RE: Uranium in GW correlations w/ Radon, etc.

["Jim Otton" <jkotton@usgs.gov>]

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 under the local conditions.  Hence correlations between uranium and

radon are likely for only wells in limited geologic and hydrologic domains

with similar conditions.  Unless the uranium has only recently been

emplaced, high uranium wells will often be high radon wells.  Positive

correlations between radium and uranium in water are unlikely under most

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 reported 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.







Hey all,



I'm a toxicologist with the Env. Tox. Program in the Maine Bur. of Health.

In Maine we have a problem with U in well water - not a horrible problem

overall - maybe a few percent of wells above 20-30 ug/l, but what makes my

palms sweat is the occasional wells in the 2,000+ ppb range.  We know that

kidney tox is the issue (altho you do get some cancer risk at these high

concentrations - especially when you take into account disequilibrium).

(Obviously, we tell them to take corrective actions).



But what I want to look into is correlations between U and radon or radium

in well water (I should say this is a little bit academic - we have a radon

issue in Maine as well - our recommendation is that everyone do a radon

test, there doesn't seem to be a lot of radium in wellwater in Maine).

While I'm waiting for our in house data to appear, I thought I'd ask the

collective wisdom in radsafe land.  I haven't found a lot in the literature

that suggests decent correlations (it really seems to depend a lot on local

geology, etc.). Do any of you have any direct experience or opinions?  In

particular, I'm more interested in correlations between U and radium in GW

at these higher levels.  Thoughts?



Thanks,

Eric



Eric Frohmberg

Toxicologist

Key Plaza, 8th Floor

11 State House Station

Augusta, ME 04333-0011

Tel: (207) 287-8141

FAX: (207) 287-3981

TTY: (207) 287-8066

eric.frohmberg@state.me.us



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