[ RadSafe ] Fwd: [RADONPROFESSIONALS] Radon in granites

Tue Aug 5 10:06:41 CDT 2008

FYI - I see that the HPS has added a response to the "radon from granite countertop" think to their web site.  The link is http://hps.org/newsandevents/societynews.html#6581.  

I'll be honest, I've not been following the whole "radon in granite" issue, so this may have previously been discussed on RADSAFE.

Jeff Brunette

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf Of Philip Egidi
Sent: Friday, August 01, 2008 11:26 AM
To: radsafe at radlab.nl
Subject: [ RadSafe ] Fwd: [RADONPROFESSIONALS] Radon in granites

This is from another listserv and may be of interest to some.
Phil Egidi

>>> Jim Otton <jkotton at usgs.gov> 8/1/2008 8:56 AM >>>
Folks,

Here are two papers on radon in granites and related rocks. One
describes emanation in granites and the other the range of uranium
content in a small area of related but geochemically diverse plutonic
rocks in Massachusetts.   BTW many granites in southwestern Maine and
central New Hampshire are significantly radioactive due to high uranium
concentrations.

These annotated items are from Allan B. Tanner’s US Geological Survey
Open File Report 92-351 “Bibliography of radon in the outdoor
environment and selected references on gas mobility in the ground”
which is available online in three parts through the USGS’ Publication
Warehouse at http://infotrek.er.usgs.gov/pubs/ 

Jim Otton

Uranium, radium and radon specialist 

U.S. Geological Survey

Hon, Rudolph, and Nancy M. Davis, 1989

Determinations of bulk emanation rates of selected granites by gamma
ray spectroscopy [abs.]:

Eos, Am. Geophys. Union Trans., 70(15): 496

Bulk radon (Rn‑222) emanation rates of eleven selected granites,
predominantly from SE New England, were measured by gamma‑ray
spectroscopy on a 16% efficiency REGE detector.  The employed technique
compares specific gamma emissions from isotopes that are precursors to
the radon stages with those that are subsequent to it.  Absolute
abundances are obtained through a calibration of absolute efficiencies
and by an instrumental neutron activation. Uranium abundances vary from
a low of 2 ppm to a high of 46 ppm.  Alkaline and peralkaline granites
show range between 5 and 10 ppm; whereas peraluminous and two- mica
granites give a typical range of 5 to 20 ppm with a few samples yielding
as high levels as 46 ppm.  Radon emanations appear to be only mildly
dependent on the absolute uranium concentrations but are strongly
dependent on the fragment sizes used in our experiments.  Emanation
rates for Rn‑222 in bulk sized fragments (>4 inches average dimension)
yield rates as high as 20 to 25% (in alkali granites) but a more typical
range is less than 10%.  With decreasing fragment sizes to less than 100
μm the emanation rates steadily increase up to 40 to 60% and possibly
even higher.  In contrast the maximum Rn‑220 emanation rates for the
finest fraction are typically less than 10%.  We interpret these results
by a very different mode of distribution of uranium and thorium in these
rocks.  Thorium as a trace element is likely to be found on lattices of
resistant accessory minerals whereas uranium is most likely in form of
uranium minerals along microfractures.  These microfractures then
provide a mechanism for the higher Rn‑222 release.

Loftenius, Christer J., and Rudolph Hon, 1989

Assessment of radon production levels in calc-alkalic plutons [abs.]:

Eos, Am. Geophys. Union Trans., 70(15): 496

Near-surface radon levels in areas associated with either peraluminous
or peralkalic granitic plutons have been well documented by a number of
the previous studies.  However, there is a lack of data for calc-alkalic
plutons which are the more common of the intrusive types.  Our study is
focussed on a calc-alkalic suite of rocks within the Sharpners Pond
Quartz Diorite Pluton of NE Massachusetts.  Geochemical study of 42
samples that includes every lithological type shows a presence of two
rock groups: (1) gabbro‑diorites with average U: 1 ppm (range: <0.5 to
4.5 ppm), average Th: 4.6 ppm (0.4 to 9.6 ppm), and (2) granites with
average U: 5.2 ppm (2.9 to 8.2 ppm), average Th: 16.8 ppm (11.1 to 22.4
ppm).  Uranium abundances fail to correlate with most of the major or
trace elements, which suggests that a significant portion of U does not
appear to be part of the original rock mineralogy, but rather it is in
the form of secondary uranium-bearing minerals along open and healed
microfractures.  Our study of radon emanation rates on 10 selected
samples indicates that the bulk sample radon emanation is highly
variable (5-25%) supported by 0.5 to 1.5 ppm of the total U present in
the rock.  In summary: (1) the absolute levels of radon emanation from
the bulk samples of calc‑alkalic intrusives are lower than those from
other types of granites; (2) levels of radon release into a pore space
in soils above such intrusions still pose a risk.  A significant part of
U does not appear to be "locked" in any of the weathering‑resistant
crystal structures (lack of correlation with Zr, rare-earth elements,
etc.) and consequently this U may become free and mobile during the
chemical weathering cycle.

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