Eric,
Radsafers,
As the former lead for the US
Geological Survey's uranium and radon program
(terminated in 1995), I might be able to offer some insights here.
The short answer is that radium has very
low solubility under most shallow ground water conditions, uranium has varying
solubility, and radon is not dependent on solubility but rather on the
concentration of radium on fracture or sediment surfaces in the aquifer and the
fraction of radon kicked into the adjacent volume of water (or air) when alpha
recoil occurs as radium decays. Thus a granite with uranium-enriched
fracture surfaces will likely yield high levels of radon to the water in those
fractures, but will only yield high concentrations of uranium if uranium is
soluble in the water. Hence correlations between uranium and radon
are likely for only wells in limited geologic and hydrologic domains with
similar conditions.
Here is the long
answer:
Naturally occurring uranium, radium, and radon behave
somewhat differently in the environment dependent on differences in their own
chemistry and differences in the solid and aqueous geochemistry of
the local rock, sediment, or soil and the ground waters they occur in. In general, uranium is
mobile under oxidizing conditions because the U +6 ion (the oxidized species of
uranium) is highly soluble and is readily complexed by anions that commonly
occur in oxidizing waters (bicarbonate,
for example). Uranium in reduced form, the U +4 ion, is very
insoluble. Similarly, thorium, which
forms only Th+ 4 ions, is very insoluble.
Uranium can be adsorbed from solution under oxidizing conditions by iron
oxyhydroxides ("rust"), organic matter (both humic substances and living/dead
microorganisms), and other agents. Adsorption thus limits uranium's
mobility where such agents are present as they commonly are. However, high concentrations of complexing agents can
keep uranium in solution.
In the
near surface environment, rocks of all kinds undergo physical and chemical
weathering. Rainwater is somewhat acidic and has elevated oxygen content
and will oxidize and move uranium readily. Uranium is held in rocks in
minerals that are either very resistant to weathering or are readily destroyed by weathering or something in between. Uranium may also
be present on mineral grain surfaces or in fractures where it is readily
oxidized and leached by ground water during the early stages of
weathering. Uranium that becomes mobilized may move extremely short
distances or rather long distances, but where adsorbed, it ends up in mineral coatings
on fractures in rocks or grain surfaces in
sediment, with the water it was derived from very close
by.
Thus as rocks weather in the near surface environment, some uranium will
become mobile and some remains with the weathered rock, or soils and sediment
formed from components of the weathered rock. Thorium in rocks occurs
locked in minerals that don't weather easily or, where it is reachable by
groundwater, it has very limited solubility. Thorium-234 that forms
by radioactive decay from U-238 in solution immediately precipitates out or is
sorbed to mineral grain surfaces, suspended particulate matter, or colloids in
solution. Ultimately, all uranium and
thorium ends up in the oceans, carried there in solution, on suspended matter,
or in sediment carried along near the stream
bottom.
Radium
has limited solubility and mobility under near-surface oxidizing
conditions. It tends to remain locked in minerals or in the weathering
products of those minerals. It is readily sorbed from solution by a
variety of mineral surfaces. Some
radium can be transported by suspended matter or colloids. It will
coprecipitate with various very insoluble minerals where sulfate is
present. Only under somewhat unusual conditions is radium somewhat
mobile- for example, highly saline, chloride-rich reducing waters found in
oilfields.
Radon
is generally chemically inert but responds to various physical processes.
Radon can become mobile when alpha recoil during its formation kicks the radon atom from a mineral
grain or grain surface into a water-filled or air-filled pore. The
fraction of radon that escapes to the pore is called the emanation
fraction. Once in the pore it can move by diffusion or advection.
Radon can be sorbed to certain high-surface-area materials such as activiated carbon.
Note that where the predecessors of radon have been concentrated along fractures
in weathering rock or on the surfaces of mineral grains in sediment or soil, the effective
concentration of radon (or uranium) available to water- or air-filled pores is
much higher than the original concentration of uranium in the parent unweathered rock.
In
summary, under near-surface, oxidizing conditions the following occurs
(only relatively long-lived isotopes are considered):
U-238- mobile
Th-234- immobile
U-234
mobile
Th-230- immobile
Ra-
226- immobile
Rn-222- mobility controlled by alpha recoil and
physical proximity to a water- or air-filled fracture
All
Th-232 decay series isotopes are immobile under near-surface oxidizing
conditions down to Rn-220, which has mobility similar to
Rn-222.
Note that the residence time of the uranium on the
walls of a fracture or soil grain surface and the 1/2 life of the decay
products has a role in the accumulation of radium and radon on the surface and
their subsequent availability to the water or air in the adjacent
opening. Freshly sorbed or precipitated uranium has little ingrown
radium or radon to contribute. Significant radium and radon ingrowth
doesn't occur for 20,000 years and secular equilibrium is approached at about
300,000 years (when the activity of the radium and radon about equals that of
the uranium present).
Note that the concentration of uranium in the water in
a rock fracture or soil pore is dependent on the concentration of uranium on the
fracture or pore surface, the ratio of the fracture or pore volume to the
surface area of the fracture or pore and the rate of dissolution and
reprecipitation of uranium (the equilibrium condition). The concentration
of radon in the water or air of a rock fracture or soil pore is dependent on the
concentration of radium on the fracture or pore surface, the ratio of the
fracture or pore volume to the surface area of the fracture or pore,
and the emanation rate of the radon. Thus the uranium or radon
concentration in water in a rock with open fractures or soil or sediment
with high porosity will tend to be less that that of a rock with tight fractures
or soil or sediment with little porosity.
Maine geology is
dominated by metamorphic rocks and granites with a widely distributed veneer of
glacial deposits. Much of the
rest of New England is similar. The granites of
Maine and adjacent New Hampshire are
generally the most uranium-enriched rocks of
the area although some of the high-grade metamorphic rocks are also
enriched. The uranium content
of New England granites varies as freported
in the following abstract:
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
...eleven
selected granites, predominantly from SE New England, were measured by gamma-ray spectroscopy....
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......
Prior to glaciation (12,000-13,000 years ago and
older), the granitic and metamorphic rocks of New England were extensively
physically and chemically weathered. It seems likely that uranium was
distributed in these rocks based on the principles outlined
above. The distribution of uranium in granites and granitic
metamorphic rocks in the southern Piedmont of the southeastern U.S. (which has
not been glaciated) has been studied to some degree and may provide some
insights to Maine and New England prior to glaciation. Where a granite has
a high percentage of readily mobilized uranium, uranium becomes leached from the
highly weathered part of the granite close to the surface and can become
concentrated on fracture walls near the weathering "front" at some depth within
the granite. Alternatively, if uranium (with accompanying thorium) occurs
mostly in highly resistant minerals in the granite, uranium may accumulate in
near surface residual soils or in stream deposits and very little uranium moves
about in the ground water. Granites coevring the spectrum in between
also occur.
When glaciers covered most of Maine and New England,
the bedrock was subjected to extensive "scraping". Weathered and
unweathered bedrock was incorporated into the basal ice and transported varying
distances to points (mostly southerly) and deposited as till. Till can
vary dramatically in grain size from clay to boulders. During melting of
the glaciers some of this till was reworked into coarse gravelly deposits.
Locally glacial lakes may have formed and finer grained sediments deposited in
them.
Groundwater in Maine can be derived from the more
permeable glacial deposits, the reworked coarse gravelly material, and from
fractures in bedrock. Where the glacial deposits and reworked material are
derived from uranium-enriched granites or metamorhphic rocks, they are likely to
be enriched in uranium themselves. Waters in these various glacial
deposits may have elevated uranium concentrations and radon levels where
the glacial deposits have elevated uranium and radium concentrations on
grain surfaces and high emanating fractions. Because glacial deposits
typically represent a blended melange of rocks derived from up-ice sources it is
unlikely that extreme radon or uranium concentrations will be found in aquifers
hosted by them. It seems likely that highest uranium concentrations in
ground water occur in areas where uranium-enriched granites with a high
fraction of leachable uranium occur and where old pre-glacial deep weathered
zones have been preserved from glaciation. Water supplies tapping
fractures in such rocks can have high uranium if the equilibrium conditions
favor uranium mobility and will have high radon in any case because radon is
independent of uranium equilibrium conditions. In some
areas there is an inverse correlation observed between well yield
(lower well yields may mean tighter bedrock fractures) and radon or uranium
concentrations in bedrock ground waters. This is probably related to
the fracture volume to fracture surface area ratio noted above.
Jim Otton
Energy Program
U.S. Geological Survey
-----Original
Message-----
From: owner-radsafe@list.vanderbilt.edu
[mailto:owner-radsafe@list.vanderbilt.edu]On Behalf Of Frohmberg,
Eric
Sent: Thursday, May 22, 2003 2:11 PM
To:
Radsafe@list.vanderbilt.edu
Subject: Uranium in GW correlations w/
Radon, etc.