[ RadSafe ] Question on total river water activity

[Dan W McCarn]

Dear Jaro:

Reasonable? Heck yes! But "It depends"...

My experience with ground waters is that measurements frequently demonstrate
significant disequilibrium between the U-238 and Ra-226 because of the
geochemical differences.  I would expect chemical uranium to be more
abundant because of the adsorption of Ra-226 onto surfaces preventing it
from staying mobilized, including the clay particles in river water. 

An example would be if the uranium is being weakly oxidized / mobilized from
a sandstone with significant interstitial clay, the radium is frequently
adsorbed onto the clays and retarded, thus causing disequilibrium.

But, in general, yes! If you measure it, it must be true!

The only time that I would really expect U-238, U-234 and Ra-226 to be "in
equilibrium" would be if a uranium-bearing mineral was completely dissolved
in a strong acid such as H2SO4.  Weak oxidation (I'll add "almost" here to
provide a caveat) always leads to disequilibrium, even between U-238 and
U-234.


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 Franta, Jaroslav
Sent: Thursday, March 10, 2011 12:43
To: The International Radiation Protection (Health Physics) MailingList
Subject: Re: [ RadSafe ] Question on total river water activity

UNRESTRICTED | ILLIMITÉ

Thanks again Dan -- very educational, and much appreciated !

The 1-3 ug/L uranium in river water seems like a reasonable figure (1-3E-6
kg(U)/m^3 water).

But when combined with a U238 specific activity of 12 MBq/kg(U), it comes to
about two times the surface water Radium activity ( "from 4 - 19 Bq/m^3").

Is that reasonable ?

Jaro
^^^^^^^^^^^^^^^^^^^^




-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Dan W McCarn
Sent: March 10, 2011 12:23 PM
To: 'The International Radiation Protection (Health Physics) MailingList'
Subject: Re: [ RadSafe ] Question on total river water activity

Hi Jaro:

You may or may not be correct about the concentration in terrestrial waters
versus marine waters. And it is important to remember the mechanisms for
weathering, transport and fixation of uranium overall.  In general,
terrestrial surface waters act as a transport medium to ocean waters, and
circulation of water in oceans tends to concentrate uranium in (primarily)
two types of marine sediments: marine black shales and phosphorites.
Examples of the larger group, the black shales, include the Kolm shale of
Sweden, the Chattanooga shale and the mid-continental Pierre shale.  Both
contain uranium in the 75-200 mg/kg range. Seawater contains rather low
concentrations of uranium - about 3 ug/L.

Terrestrial waters also contain low concentrations but the concentrations
may be insufficient to explain the 3 ug/L concentration in seawater.  As a
geologist / geochemist, I consider 10 ug/L to be quite "elevated" in
groundwater, and I would expect that the concentration in river waters is
around 1-3 ug/L.  At least one paper on Indian rivers,
http://www.springerlink.com/content/qw072388060k1077/
indicates that seawater concentrations are higher than river waters.  The
concentration of Mississippi River water at New Orleans, for instance, is
about 2 ug/L.  Concentrations in the Amazon River are significantly lower
than seawater concentration, perhaps by an order of magnitude.  On first
flush, I would have thought that the concentration of uranium in river
waters would be greater than seawater, but the initial evidence that I have
found do not suggest that.  I am at a loss to explain the difference, but
other mechanisms are present, e.g. evaporation, other anions which complex
with uranium, volcanic & other igneous rock in contact with seawater, which
may supply uranium.

One possibility for the elevated concentration is the presence of elevated
fluoride in seawater versus terrestrial waters.  Bicarbonate (HCO3-) is the
most important anion that complexes with uranium in surface waters (U+6 in
oxidizing waters), but fluoride ion very effectively mobilizes uranium in
aqueous systems (U+4 and U+6) and it's presence may prevent precipitation
below 3 ug/L since U+4 can be in aqueous solution when fluoride is present.
Terrestrial surface waters contain low concentrations of fluoride (0.01-0.3
mg/L) whereas ocean waters contain about 1.3 mg/L fluoride.  This may be
sufficient to maintain a higher concentration in seawater, but that is only
my speculation.  Note that phosphorites also contain elevated concentrations
of fluoride as well as uranium.

Note that there is reason to believe that the uranium series may not be in
equilibrium in terrestrial waters because of the relative "ease" that U-234
is weathered in weakly oxidizing environments relative to U-238.  I would
expect the U-234/U-238 activity ratio to be slightly higher than 1. 


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 Franta, Jaroslav
Sent: Thursday, March 10, 2011 06:45
To: The International Radiation Protection (Health Physics) MailingList
Subject: Re: [ RadSafe ] Question on total river water activity

UNRESTRICTED | ILLIMITÉ

Thanks Dan,

Regarding U activity in river water, I figure its constrained to a minimum
of at least several times the level in ocean water -- because that's where
it comes from, right ?

Jaro
^^^^^^^^^^^^^^^^^^^^



-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Dan W McCarn
Sent: March 10, 2011 1:43 AM
To: 'The International Radiation Protection (Health Physics) MailingList'
Subject: Re: [ RadSafe ] Question on total river water activity

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 TBq/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.

 Jaro
^^^^^^^^^^^^^^^^^^^^^^^^^


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