RE: Radon profiles in the ground/soil

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

Bjorn,Radsafers,

USGS geoscientists and researchers from many other institutions have

evaluated the variations in soil-gas radon concentrations with depth and the

many factors that influence soil-gas radon generation and movement through

soils.

In soil-gas sampling conducted by various USGS researchers, we typically

drove soil-gas probes into the ground 3/4 to 1 m to reach depths where the

soil-gas radon concentrations were consistently not influenced by the

diffusion effect.  This worked for all but the most permeable soils (coarse

sandy soils or sand and gravel soils on the floodplain of a big river, for

example).  See references and a suggestion for finding webpages below.



Caves are notorious for having elevated radon levels.  The radon activities

in cave air was first looked at in the 1970s and the National Park Service

and various States started limiting time underground for tour guides.  In

some parks cave air was being used for summertime cooling of buildings.

That practice was stopped. The highest levels of airborne radon that I am

aware of come from caves on the Wyoming-Montana border where levels reach as

much as 3,000,000 pCi/L.  These caves have uranium minerals coating the

walls and accumulating in sediment on the cave floor.  Old uranium mines in

SE Utah have been gated off from human access to prevent human exposure to

radon and falls into shafts, and to protect the many bat colonies that now

live in these features.  I am unaware of any studies that have tried to

evaluate the effect of soil-gas or cave radon on indigenous species.



A search on Google for "radon, soil, gas," yields many good results

including some pages with mathematical models.

Rogers and Associates Engineering developed a radon emanation and transport

model (RAETRAD) that models transport through soils and entry into

structures.  This model has been used in various radon programs including

the Florida Radon Research Program.



Here are some of the papers that deal with the soil-gas radon topic

including Allan Tanner's summary papers (with abstracts where available).



Tanner, Allan B., 1964

Radon migration in the ground, a review, Chap. 9, in Adams, John A.S., and

Wayne M. Lowder, The Natural Radiation Environment:

Chicago, Chicago Univ. Press, p. 161?190



Tanner, Allan B., 1980

Radon migration in the ground: a supplementary review, in Natural Radiation

Environment III, Gesell, Thomas F., and Wayne M. Lowder, eds.:

Springfield, Va., NTIS, U.S. Dept. Energy Rept. CONF?780422, Vol. 1, p.

5?56.

...Water is the most important agent in enabling radon isotopes to escape

from solid material: Water absorbs kinetic energy of the recoil atom of

radon; it is an active agent in altering and hydrating mineral surfaces,

thus enhancing their emanating power; and it decreases the adsorption of

radon on mineral surfaces.  Once in rock and soil pores, radon atoms migrate

by diffusion and by transport in varying proportions.  In diffusion and

transport calculations, it is desirable to use the radon concentration in

the interstitial fluid as the concentration parameter and to include

porosity explicitly.  Most values of diffusion coefficient in the literature

are for an effective diffusion coefficient, equal to the true diffusion

coefficient divided by porosity, and should be corrected before applying

boundary conditions.  The transport component is important in dry, permeable

soils in the upper layers, but is much reduced below depths of several tens

of meters.  Research in disequilibria among radionuclides of the uranium and

thorium series suggests that much assumed migration of 222Rn is, in fact, a

more general migration of uranium and radium isotopes.



Tanner, Allan B., 1986

Geological factors that influence radon availability, in Indoor Radon,

Proceedings of the APCA International Specialty Conference , Philadelphia,

Pa., February 24?26, 1986:

Air Pollution Control Assoc., P.O. Box 2861, Pittsburgh, PA 15230, Pub.

SP?54, p. 1?12

There is great need for the characterization of localities with respect to

their potential for supplying radon to structures, especially where the

number of existing structures is insufficient for valid sampling.  Although

the interaction between a structure and the ground is not quantitatively

known, it is practical to assume that the ground can be characterized as to

the rate at which radon can be drawn from it by a structure: a measure of

radon availability.  Radon availability is related mainly to the

concentration of radon in the spaces in rock fractures and soil pores and to

the permeability of the ground to gases.  The fraction of radium

disintegrations producing radon that reaches those spaces usually falls

within the range of 0.15 to 0.55.  Permeability and the diffusion

coefficient are reduced markedly as the sizes of those spaces are reduced

and as the proportion of the spaces filled by liquids is increased.

Coupling this knowledge with that of the geology, soil, hydrology, and

topography of a locality should permit qualitative evaluation of radon

availability.  Rock types that usually have above-average concentrations of

radon in the pore and fracture spaces include granites, some gneisses,

phosphatic rocks, marine shales, and some recrystallized limestones and

dolomites.  Construction of buildings in contact with such rocks, if

fractured, requires special radon barriers.  Residual soils, notably terra

rossas, are often enriched in radium.  Ground with coarse grain size (such

as gravels and coarse sands), particularly if well-drained, is highly

permeable and apt to make more radon available than would be expected on the

basis of its radium content.  At the other extreme, muds and clays tend to

be of low permeability, especially if wet.  Ground that does not pass a

percolation test should have low radon availability unless enriched in

radium.  Buildings located on hillsides and ridges are more apt to be

located on soils that are coarser and better drained than those in adjacent

valleys.  Other things being equal, radon availability should be greater on

hillsides and ridges.



Tanner, Allan B., 1990

The role of diffusion in radon entry into houses, in The 1990 International

Symposium on Radon and Radon Reduction Technology, Atlanta, Ga., 19-23

February 1990:

Preprints, Vol. III, no. V-2, 9 p.

Pressure-driven flow of radon-bearing soil gas is commonly accepted as the

usual mechanism whereby radon moves from outside house foundations to cause

elevated indoor radon concentrations.  It is less clear how radon moves to

the backfill-and-subslab zone just outside the foundation.  Fourteen houses

having elevated indoor radon concentrations were investigated by the U.S.

Environmental Protection Agency and its contractors.  The permeability of

the ground to gas flow was measured next to and several meters from each

house foundation.  For 6 of the 14 houses none of the intrinsic permeability

values exceeded 7.6x10-12 m2, below which diffusion is likely to be the

dominant mechanism of radon movement.  Because it can be significant in

unsaturated soils of moderate-to-low permeability, diffusion should not be

ignored in considering radon movement to house foundations.



Rose, A.W., J.W. Washington, and D.J. Greeman, 1988

Variability of radon with depth and season in a central Pennsylvania soil

developed on limestone:

Northeastern Environmental Science, 7(1): 35-39.



Gundersen, L.C.S., The Effect of Rock Type, Grain-Size, Sorting,

Permeability, and Moisture On Measurements of Radon in Soil Gas - a

Comparison of 2 Measurement Techniques, Journal of Radioanalytical and

Nuclear Chemistry-Articles, 161 (2), 325-337, 1992.

Soil surveys of radon conducted in the Coastal Plain of New Jersey, Alabama

and Texas indicate that soil composition and grain size exert the strongest

control on the concentration of radon measured. Soil-gas radon was measured

in-situ using two techniques; one developed by G. Michael REIMER of the U.

S. Geological Survey; the other developed by Rogers and Associates

Engineering Corp. for use by the Environmental Protection Agency. The Reimer

technique aquires a small-volume, grab sample of soil gas, whereas the

Rogers and Associatess technique acquires a large-volume, flow-through

sample of soil gas. The two techniques yield similar radon concentrations in

well-sorted sands, but do not correlate as well for poorly sorted soils and

clays



Marvin, Richard K., Richard R. Parizek, and Arthur W. Rose, 1988

Effects of water table fluctuations on radon-222 concentration and mobility

in overlying soil [abs.]:

Geol. Soc. America, Abstracts with Programs, 20(7): A354

In order to determine the effect of water table fluctuations on the

concentration and potential transport of radon in overlying soil, four

soil-gas monitoring stations were installed along a floodplain near State

College, Pennsylvania.  Each station consists of a nest of sampling tubes

arranged in a backfilled borehole.  Gas samples can be extracted at 1-m

intervals at depths <7 m.  During October 1987, an 11,000-m2 area near one

of the gas sampling stations was flooded by water during a 72-hour pumping

test.  The well was tested at a rate of 4,000 L/min and all the water was

recycled to the water table during the test.  This resulted in a 3-m

water-table rise over 3 days.  The rapid rise in the water table elevation

produced an increase in radon concentration from 450 to 1500 pCi/L at depths

from 1 to 4 m.  The subsequent decline of the water table resulted in a

decrease in radon concentration s from 1500 to 50 pCi/L.  The concentration

increase is due to radon-enriched soil gas from micro and macro pores

migrating away [upwards] from the advancing wetting front.  The

corresponding [subsequent] decrease is produced by the flux of fresh air

into the soil as the pores dewater.  Both the increase and decrease in radon

concentrations are significant at the 99% confidence level when compared

with concentration changes over the data collection period from September

1987 to May 1988.  Gradual seasonal water-table fluctuations (<0.1 m/day) do

not appear to affect the radon soil-gas concentration profiles.



Asher-Bolinder, Sigrid, D.E. Owen, and R.R. Schumann, 1989

Soil-characteristic and meteorologic controls on radon in soil gas in the

semiarid western United States [abs.]: Eos, Am.  Geophys. Union Trans.,

70(15): 497

The interaction between soil characteristics and meteorologic events affects

soil structure and soil moisture.  In turn, soil structure and moisture

interact to influence both radon transport and emanation within the soil.

Unirrigated smectitic clay-loam soil on a terrace deposit in Lakewood,

Colorado, was monitored daily for soil?gas radon at depths of 50, 75, and

100 cm.  Radon data were integrated with hourly weather data, and

tensiometers measured soil moisture at study depths.  Soil moisture during

spring, summer, and fall ranged from 10 to 30 weight percent.  Radon

concentrations varied by an order of magnitude throughout the year;

day?to?day variations of 100-200 percent were controlled mainly by

barometric pressure changes.  Transitory soil?moisture caps blocked soil-gas

exchange with the atmosphere and also raised radon content.  Larger seasonal

radon variations were controlled primarily by interaction between soil

cracks when present, and radon emanation, both of which were controlled

mainly by soil moisture.  Grain-to-grain permeability is limited in this

smectitic soil.  Insolation and evaporation due to wind both control

soil-crack development.  In summer to early fall soil cracks (as deep as 90

cm and as wide as 2 cm at the surface and 0.1-03 cm at depth) developed in

the soil.  Crack polygons 15?25 cm across persisted through summer and fall

except when precipitation temporarily healed cracks at the surface.  Radon

concentrations at 100-cm depth in soil with surface cracks >2 mm wide were

nearly half the concentrations noted when no cracks were present.  Thus,

atmospheric air diluted soil gas at depth in deeply cracked soil.  Emanated

radon from the six soil horizons varied by an order of magnitude, depending

on water content.  Soil horizons contained 2.0-2.8 pCi/g radium and emanated

<100 to >1000 pCi/L-kg soil of radon in laboratory study.  Radon emanation

was maximized when water was 9-17 weight percent of the sample.  Temperature

effects were not discernible at 5, 20, 36°C, characteristic of much of the

yearly weather range.  Samples at -18°C, the low temperature extreme, showed

lessened apparent emanation because ice within soil pores blocked transport

from the soil.



Asher-Bolinder, Sigrid, Douglass E. Owen, and R. Randall Schumann, 1990

Pedologic and climatic controls on Rn-222 concentrations in soil gas,

Denver, Colorado:

Geophys. Research Letters, 17(6): 825-828

Soil-gas radon concentrations are controlled seasonally by factors of

climate and pedology [soil character].  In a swelling soil of the semiarid

western United States, soil-gas radon concentrations at 100?cm depth

increase in winter and spring due to increased emanation with higher soil

moisture and the capping effect of surface water or ice.  Increased soil

moisture results from a combination of higher winter and spring

precipitation and decreased insolation [solar heating] in fall and winter,

lowering soil temperatures so that water infiltrates deeper and evaporates

more slowly.  Radon concentrations in soil drop markedly through the summer

and fall.  The increased insolation of spring and summer warms and dries the

soil, limiting the amount of water that reaches 100 cm.  As the soil dries,

radon emanation decreases and deep soil cracks develop.  These cracks aid

convective transport of soil gas, increase radon's flux into the atmosphere,

and lower its concentration in soil gas.  Probable controls on the

distribution of uranium within the soil column include its downward

leaching, its precipitation or adsorption onto B-horizon clays, concretions,

or cement, and the uranium content and mineralogy of the soil's granitic and

gneissic precursors.



Rose, Arthur W., and John W. Washington, 1989

Controls of seasonal variability in Rn content of soil gas [abs.]:

Geol. Soc. America, Northeastern Section, Abstracts with Programs, 21(2): 63

Measurements of soil gas radon over a one-year period at five sites in

central Pennsylvania show a large seasonal variation.  The radon (222Rn)

concentrations reach values of 1500 to 4000 pCi/L in summer at depths of a

meter or more.  Winter values are only 1/3 to 1/10 of the summertime high

values.  The seasonal decrease is most pronounced in deep soils (greater

than 1 meter).  In summer the depth profiles approximate a diffusional-type

pattern, but during other periods of the year the depth profiles are

irregular, often showing peaks in the middle part of the profile.  Thoron

(220Rn) shows patterns over time similar to 222Rn.  Depth profiles of thoron

are not clearly diffusionally controlled at any time of the year.

Presumably the muting of the diffusion pattern for thoron is related to its

short half-life.  The radon variability both with time and depth correlates

with soil moisture as determined by resistance blocks, tensiometers, neutron

moisture gauge, and visual observation.  In summer the soils are usually dry

due to high evapotranspiration; in late fall the soils are relatively wet

because of low evaporation and perhaps increased precipitation.  The exact

relationship between the low winter radon values and high moisture is not

clear, but appears to involve inhibited diffusion through water-filled pores

or adsorption-like effects.  Several other researchers have reported similar

seasonal effects in humid-region soils, but in semi-arid regions, high radon

is reported in winter or early spring, apparently during periods of high

moisture.  Evidently several processes cause seasonal variability in

soil-gas radon values.



Washington, J.W., and A.W. Rose, 1989

Effects of variation in soil temperature and moisture on radon in soil gases

[abs.]:

Geol. Soc. America, Abstracts with Programs, 21(6): A145

Radon in soil gases partitions between the air and water phases of soil.

The partition ratio, Rn(air)/Rn(water), depends strongly on temperature,

ranging from 1.87 at 0°C to 2.73 at 10°C, 3.88 at 20°C, and 4.93 at 30°C.

For a soil with a given emanation coefficient, radium, and porosity, radon

concentration in the air phase increases by a factor of nearly 4 at 20°C as

the moisture saturation increases from 0 to 100%.  At conditions near

saturation, changes in temperature and moisture can cause large changes in

radon in the air phase.  For example, cooling from 20°C and 99% saturation

to O.l°C and 50% saturation decreases radon in the air phase to about 37% of

its initial value.  In contrast, changes of temperature and moisture in the

dry region have relatively little effect.  Because of these effects, soils

in humid regions that experience sub?freezing temperatures are characterized

by significant seasonal variations in radon content.  This behavior is

observed for some soils from central Pennsylvania which typically are near

saturation and 0°C to 3°C at 0.5 m and deeper during winter, and at 15°C to

20°C with moisture saturation of about 0.8 during summer.  The lowest radon

values are in the winter for these soils, which are dominated by temperature

effects.  In contrast, reported seasonal variations for soils that are less

moist and remain well above freezing typically show highest radon in winter,

interpreted to result from the increased soil moisture during winter.  Radon

entering homes is also expected to vary as a function of climate.



Washington, J.W., A.W. Rose, and D.J. Greeman, 1989

Effect of inhomogeneity of soil properties on radon transport in soil gases

[abs.]:

Eos, Am. Geophys. Union Trans., 70(15): 497

Past concepts and models of radon in soil gas have assumed a depth

distribution determined by diffusion and flow in soil with uniform radium

content, emanation coefficient, porosity. diffusion coefficient, moisture

content, and permeability.  Measurements for six soil profiles in

Pennsylvania and North Carolina show large variations in these properties;

also, variation of moisture content with time causes variation of other

properties with time.  Field permeability measured with a 10-cm diam. ring

pushed 10 cm into pit walls commonly varies by xlO to x50 within a profile.

The A horizon usually has much higher permeability than the B horizon.

Diffusion coefficients measured in the field show a similar large range,

correlating with permeability.  Radium commonly varies by x2 within

profiles.  Moisture content varies widely with depth, and is generally much

higher in winter than in summer.  Emanation coefficients measured in the lab

at a range of moisture tensions show values as low as 0.03 for

near?saturated conditions, compared with values of 0.10 to 0.40 for

disaggregated soils suspended in water.  Both models and observations of

radon activity in soil gas reflect these inhomogeneities by complex vertical

profiles that change with time.



Happy Holidays to all.



Jim Otton

U.S. Geological Survey



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