[ RadSafe ] Question on total river water activity

Dan W McCarn hotgreenchile at gmail.com
Thu Mar 10 00:43:23 CST 2011

Hi Jaro:

Here are a few thoughts... As a Shell Oil colleague once suggested as an
answer to all general questions, "It depends!"

For starters, you have to characterize the soils, hydrogeology, surface / GW
hydrology and land use of the area. For seconds, let's break the uranium
decay chain into 2 pieces: From U-238 to Th-230 and from Ra-226 to Pb-206.  

Because the geochemistry of uranium is so different from radium, the
concentrations of U-238 may likely be decoupled and uncorrelated with
Ra-226.  Since the multiple alpha decays have imparted kinetic energy into
the progeny, radium can weather preferentially as can U-234 since they will
be dislocated in the crystal lattice of the uranium-bearing mineral. Uranium
is redox sensitive with the +4-valent state having very low solubility
(10E-12 M) compared to the +6-valent state having 10E-2 to 10E-3 M
solubility.  Thorium tends to very low solubility.  Radium tends to be
adsorbed / desorbed onto clays or organic matter and is much harder to
simply characterize.

So do not count on trying to correlate the two. The de-gassing of Rn-222 in
surface waters controls how much remains in the water and will determine the
balance of radioactivity of the remaining members of the radium decay chain.

Production of phosphate fertilizer does retain uranium unless it is
recovered via solvent extraction, however if the WPA process is used (wet
process phosphoric acid), the gypsum (CaSO4 2H2O) produced usually
co-precipitates most of the radium.  Concentration of uranium may vary
between 75-200 mg/kg (ppm) in phosphate ore.  (As a side note, the uranium
series and the radium series are in secular equilibrium in phosphate ores.)
Uranium geochemistry has pretty fast kinetics. So the runoff from farm land
will reflect this in part. Depending on the transmissivity of the soils,
uranium may (or even likely will) migrate vertically into the soil column
rather than appearing as runoff.  If the soil contains a well-developed
organic-rich humus layer, which is both reducing and has a high
cation-exchange capacity, both uranium and radium may be retained.  

Dan ii

Dan W McCarn, Geologist
108 Sherwood Blvd
Los Alamos, NM 87544-3425
+1-505-672-2014 (Home - New Mexico)
+1-505-670-8123 (Mobile - New Mexico)
HotGreenChile at gmail.com (Private email) HotGreenChile at gmail dot com

-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Jaro Franta
Sent: Wednesday, March 09, 2011 19:05
To: radsafe at health.phys.iit.edu
Subject: [ RadSafe ] Question on total river water activity

Dear Radsafers,

Taking the example of a large river like the St. Lawrence, what would be the
total river water activity discharge rate ?

Eisenbud's Environmental Radioactivity says that typical "surface water"
Radium activity (only!) ranges from 4 - 19 Bq/m^3
It doesn't differentiate between rivers or lakes as "surface water".

The St.Lawrence has a flow rate of 9,850 m^3/sec = 851E6 m^3/day or 0.3E12
m^3/y -- yielding about 3 GBq/y of Radium.

How much activity would the other uranium decay chain nuclides add to this
rough estimate ?

Also, would there be significant seasonal changes due to fertiliser runoff
from agricultural areas ? (phosphate fertiliser contains elevated
concentrations of uranium & daughters).

Finally, are there any published surveys that provide detailed statistics ?

Thanks in advance.


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