flow rates give dilution factors of approximately 400, 10000, and
100000 respectively for low, normal, and flood flows.
The assumptions on size of recharge basin, rainfall, and evapotranspiration rate
are pretty solid. The assumption of equilibrium conditions is open to question
since for many years Hanford operations discharged large volumes of slightly
contaminated effluent directly to the ground. These discharges are the source
of most of the groundwater contamination and were sufficient to produce water
table "mounds" underneath the two processing areas on the reservations high
plateau. Because of those discharges, which were terminated around 10-15 years
ago, groundwater flow to the river may include a contribution from those water
table "mounds".
These numbers suggest that, even if the high tritium concentrations are not
diluted by surrounding groundwater on their way to the river, the river flow
will be sufficient to dilute the tritium to at or below the federal drinking
water limit. Some mitigation might be required during times of lowest river
flow.
As a resident of Richland, which gets its water directly out of the river a few
miles downstream from the contamination, I have more than a modest interest in
this issue.
Best regards.
Jim Dukelow
Pacific Northwest National Laboratory
Richland, WA
jim.dukelow@pnl.gov
These comments are mine and have not been reviewed and/or approved by my
management or by the U.S. Department of Energy.
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