The Cesium-137 Method for MeasuringErosion: Case Study in a Close Arid Basin

Item Type Thesis-Reproduction (electronic); text

Authors Hartley, Daniel Robert.

Publisher The University of Arizona.

Rights Copyright © is held by the author. Digital access to this materialis made possible by the University Libraries, University of Arizona.Further transmission, reproduction or presentation (such aspublic display or performance) of protected items is prohibitedexcept with permission of the author.

Download date 11/06/2021 15:43:13

Link to Item http://hdl.handle.net/10150/192123

http://hdl.handle.net/10150/192123

THE CESIUM-137 METHOD FOR MEASURING EROSION:

CASE STUDY IN A CLOSED ARID BASIN

by

Daniel Robert Hartley

A Thesis submitted to the Faculty of the

DEPARTMENT OF HYDROLOGY AND WATER RESOURCES

In Partial Fulfillment of the RequirementsFor the Degree of

MASTER OF SCIENCEWITH A MAJOR IN HYDROLOGY

In the Graduate College

THE UNIVERSITY OF ARIZONA

2004

2

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfilment of requirements for an advanced degreeat The University of Arizona and is deposited in the University Library to be madeavailable to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided thataccurate acknowledgment of source is made. Requests for permission for extendedquotation from or reproduction of this manuscript in whole or in part may be granted bythe head of the major department or the Dean of the Graduate College when in his or herjudgment the proposed use of the material is in the interests of scholarship. In all otherinstances, however, permission must be obtained from the author.

Signed:

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

I i prt..1 2_0-D vVictor Baker Date

Professor of Hydrology

3

ACKNOWLEDGMENTS

This research was funded by the Grand Canyon Monitoring and Research Center and the

Mojave Ecosystem Project, through the U.S. Geological Survey (USGS). Usage of

gamma counters was provided by the University of Arizona Radiation Control Office

(RCO) and the USGS. The USGS purchased a gamma counter to help support this

project.

I wish to thank Dr. Robert Webb for his guidance, support, encouragement, and his

participation in surveying and sampling in the field. Peter Griffiths designed the

spreadsheet algorithm for computing 137Cs activity, and assisted with gamma counting,

sieve analyses, and other tasks. Drs. Victor Baker and Richard Hawkins served on my

thesis committee and reviewed this manuscript along with Robert Webb. Dr. Baker

graciously wrote letters to the Graduate College on my behalf. At the RCO, I received

assistance from Dr. Charles Sondhaus, Dan Silvain, Carl Irwin, and Mike Susalla. Dr.

Satya Harpalani in the Department of Mining and Geological Engineering at the

University of Arizona provided a ball mill for pulverizing and homogenizing selected

samples. The Tohono O'odham Nation's Water Resources Study Program provided the

laptop computers on which most of this thesis was written.

I especially wish to thank my wife, Kathy, for the long hours she spent formatting this text

and for the many sacrifices has she made for this degree, and my mother-in-law, Roz

Eppstein, for her generosity in providing housing for me and my family for two years

while I was in school. This endeavor would not have been possible without them. I wish

to thank my parents, Robert and Nancy Hartley, and my father-in-law, Arthur

Steinbrenner, and his wife, Linda, who also provided financial help.

TABLE OF CONTENTS

LIST OF FIGURES 7LIST OF TABLES 9

ABSTRACT 10

SECTION 1. INTRODUCTION 111.1 TRADITIONAL EROSION MEASUREMENT 111.2 RADIOACTIVE TRACERS 121.3 WHY CEsrum-137? 141.4 RADIATION UNITS 16

SECTION 2. FALLOUT CHARACTERISTICS 172.1 ' 37Cs AND 90SR FORMATION 172.2 137CS TO 90SR RATIO 182.3 CONTINENTAL AND GLOBAL FALLOUT 192.4 CORRELATION WITH PRECIPITATION 222.5 TIME DISTRIBUTION OF 137CS DEPOSITION 24

2.5.1 Start of Cesium Deposition 242.5.2 Fallout Rates over Time 302.5.3 Fallout Accumulation 322.5.4 137Cs Accumulation with Erosion 352.5.5 End of Fallout 37

SECTION 3. FATE OF 137Cs IN SOIL 383.1 ADSORPTION OF 137CS 383.2 DEPTH DISTRIBUTION OF 137CS 41

SECTION 4. USE OF RADIONUCLIDES TO STUDY SOIL MOVEMENT . . . . 504.1 HISTORY 504.2 ASSUMPTIONS 514.3 QUANTIFYING SOIL LOSS 524.4 DATING SEDIMENT PROFILES WITH 137CS 554.5 OTHER RADIOACTIVE TRACERS 57

SECTION 5. SAMPLING METHODS OF OTHER RESEARCHERS 595.1 DEPTH PROFILES 595.2 REFERENCE SAMPLES 60

5.2.1 Site Selection Criteria 605.2.2 Reference Sampling Methods 60

4

TABLE OF CONTENTS - Continued

5.3 SAMPLE PREPARATION 625.4 SAMPLING PATTERNS FOR EROSION MEASUREMENT 63

SECTION 6. METHODS AND TESTING OF METHODS 646.1 STUDY AREA 646.2 SOIL SAMPLING METHODS 68

6.2.1 Selection of Reference Sample Locations 686.2.2 Vertical Depth Profiles 706.2.3 Erosion Samples 71

6.3 SAMPLE PREPARATION 736.4 GAMMA RAY SPECTROSCOPY 76

6.4.1 Gamma Detectors 766.4.2 Sample Geometries and Sample Loading 776.4.3 Count Times 806.4.4 Efficiency Calibrations 806.4.5 Comparisons between Detectors 836.4.6 Peak Width Definition 876.4.7 Background Peak in Radiation Control Detector 88

6.5 POND AND CHANNEL SEDIMENT VOLUME 926.6 BULK DENSITY 936.7 DIGITAL ELEVATION MODEL 94

SECTION 7. RESULTS 957.1 137CS DEPTH DISTRIBUTION 957.2 137CS DISTRIBUTION BY PARTICLE SIZE 1037.3 ' 37Cs DESORPTION 1097.4 EROSION CALCULATIONS FROM POND SEDIMENT VOLUMES 1097.5 REFERENCE SAMPLES 1317.6 I37CS DISTRIBUTION ACROSS SILURIAN BASIN 1 1177.7 137CS IN CHANNEL BED SEDIMENTS 121

SECTION 8. DISCUSSION AND CONCLUSIONS 1268.1 EROSION ESTIMATIONS 1268.2 ASSESSMENT OF THE I37CS METHOD 132

SECTION 9. RECOMMENDATIONS 1359.1 SAMPLING RECOMMENDATIONS FOR ALL 137CS EROSION STUDIES 1359.2 ADDITIONAL INVESTIGATIONS 136

5

TABLE OF CONTENTS - Continued

APPENDICES 137

APPENDIX 1. RADIATION UNIT CONVERSION CHART 138

APPENDIX 2. WORLD-WIDE NUCLEAR EXPLOSIONS THAT RELEASEDRADIOACTIVITY TO THE ATMOSPHERE 139

APPENDIX 3.1. 137Cs GAMMA COUNTING DATA FOR MOJAVE DESERTSAMPLES 156

APPENDIX 3.2 MOJAVE DESERT DEPTH PROFILES AND INTENDEDREFERENCE SAMPLES 162

APPENDIX 3.3 MOJAVE DESERT WATERSHED SOIL SAMPLES, 137CsACTIVITY PER GRAM 163

APPENDIX 3.4. MOJAVE DESERT WATERSHED SOIL SAMPLES, 137CsLOSS/GAIN 168

APPENDIX 3.5. MOJAVE DESERT POND SAMPLES 172

REFERENCES 173

6

LIST OF FIGURES

Figure 1. Estimated trajectories, at several altitudes, of the radioactive cloudresulting from the test CHARLIE of April 22, 1952. 20

Figure 2. Cumulative average global distribution of 'Sr fallout at theend of 1990, by 100 latitude bands 22

Figure 3. Quarterly and cumulative 137Cs deposition in Los Angeles estimatedfrom 90Sr measurements 33

Figure 4. Quarterly and cumulative 137Cs Deposition for El Paso, TX,(1959-1976) and Roswell, NM (1951-1959) 34

Figure 5. Cumulative 137Cs deposition curves with decay for sites listed inTable 2 36

Figure 6. Typical 137Cs depth profiles in undisturbed soil 42

Figure 7. 137Cs depth distribution in the Gold Fork River Basin, Idaho, 1996 43

Figure 8. 137Cs depth profile by Walling et al. (1986) 46

Figure 9. Change in 137Cs depth distribution over time in Croatia afterChernobyl accident. 47

Figure 10. 137Cs depth profiles measured by Lou. bran et al. (1993) 49

Figure 11. Location map showing Silurian Basins and Nevada Test Site 65

Figure 12. USGS Digital Orthophoto Quadrange views of Silurian Basin 1. 66

Figure 13. View of Silurian Basin 1 67

Figure 14. A sample location in Silurian Basin 1 with sampling template in place. 72

Figure 15. Comparison of multiple aliquots from 2 samples 75

Figure 16. A sample gamma-ray spectrum from the USGS well detector. 80

Figure 17. Comparison between two gamma detectors 86

Figure 18 The ' 37Cs energy range of a 20-hour gamma-ray spectrum showing abackground peak. 89

Figure 19. The 137Cs energy range of a spectrum from cesium-free soil showing abackground peak and two additional peaks. 91

Figure 20. Mojave Desert depth profiles in soil, with best-fit regressionequations 96

Figure 21. Percentage of sample that is gravel in Mojave Desert soil depthprofiles 99

7

LIST OF FIGURES - Continued

Figure 22. Depth profile within Silurian Basin 1 100

Figure 23. 'Cs depth profiles of channel bottom sediments in Silurian Basin 1. 101

Figure 24. 'Cs profiles of Silurian basin pond sediments. 102

Figure 25. Comparison of "Cs activity in < 8 mm and

LIST OF TABLES

Table 1. Is the initial 'Cs deposition still detectable today? 28

Table 2. Peak 'Cs fallout times and cumulative peak times at selectedlocations, calculated from monthly "Sr fallout measurements. 31

Table 3. Calibration standards and efficiency calibration results for gammadetectors. 82

Table 4. Comparisons between the USGS well detector and a USGS planardetector in Menlo Park, CA 83

Table 5. Comparisons between the USGS well detector and the RCOdetector 84

Table 6. Sediment yield calculations from measured pond sediment volumesand the percentage of sediments containing 137Cs 110

Table 7. Mojave Desert reference samples to determine total fallout 137Cs . .. 114

Table 8. Percent of samples indicating 'Cs loss of 5% or more. 121

Table 9. 137Cs erosion calculations 129

9

10

ABSTRACT

Fallout 'Cs was used to study erosion in a 1-ha closed arid basin created by a railroad

embankment in the Mojave Desert. A literature review discovered a web site that gives

the start date of Nevada Test Site fallout for any county in the U.S. Calculations showed

that the peak 137Cs concentration in sediment deposits occurred in 1966 when

accumulations in soil were greatest, not in 1963 as is commonly assumed. 137Cs

calculations using some unverified assumptions yielded soil erosion of 10,000 kg over 46

years, compared to an estimated volume of 15,000 kg of reservoir sediments. The highest

erosion rates occurred in the disturbed borrow pit and on steep channel side slopes, while

desert pavement areas had deposition. Two main difficulties with 137Cs erosion estimates

are relating point measurements to larger areas and converting 137Cs loss to soil loss.

11

SECTION 1. INTRODUCTION

1.1 TRADITIONAL EROSION MEASUREMENTS

The traditional approach to the measurement of soil erosion has been to sample sediment

concentrations in runoff in rivers and small runoff plots. This approach requires years of

effort to overcome the tremendous interannual variation in natural hydrologic systems. As

an example, 10,000 plot years of data have gone into development of the empirical

Universal Soil Loss Equation (USLE) and its update, the Revised Universal Soil Loss

Equation (RUSLE) (Osterkamp and Toy, 1997). Rainfall simulators have been employed

on small plots to speed up the process, but they raise the question of how well artificial

rain mimics the erosive properties of natural rain. Most of these data have been collected

in humid agricultural regions. In arid and semi-arid regions where rangeland erosion is a

concern and much of the sparse rainfall comes from localized thunderstorms, sampling

efforts are compounded by the need for personnel to be in the right place at the right time

in order to find any water to sample in ephemeral channels.

Current erosion-prediction technology, based on this traditionally-collected data, is faced

with a problem of scale. Osterkamp and Toy (1997) explain this problem very clearly, and

the following discussion will draw upon their ideas. Most erosion-prediction algorithms

focus on the hillslope, agricultural field, and small watershed scale where the dominant

processes are raimplash particle detachment and overland-flow transport. In larger

watersheds, the processes of gully, channel, and bank erosion, sediment storage, and in

semiarid environments, channel transmission losses (infiltration of runoff into the channel

12

bed) become important. On a continental scale, the annual sediment discharge to North

American coasts is estimated at 600 million metric tons, while an estimated 6 billion metric

tons are eroded annually from the land surface (Osterkamp and Toy, 1997). In other

words, 90% of eroded sediment is stored in fluvial systems before it reaches the sea.

This invokes the concept of a sediment delivery ratio, or the percentage of sediment

eroded from hillslopes that reaches the basin outlet. However, sediment delivery data

comparable to the aforementioned plot-years of data have not been systematically

collected. Also, the understanding of processes that we can gain from traditional sampling

efforts is limited by the fact that sediment sampling at discrete points in a channel system

gives no information on where sediment is being stored in between sampling stations or

for how long, nor on what portion of the sediment comes from hillslope erosion versus

gully or channel erosion. Granted, other methods such as repeat photography, repeat

surveying, floodplain coring, and the application of hydraulic sediment-transport equations

can shed some light on these questions. But two things that would be useful to aid our

understanding of erosion and sediment delivery processes are a method for quickly

gathering long-term erosion data, and a dated tracer for sediment that would allow the

tracking of its movement through the fluvial system.

1.2 RADIOACTIVE TRACERS

With the advent of the atomic age and atmospheric nuclear weapons testing, radioactive

isotopes were introduced into the environment worldwide. When it became known that

13

this radioactive fallout was being spread over wide areas, the public desired to know its

effect on human health. This led to global programs of fallout measurements, such as that

of the U.S. Health and Safety Laboratory (HASL), now known as the U.S. Environmental

Measurements Laboratory (EML) (Hardy, 1977; Larsen, 1984; Larsen and Juzdan, 1986;

Juzdan, 1988; Monetti and Larsen, 1991; Monetti, 1996), and numerous studies on the

interaction of radioisotopes with the environment. Although conclusions on the effects of

low-level radiation on man have been few (Harley, 1976), much knowledge has been

gained about the physical and biological processes that distribute this introduced

radioactivity throughout the environment. Attainment of this knowledge has been aided

by advances in semiconductor technology which have led to the increased resolution of

today's high-purity-germanium gamma ray detectors. This has facilitated the

measurement of low levels of gamma-emitting radioisotopes.

Atomic fission chain reactions produce numerous radioactive isotopes, but most are too

short-lived to be very useful in erosion studies. The most abundant long-lived isotopes are

'Cs and 90Sr with half-lives of 30.07 years and 28.79 years (Firestone and Shirley, 1996),

respectively, and several isotopes of plutonium with half-lives of thousands or millions of

years. About 0.18 megacuries (6.7 Petabequerels (PBq, see Appendix 1)) of 137Cs are

produced for every megaton of fission energy release (Davis, 1963). Carter and Moghissi

(1977) tallied armounced worldwide atmospheric, high altitude, and underwater tests

through 1975 with a total yield of 276 megatons, not including unannounced tests or

underground tests that vented to the atmosphere. Atomic fission also produces 'Cs, but

14

with a half-life of 2.06 years (Firestone and Shirley, 1996), this isotope is only useful for

the first decade or so after fallout deposition. The distribution in soils of all three elements

has been widely studied (e.g. Hardy and Krey, 1971; Hardy, 1974; Miller et al., 1974;

Krey and Beck, 1981; Cox and Faukhauser, 1984; Loughran et al., 1993). "Sr has been

considered the most dangerous of these isotopes because it behaves chemically in a

manner similar to calcium, whereas cesium behaves chemically like potassium and is not as

biologically active. 90 Sr has therefore been the focus of more fallout-measurement and

food-chain studies than has cesium or plutonium.

Naturally occurring radioisotopes have also been utilized as tracers for sediment

movement. 'Be is produced by cosmic rays in the upper atmosphere and has a half-life of

53.4 days. It has been used to identify sediments in a stream channel that are freshly

eroded from the landscape (e.g. Bonniwell et al., 1999; 011ey et al., 1993). 210Pb fallout

results from 226Ra decay to gaseous 222Rn, which undergoes a series of decays with short

half-lives to 210Pb, with a half-life of 20.4 years, which then falls onto the soil surface. The

ratio of 7Be/210Pb can be used to indicate the "newness" of sediment in a stream channel

(Bonniwell et al., 1999). 011ey et al. (1993) used the ratio of 2261Za to 232Th in sediment,

which varies according to source rock type, to identify the sediment source.

1.3 WHY CEstum-137?

A number of properties of 'Cs make it particularly suited for erosion studies:

• Its long half life and relative abundance in fallout ensure that it will be present

15

in measurable quantities for some time to come. (Seven half lives are required

to decay to 1/128th of the original amount, ten half-lives to decay to 1/1024th

of the original amount.)

• It is produced only by atomic fission, so none existed before the first nuclear

chain reactions were generated in the 1940's. It enters the environment as a

result of nuclear detonations and nuclear reactor releases. Thus, its

appearance in a watershed can be dated.

• It was distributed globally after the first hydrogen bomb test on October 31,

1952, and continued atmospheric thermonuclear testing through 1962

propelled radioactive fallout into the stratosphere.

• It binds rapidly and nearly irreversibly to soil particles, especially illite clays, in

the top few centimeters of soil so that, for the most part, 137Cs atoms move

only when soil particles move. This makes it a dated tracer for sediment

movement.

• It produces a high energy gamma ray (661.660 kEV, Firestone and Shirley,

1996) that is easily measured in a shielded gamma counter and is not subject

to self-shielding by other parts of the sample.

• Analyses of 137Cs require no wet chemistry or complex sample processing.

Usually drying and sieving are the only treatments applied. This ease of

measurement enhances the utility of 137Cs as a tracer and erosion indicator.

These properties enable one to obtain a SO-year-average erosion rate from one set of soil

16

samples, making it one of the most cost-effective methods for measuring soil erosion

(Loughran, 1990).

Although 'Sr has a similar half life and is also abundant in nuclear fallout, it is more

mobile in soil than 137Cs (Ruse et al., 1990; Davis, 1963), and its beta radiation is subject

to self-shielding by the sample, so it is less easily measured. Therefore, it is less suitable

for use in erosion studies.

In order to understand the potential uses and possible limitations of 137Cs, I will review

literature that examines the processes and timing by which 'Cs is produced, travels

through the atmosphere, falls onto the land surface, sorbs to soil particles, and is

distributed in the soil profile. Next, I will review ways other researchers have used 'Cs

before presenting new data collected for this study.

1.4 RADIATION UNITS

The traditional unit for measuring radioactivity has been the Curie. It is defined as 3.7 x

10 1 0 disintegrations per second, based on the radioactivity of radium. The official S.I. unit

of radioactivity is now the bequerel, a more basic unit defined as one disintegration per

second. Appendix 1 contains a conversion table for the various units of radioactivity and

radioactivity per unit area found in the literature.

17

SECTION 2. FALLOUT CHARACTERISTICS

2.1 137Cs AND 90Sr FORMATION

137Cs is formed relatively late in the nuclear explosion from gaseous precursors by the

reactions:

137 2.5sec. 137 24.5sec. 137 13.8min. 137 ,— ---> Ae— — -- U> 55 s53 54

30.07yrs. 137 2.6m n. 137— — --+ 56 Ba(excited) –> 56 C')

(Firestone and Shirley, 1996; Heilman, 2002)

Analyses of 'Cs actually measure the gamma ray emitted when the excited state of 13713a

decays to the stable condition. Because of the 4.3-minute half-life delay in the formation

of ' 37Cs, with most of that delay spent as xenon gas, relatively little 'Cs is included in the

larger particles of radioactive debris which fall out rapidly near ground zero. Instead, it is

associated with small fallout particles that can travel great distances (Davis, 1963).

"Sr is of interest for erosion studies because data from global "Sr monitoring programs

can be used to estimate 137Cs deposition over time. "Sr also forms from gaseous

precursors:

32.3sec. 90 , 158 sec. 909 AQ01.9 sec. 90 y

3 36 Ad-- ---> 37 --> 38 Sr

28.78yrs. 90 64.1hrs. 9039y –> 40 Zr(stable)

(Firestone and Shirley, 1996; Heilman, 2002)

Its 3.2-minute half-life delay in formation from krypton gas gives 90Sr similar

characteristics in the fallout cloud as "Cs, so 90Sr fallout deposition patterns are similar to

18

those of 137Cs. McHenry and Ritchie (1977) found that the amount of measured "Sr

fallout correlated highly with the distribution of 137Cs in the surface soils of watersheds in

the western U.S.

2.2 137Cs TO 90Sr RATIO

To estimate 'Cs deposition from "Sr fallout measurements, it is necessary to know the

ratio of 137Cs to "Sr in fallout. This ratio is not constant, so any constant value used is an

approximation. Sherrill et al. (1975) analyzed 'Cs and "Sr in 137 individual rain samples

collected in Arkansas from 1967 to 1969. The ratio between the two ranged from 0.26 to

8.2 in individual samples, and from 0.8 to 2.0 for bimonthly averages, with most of the

averages near 1.5. The magnitude of the ratio seemed to depend on the sizes of particles

washed out of the atmosphere by a storm, with heavier rainstorms being more likely to

capture smaller particles and therefore tending to have larger 137Cs: 90Sr ratios (Sherrill et

al., 1975).

Literature on production of the two isotopes is conflicting in that ratio between

production quantities given in HASL/EML reports do not match the production ratio

given directly in a different HASL report. As mentioned earlier, Davis (1963) cites the

1959 report, HASL-42, as determining that 0.18 megacuries of mCs are produced per

megaton of fission energy release, which is equivalent to 6.7 PBq. Monetti (1996) in

EML-579 reports that each megaton of nuclear explosive power generates 3.7 PBq of

90Sr. These numbers give a ratio of 1.8. Sherrill et al. (1975) quotes the 1965 report,

19

HASL-165, as giving the production ratio as 1.5. But that ratio depends on which

fissionable isotopes make up the bomb which produces the fallout. Sherrill et al. (1975)

give a I37Cs:90Sr ratio of 0.995 for the fission of 235U, 1.9 for 238U, and 2.75 for 'Pu.

Ritchie and McHenry's (1990) 137Cs: 90Sr ratio of 1.45 at formation is exactly the average

of the production ratios for the uranium isotopes.

Soil samples collected at 15 locations across Utah by EML in 1971 and analyzed for both

137Cs and 90Sr are available in their on-line data base (EML, 2003). 137Cs:90Sr ratios in

those samples range from 1.19 to 2.24, with both a mean and median of 1.57. Earlier

Utah samples in the data base were sampled to shallower depths which may not have

captured all 90Sr, so most of those ratios are higher. Hardy (1974) collected soil samples

at five locations each in the New York City and San Francisco Bay areas in 1972 and

1973. 137Cs:90Sr ratios ranged from 1.7 to 1.9 with an average of 1.8 in the New York

area, and from 1.5 to 1.7 with an average of 1.6 in the San Francisco area. Davis (1963)

cites reported 137Cs:90Sr ratios of 1.5, 1.62, and 1.8. McHenry et al. (1973) and Krey et al.

(1990) both use a ratio of 1.6 for computing 137Cs fallout from 9°Sr records. Later in this

report, I will use a factor of 1.6 for estimating 'Cs fallout from 9°Sr measurements.

2.3 CONTINENTAL AND GLOBAL FALLOUT

There are two scales of fallout: continental and global. Continental-scale fallout occurs

downwind from atmospheric nuclear detonations with yields in the kiloton range. In

North America , this includes atmospheric tests at the Nevada Test Site (NT S) from 1951

Lifig.447e2,452)

..... F . . ..

••••

. .. rt • • •

. ' .. . ... ....

. . . . . . . . . . ..

. ...

.....

...

. .

.....

. .......•

....... ...... ! ......... .

..... . . . .............

.

•

........ . r •

20

Figure 1. Estimated trajectories, at several altitudes, of the radioactive cloud resultingfrom the test CHARLIE of April 22, 1952. Numbers indicate the position of eachtrajectory at 00:00 GMT for several days following the explosion. This was the secondtest to deposit fallout over Arizona and eastern California.

to 1962, as well as the July 16, 1945, TRINITY test near Alamogordo, New Mexico.

Beck (1984) plotted the paths followed by NTS fallout on wind currents at different

altitudes across the western U.S. The National Cancer Institute (1997) incorporated that

information in to their study of the public's exposure to 'I fallout. Fallout paths from the

31 kiloton test on April 22, 1952, which was the second test to produce fallout in the

southeastern California desert, are reproduced from the National Cancer Institute (1997)

in Figure 1.

21

Enhanced deposition of continental-scale fallout occurs when thunderstorm activity

intersects the radioactive cloud from an atmospheric atomic detonation. Hoecker and

Machta (1990) discuss an example of this at Albany, New York, in 1953 following the

SIMON test at the Nevada Test Site. Church et al. (1990) mention other cases of this in

Duchesne County, Utah, and in Wyoming, also involving Nevada Test Site fallout.

Global-scale fallout occurs after thermonuclear hydrogen bomb tests yielding in the

megaton range propel fallout into the stratosphere, where is spreads worldwide.

Stratospheric air descends in the largest volumes in middle latitudes. This exchange is

accelerated during the spring. Therefore global fallout rates have been greatest in the

spring near 45 degrees north and south latitude (Davis, 1963; Larsen, 1984), with the

Northern Hemisphere receiving much more fallout than the Southern Hemisphere because

of the greater number of atmospheric nuclear tests north of the equator, although some

stratospheric fallout does cross the equator (Monetti, 1996). Figure 2 shows the

cumulative average global distribution of 'Sr fallout, accounting for decay, by 100 latitude

bands, based on data provided by Monetti (1996). The half-residence time of fallout in the

stratosphere has been estimated at between 6 months and a year, depending on the latitude

of the test, with fallout from equatorial tests at the long end of that range, and arctic tests

at the short end (Harley, 1976; Larsen, 1984). The particle sizes of stratospheric fission

debris are too small for there to be significant deposition by gravity, so most global-scale

fallout, including 'Cs, reaches the earth surface in rain or snow (Davis, 1963).

1800

et" 1500

,))1200

Cl.t.)

CJD

900I)

a)

600

300

H0 -10 -20 -30 -40 -50 -60 -70 -80 -90

South

"2 9

10 ° Latitude Bands

Figure 2. Cumulative 90 Sr deposition in 1990 by

10 0 latitude bands (data from Monetti, 1996)

2.4 CORRELATION WITH PRECIPITATION

In most areas, precipitation is the primary means by which long-range fallout reaches the

ground. In the eastern United States (

23

latitude and longitude. Beck and Anspaugh (1991) give the equation:

cP ( p Y4

and

1 = (1- el

where x —7.8)

I = Inventory of global 137Cs fallout on 1/1/83 (nCi/m2)

P = average annual precipitation (cm)

c = empirical coefficient which varies with latitude and longitude from 0.7 to 2.3according to a table of locations in the western U.S. published by the authors.

The only significant fallout deposition after 1983 was in Europe as a result of the

Chernobyl accident in 1986, so after correcting for decay from 1983 to the present, this

equation, if correct, would predict the current global 137Cs fallout inventory at locations in

the western U.S. Cox and Fankhauser (1984) found a correlation between annual rainfall

and 'Cs concentration in the top 5 cm of soil in Hawaii, but with a wide scatter. They

referenced a linear correlation between the two in Puerto Rico. Loughran (1994) reported

a correlation (r = 0.78) between mean annual precipitation and reference levels of 137Cs in

southeastern Australia. Ritchie et al. (1970), in a study in the Smoky Mountains, found

the ratio of soil 'Cs concentrations at different elevations to be within 5% of the ratio of

annual precipitation amounts at those same elevations. Hardy (1975) found a ratio of 2.35

between "'Cs accumulation in New York City versus San Francisco, and a ratio of 2.24

between the annual precipitation in those two cities.

24

2.5 TIME DISTRIBUTION OF l37CS DEPOSITION

2.5.1 Start of Cesium Deposition

Knowing the start date for deposition of 137Cs is essential to know the time period over

which erosion is being measured. Richie and McHenry (1990) put the initial input of 'Cs

into the environment at 1952 ± 2 years, with "measurable amounts in soils generally

beginning in 1954." Walling et al. (1986) and Walling and He (1997), studying in the

U.K., use 1954 as the year that 'Cs fallout was "first documented." Loughran (1994),

working in Australia, lists fallout commencing in 1954 as one of his assumptions. Van

Metre et al. (1997) use 1952 as the year that 137Cs first appeared in reservoir sediment

cores in the central and southeastern U.S. And Ely et al. (1992) refer to 1950 as the

approximate arrival date for 'Cs in Arizona watersheds, recognizing that the actual date

is between 1945 and 1955. A review of the literature on atmospheric nuclear testing and

fallout measurements shows that 'Cs deposition began at different times in different

locations, and that this time can be precisely determined for many areas.

The first thermonuclear test on October 31, 1952, in the mid-Pacific Ocean (11 0 N

latitude, 162°87 E longitude) (Geoscience Australia, 2003), propelled 137Cs well up into

the stratosphere (Harley, 1976), where it spread worldwide. This event marked the

beginning of global-scale fallout. Since exchange between the stratosphere and the

troposphere below is most active in the spring, as mentioned earlier, it was likely spring of

1953 before much fallout from that test reached North America, Europe, and other distant

areas. The next thermonuclear test was on February 28, 1954, and began the Castle series

25

of 5 thermonuclear tests in 75 days (see Appendix 2). That series is probably the basis for

the use of 1954 as the start date of 'Cs deposition.

Continental-scale fallout occurred before 1953 in North America from tests at the Nevada

Test Site and downwind of other nuclear tests in the Pacific Ocean and in Kazakhstan.

Appendix 2 lists nuclear explosions that released radioactivity into the atmosphere.

Testing at the Nevada Test Site (NTS) started on January 27, 1951, and produced

radioactive snow measured at the Eastman Kodak factory in Rochester, New York on

January 29, 1951. This finding prompted the development of a nationwide, and later

worldwide, fallout monitoring program (Harley, 1976). Virtually every part of the

continental United States received fallout from atmospheric nuclear testing at the NTS,

but except for areas immediately downwind of the NTS, global fallout was more

significant for total 137Cs deposition (Beck and Anspaugh, 1991).

The start date of 137Cs deposition from the NTS for any county in the continental U.S. can

be determined using results published on the internet from a study by the National Cancer

Institute (NCI) (1997) of iodine-131 exposure from NTS nuclear tests. The NCI

published an individual dose calculator on the internet (http://spike.nci.nih.gov

Nallout/html) for 'I exposure. To use this for 137Cs deposition, one first selects the state

and county of interest. The option of viewing 1311 ground deposition by test then appears

at the bottom of the list of choices. The earliest date of ' 3 'I deposition will also be the

start date of 'Cs deposition from the NTS. The 'I information will not, by itself,

26

indicate the amount of 137Cs deposited. Since the half-life of 'I is only eight days, its

levels in the fallout cloud drop rapidly after just a few days, while 137Cs levels remain fairly

constant over that same time interval. Therefore, the relation between 1311 and 'Cs

fallout is highly time-dependent. But for the southwestern U.S. where 'I deposition

occurred within the first day or so after the nuclear test, estimates of 13I I fallout should

give a good relative indication of i37Cs fallout.

Another factor affecting the date of first 'Cs deposition in North America is the Trinity

nuclear test at Alamagordo, New Mexico on July 16, 1945. Measurements and

meteorological modeling following the test showed the radioactive cloud, which reached

an altitude of 12 km (Eisenbud and Harley, 1953), moving to the northeast into

southeastern Colorado (Cederwall and Peterson, 1990). The Eastman Kodak company

discovered evidence of long-range fallout from this test, also. In the fall of 1945, some of

their X-ray film was fogging before it was unpacked. They traced the problem to water

from the Wabash and Iowa Rivers supplying paper mills in Indiana and Iowa, respectively,

which in August and September of 1945 produced contaminated strawboard used to

package the film. The isotope causing the damage was identified as cerium-141 from

atomic bomb fallout (Webb, 1949). So, at a minimum, Trinity fallout in 1945 covered the

region from central New Mexico to Iowa and Indiana. The problem is in knowing

specifically in which areas that occurred, and whether enough fell to be detectable today.

Models used to estimate fallout deposition from the NTS (e.g. Cederwall and Peterson,

1990; Hoecker and Machta, 1990; National Cancer Institute, 1997), along with weather

27

data from the time period following the test, should be capable of estimating the fallout

distribution across the United States from the Trinity test, but I have not found any

published results from such an effort.

Both Ritchie and McHenry (1990) and Loughran (1994) point out that it is possible for

the initial 137Cs deposition to be too light to be detectable today, now 1.7 half-lives later,

due to radioactive decay. This would mean that the quantity of 137Cs in a modern soil

sample would represent a time period that started sometime later. Beck (1984) published

data that allows one to determine if that is the case for 25 locations in the western U.S.

For those locations, he estimated 137Cs deposition from each NTS nuclear test through

1957 based on fallout monitoring following each test. Table 1 lists estimated first year

137Cs deposition from Beck (1984) for four of those locations, selected for their proximity

to study sites in eastern California and northern Arizona and for their range of values, and

calculates decay to January 1, 2004. In soil sampling, one measures radioactivity per gram

of soil, so Table 1 computes the i37Cs concentration per gram with the following

assumptions:

• the soil bulk density is 1.5 g/cm3 ,

• the surface deposition is distributed through the top 5 cm of soil, and

• a sample of the top 5 cm is completely mixed.

First year deposition at Las Vegas, Nevada, after being mixed in a 5 cm layer of soil, was

too light to have been detected in 1952 by the USGS well detector used in this study.

Fallout was captured before it touched the soil to make these estimates oft37Cs

Table 1. Is the initial 137Cs deposition still detectable today?Examples from Nevada Test Site fallout in the western U.S.

Detection Limit for Original Estimation: 0.005 mCi/km2Detection Limit for USGS Well Detector from this report: 0.15 mBq/gSums below 0.15 mBq/g are in italics.

Estimated first year 137 Cs

Test Date mCi/km2 rnBq/cm2 a (%)

depositiontop 5 cm*

mBq/g

Decayed to January 1,2004top 5 cm*

mCi/km2 mBq/cm2 mBq/gLas Vegas, NV

04/22/1952 0.04 0.15 50 0.020 0.01 0.04 0.00605/01/1952 0.05 0.19 50 0.025 0.02 0.06 0.007

sum 0.09 0.33 36 0.044 0.03 0.10 0.013

Fresno, CA04/22/1952 0.08 0.30 28 0.039 0.02 0.09 0.01205/07/1952 0.02 0.07 26 0.010 0.01 0.02 0.00305/25/1952 0.04 0.15 31 0.020 0.01 0.05 0.00606/01/1952 0.22 0.81 28 0.109 0.07 0.25 0.03306/05/1952 0.23 0.85 27 0.113 0.07 0.26 0.035

sum 0.59 2.18 15 0.291 0.18 0.66 0.089

Flagstaff, AZ04/15/1952 0.02 0.07 100 0.010 0.01 0.02 0.00304/22/1952 0.15 0.56 37 0.074 0.05 0.17 0.02205/01/1952 0.02 0.07 100 0.010 0.01 0.02 0.00305/07/1952 0.02 0.07 100 0.010 0.01 0.02 0.00305/25/1952 0.74 2.74 30 0.365 0.23 0.83 0.11106/01/1952 0.11 0.41 28 0.054 0.03 0.12 0.017

sum 1.06 3.92 22 0.523 0.32 1.19 0.159

Salt Lake City, UT11/19/1951 0.03 0.11 100 0.015 0.01 0.03 0.00411/29/1951 0.03 0.11 100 0.015 0.01 0.03 0.00404/01/1952 0.10 0.37 24 0.049 0.03 0.11 0.01504/22/1952 0.03 0.11 100 0.015 0.01 0.03 0.00405/01/1952 0.03 0.11 100 0.015 0.01 0.03 0.00405/07/1952 3.22 11.91 36 1.589 0.98 3.62 0.48305/25/1952 0.28 1.04 25 0.138 0.09 0.32 0.04206/01/1952 0.03 0.11 100 0.015 0.01 0.03 0.00506/05/1952 0.14 0.52 21 0.069 0.04 0.16 0.021

sum 3.89 14.393 30 1.919 1.18 4.38 0.583

* assuming 137 Cs is distributed through the top 5 cm of soil, and assuming

a soil bulk density of 1.5 g/cm3

28

Source: Beck, 1984.

29

deposition, so the radioactivity was concentrated and smaller amounts could be detected.

Even though Las Vegas is the closest of the four sites to the Nevada Test Site, it has the

lowest level of' 37Cs deposition because it was upwind during most of the tests. First year

137Cs deposition at Fresno, California has decayed below detection in a soil sample, while

deposition in Flagstaff, Arizona is now right at the detection limit.

137Cs measurements for this study were made from 1998 to 2000, and decay-corrected to

January 1, 2000. First-year deposition in Flagstaff would still be detectable at that time.

In general, Table 1 shows that a light i 'Cs deposition in 1952 will have decayed below

detection in a recent soil sample.

In reservoir sediments, 137Cs is less likely to fall below detection levels for two reasons.

First, finer particles with more 137Cs are preferentially eroded and transported. Second,

'Cs in a natural soil is concentrated at the surface where sheet and small rill erosion takes

place. Both of these processes concentrate 'Cs in sediment deposits.

If 137Cs fallout were distributed and sampled in just the top 2 cm of soil instead of the top

5 cm, them all values in Table 1 would be multiplied by 2.5. In that situation, first-year

'Cs fallout in Las Vegas would still be undetectable from the beginning, but it would stay

above the detection limit at the other locations.

Data collected and models used by the NCI (1997) for their 131J study should be sufficient

to estimate initial 137Cs deposition from the NTS as well as 1311 deposition for anywhere in

30

the continental U.S. using methods similar to those used by Beck (1984), but I have not

seen such calculations published.

2.5.2 Fallout Rates over Time

Periods of heavy 137Cs deposition occurred the year after heavy atmospheric nuclear

testing. The greatest atmospheric nuclear testing activity occurred in 1962 (see Appendix

2) prior to the signing of the Nuclear Test Ban Treaty, which forbade signing nations from

producing radioactive debris that would spread beyond their borders. The resulting peak in

fallout deposition came in 1963 in most locations. Another increase in nuclear testing

occurred in 1958 prior to a nuclear test moratorium in 1959, and a secondary spike in

fallout deposition occurred in 1959. Table 2A lists the times of peak fallout rates over

monthly, quarterly, and annual time intervals for selected sites in the western U.S. and for

Klagenfurt, Austria, the EML global fallout monitoring site most impacted by Chernobyl

fallout. Table 2B lists the time ranges of data available for this analysis, as well as the

calculated magnitudes of the peak and ending 137Cs accumulations and the annual

precipitation at the monitoring sites.

As Table 2A shows, the timing of the peak deposition rate depends in part on the time

interval over which it is tabulated. But for these sites, 1963 was the most common year

for the peak annual and monthly rates. In Los Angeles, nearly 40 cm of rain in February

of 1962 produced the peak monthly and quarterly fallout rates, but the peak annual rate

was still in 1963. In Houston, the peak monthly rate was in 1963, but fallout in the second

31

Table 2A. Peak 137Cs fallout times and cumulative peak times at selected locations,calculated from monthly 90Sr fallout measurements. The timing of peak rates variedaccording to the time interval and local conditions. Cumulative peaks all occurred in1966. Simulated erosion caused the cumulative peak to occur earlier.

CumulativePeak with

Peak Deposition Time by: Cumulative ErosionLocation Month Quarter Year Peak FunctionW. Los Angeles, CA Feb-1962 1-1962 1963 Mar-1966 Sep-1965Salt Lake City, UT Apr-1963 2-1964 1964 Apr-1966 May-1965Denver, CO Jun-1964 3-1964 1963 Jul-1966 Jun-1965Helena, MT Jun-1963 2-1963 1963 Jul-1966 Aug-1965El Paso, TX/Roswell, NM Apr-1953 2-1953 1953 Aug-1966 May-1966Houston, TX Jun-1963 2-1959 1963 Aug-1966 Jul-1965Klagenfurt, Austria May-1963 2-1963 1963 Aug-1966 Aug-1964

Table 2B. Time ranges of continuous 90 Sr and precipitation monitoring for the aboveselected sites, along with magnitudes of the calculated cumulative peaks, endingaccumulations, and annual precipitation for the listed time period

90Sr & Precipitation Peak Cum. Dec-20031370 137CSMonitoring

AnnualPrecipitation

Location Stare End (Bq/m2) (Bq/m 2) cm/yrW. Los Angeles, CA Dec-1956 Dec-1990 1742 872 40.5Salt Lake City, UT Dec-1956 Jan-1976 5271 2516 44.5Denver, CO Jan-1959 Jun-1976 2987 1423 39.6Helena, MT Mar-1959 Jun-1976 2676 1276 27.6El Paso, TX Jan-1959 Jun-1976 1052 504 19.2Houston, TX May-1958 Apr-1990 2272 1172 119.5Klagenfurt, Austria Aug-1957 Dec-1990 3254 1773 99.4

1 Fallout before the start dates was estimated from soil samples and/or gummed filmmonitoring of NTS fallout, except at Klagenfurt, where no pre-1957 fallout was added tothe cumulative curve.

Source data is from the U.S. Environmental Measurements Laboratory on-line data bases(http://www.emldoe.gov/databases); Beck (1984); and Beck, et al (1991).

32

quarter total of 1959 following 1958 nuclear testing topped quarterly totals from 1963.

For El Paso, data prior to 1959 come from ' 37Cs fallout estimates by Beck (1984)

calculated from gummed fi lm measurements in Roswell, New Mexico, following NTS

tests. The Simon test on April 25, 1953, produced the most fallout of any test ever

conducted at the NTS (Beck et al., 1990). Fallout in New Mexico from that test exceeded

global fallout deposited in1963. Meteorological modeling by Hoecker and Machta (1990)

shows the edge of the main radioactive cloud from the Simon test just approaching El

Paso, so deposition from Simon in El Paso may have been somewhat less. In Klagenfurt,

Austria, fallout from the Chernobyl accident in 1986 was less than the 1963 global fallout

rate, but that was not the case in some other parts of Europe (e.g. Croatia, see Filipovic-

VinceKovic et al., 1991). Figures 3 and 4 display quarterly fallout rates for Los Angeles

and El Paso/Roswell, respectively, with 137Cs deposition calculated from 'Sr monitoring

data assuming a 137Cs :"Sr ratio of 1.6.

2.5.3 Fallout Accumulation

The second-to-last column in Table 2A and the solid curves in Figures 3 and 4 show the

timing of the calculated maximum cumulative 'Cs deposition, accounting for radioactive

decay. These peaks are all in 1966. Figure 5 displays the cumulative deposition curves

for all sites listed in Table 2. It is not meaningful to compare portions of the curves

representing the 1950's due to inconsistencies in the availability of data. Continuous "Sr

monitoring started at different times in the late 1950's, as listed in Table 2B. To account

for missing 1950's fallout, a quantity of 'Cs was added to the calculation starting

1800

CumulativeDepositionwith Decay

1600

1400

„--, 1200

-Er

1000 —0

CL.)

ci) 800

-o

600

400

200

N

I i.1.1111.t .11.....i Ili' III..idIII 11.111[1i, 111111i I I i it] I 10

Cumulative withn

Erosion Function:[decayed fallout*precip (cm) / 1000]

1950 1960 1970 1980 1990 2000

Figure 3. Quarterly and cumulative 137Cs deposition

in Los Angeles estimated from 90Sr measurements

33

CumulativeDepositionwith decay

Cumulative withErosion Function:[decayed fallout*precip (cm) / 1000]

I , 11 IF t IIi i I i i i i i j i I I i i I I i i i i I II. .1,,

34

1000

800

CC1

• 7)

a)

600

4-cti400

[3.1

200

0

1950 1960 1970 1980 1990 2000

Figure 4. Quarterly and cumulative 137Cs Deposition for El Paso, TX,

(1959-1976) and Roswell, NM (1951-1959), estimated from 90 Sr

measurements and gummed film data, respectively.

35

arbitrarily in 1955 such that the calculated accumulation would match the total quantity of

137Cs in uneroded soil samples collected by EML in the late 1950's (available in EML,

2003, on-line data base). This was done for all sites except El Paso and Klagenfurt.

The applicability of the soil sampling data also varied. For Houston the amount of 137Cs

added was based on an average of two samples collected over 100 km away in different

directions (Danevang and Brenham, TX), and the amount for Helena was based on a soil

sample from Bozeman, 126 km away. Other soil samples were collected much closer to

the fallout monitoring station. NTS fallout estimates were added in where available,

which was at all sites except Houston, Helena, and Klagenfurt, with NTS fallout data from

Roswel substituted for El Paso.

Despite differences in the early parts of the curves in Figure 5, the curves all have similar

shapes after the 1950's. This shape is characterized by a steep rise in 1962 and 1963, a

peak in 1966, and an increase in the downward slope after about 1973. The 1953 spike in

137Cs deposition at El Paso/Roswell does not change the overall cumulative pattern for

that area_

2.5.4 137Cs Accumulation with Erosion

The dashed lines in Figures 3 and 4 represent 137Cs accumulation with an arbitrary erosion

function based on monthly or quarterly precipitation. This obviously reduces the

magnitude of the peak accumulation, but it also causes the peak to occur earlier. The last

column in Table 2A lists the times of peak accumulations with this erosion function. The

1990 20001970 198019601950

36

Salt Lake City

— Klagenfurt, Austria

— — Denver

Helena

— Houston

--- Los Angeles

El Paso

-

-

-

-

-

Il IIIIIIIJ

r,

[

5000

4000

3000

2000

1000

0

,--.,Ecr

cr.)s....,

no

'ôo.ci)

(:)cn

r—C)en—

a)›-—

A-,cd

nEn

L.)msa>cdE—

Li.ln

Figure 5. Cumulative 137

C s deposition curves with decay for sites

listed in Table 2. Peak accumulations occur in 1966.

37

slopes of the accumulation curves with erosion in Figures 3 and 4 decrease after the

rainfall data at the fallout monitoring stations ends. This is obviously an artificial

condition.

2.5.5 End of Fallout

"Sr fallout decreased to undetectable levels in the mid-1980's. No detectable "Sr fallout

was measured at the Los Angeles monitoring station from the fourth quarter of 1983

through the end of 1990, when monitoring was discontinued (EML, 2003; see Figure 3).

In Houston, only one quarter (first quarter, 1988) had detectable "Sr fallout after the first

half of 1986, through the end of 1990. '37Cs fallout dropped below detectable levels in

North America after 1983 (Ritchie and McHenry, 1990).

38

SECTION 3. FATE OF 137Cs IN SOIL

3.1 ADSORPTION OF 137Cs

When 137Cs fallout reaches the ground, studies show that it is quickly and almost

irreversibly adsorbed onto soil and sediment particles. Comans and Hockley (1991)

combined 10 p.g/L of cesium with 100 mg/L of calcium-saturated illite in a vessel that

was continuously tumbled. After 30 minutes, 70% of the cesium was removed from

solution. When the cesium concentration was lowered to 1 [ig/L, 95 to 98% of the

cesium was removed from solution in "a few hours." For a comparison to fallout

conditions, the highest "Sr concentration in rainfall in Los Angeles reported by Hardy

(1977) was 767 pCi/L in May, 1963. A 137Cs:"Sr ratio of 1.6 and a conversion factor of

0.01595 pg/pCi of 137Cs give a maximum 137Cs concentration of 20 pg/L, which is less by

a factor of 50,000 than the lowest cesium concentration used in that experiment. Runoff

from desert soils typically carries much more than 100 mg/L of sediment (e.g. fig. 12 of

Renard and Laursen (1975) shows sediment concentrations between 10,000 and 100,000

ppm at Walnut Gulch Experimental Watershed in southeastern Arizona), and rainfall

infiltrating into the soil obviously encounters a vast excess of sediment. Under these

natural conditions of very minute cesium concentrations and abundant sediment, all

cesium is expected to be rapidly adsorbed onto sediment particles.

Comans and Hockley (1991) developed kinetic models of cesium sorption that predict

that "effectively instantaneous and reversible kinetic processes control cesium sorption

over time scales of a few days and less." Their experimental data matched the model

39

predictions. The decreasing extractability of cesium with ammonium nitrate over time

led them to conclude that cesium slowly moves to clay interlayer sites over time scales of

weeks to months. When that happens, it becomes irreversibly bound.

Cesium sorption is little affected by the pH ranges commonly found in soils. Davis

(1963) reported on soil column experiments where "the soil-exchange capacity for cesium

was approximately constant above pH 3.5," and referred to another study that reported

that "even cesium in highly acid wastes is quickly fixed in soil."

Cesium is preferentially adsorbed onto the smallest soil particles. Davis (1963) reports

on two studies that address this issue. One showed that "about 50% of the 137Cs in

slightly alkaline clay (pH 7.5) was adsorbed onto particles smaller that 2 microns, which

represented only 5% of the total mass." In the other study, 30g samples of different soils

were mixed in one liter of solution containing 1.2 x 10' meq of cesium. The sand sample

adsorbed 50% of the available cesium, while sandy looms and clays effected 90%

adsorption.

Experiments have shown that the most effective clay mineral for adsorbing cesium is

illite. Tamura and Jacobs (1960) found that illite was a more effective sorbent than

montmorillonite, kaolinite, or hydrobiotite. Komarneni (1978) found no irreversible

fixation of cesium in pure kaolinite--all cesium was released after three extractions.

Tamura (1964) reported on tests by Graham and Killion that showed that illite and

Putnam clay sorbed more cesium than monttnorillonite, kaolinite, or fibrous peat.

40

Soil organic matter is generally known for its adsorption capacity, and cesium does sorb

to it. Tests by Riise, et. al. (1990) on soil cores from central Norway after the Chernobyl

accident found a higher concentration per gram of 137Cs in litter than in the top centimeter

of soil. But because the density of organic matter is so low, the percentage of "Cs bound

to the organic material was still fairly low. In sequential extractions performed on the top

2 cm of 7 soil cores, the same authors found only 3 to 13% of the total 'Cs in the

organic fraction.

Studies have shown that some cesium sorbed onto sediments can be displaced by NH 4+,

and to a lesser extent, potassium. Extractions by Riise, et. al. (1990) with ammonium

acetate freed between 2 and 10% of the radiocesium in 7 cores. Shulz (1965) grew

Romaine lettuce in pots containing 1600 g of soil treated uniformly with 40 p.Ci of "Cs

(25 'Xi/kg). After 3 weeks, the pots were given treatments of CaC12 , Ca(NO3)2 , K2SO4,

or (NH4)2SO4 fertilizer, with a control group of no fertilizer, and the lettuce was harvested

after another 2 weeks. With applications equivalent to 100 and 300 lbs/acre, the

potassium fertilizer increased "Cs uptake by the lettuce an average of 134% and 186%,

respectively, from the control, while the ammonium fertilizer increased 'Cs uptake an

average of 225% and 625% with the same application rates. Comans and Hockley (1991)

reported that "a few percent of sediment-bound radiocesium can still be mobilized by the

enhanced NH4+ levels associated with anoxic conditions, even after many years of

contact." Loughran (1994) reported on two studies that showed that cesium sorption

decreased with increasing salinity, probably due to competition for sorption sites by IC.

41

and Na+ ions.

Despite the fertilizer experiments, cesium uptake by plants is low (Ritchie and McHenry,

1990). Davis (1963) reports on indoor experiments by Wijk and Braams with grass

grown on clay and sand soils that were contaminated with 10 Ci/kg of wCs. "Only a

few tenths of 1% of the radiocesium was actually assimilated within a period of several

months."

Strontium-90 has been used to measure soil movement. Ritchie and McHenry (1990) cite

three such studies in the 1960's. However, numerous studies—Davis (1963) cites

three—show that "Sr is bound much less tightly to soil particles than is 137Cs. As a result,

it migrates deeper into the soil profile, and it is more easily removed from the soil to

become active in food chains. This makes 90Sr less suitable for soil erosion studies.

Also, its beta radiation is not as easily measured as the gamma radiation from 'Cs.

3.2 DEPTH DISTRIBUTION OF 137CS

As 'Cs falls onto the land surface, usually dissolved in rainwater, most of it sorbs to soil

particles very close to the surface. What does not, percolates into the soil, decreasing in

concentration with depth as it sorbs to lower soil layers. The result is an exponential

distribution of 'Cs with depth. The depth and shape of the 137Cs distribution curve

depend on the hydrologic and minerologic characteristics of the soil and on the volume of

water available for infiltration. These parameters have a wide variation, so the depth

distribution of mCs has a wide variation as well, but common characteristics do exist.

A

100 200 300 400137Cs Concentration (mBq/g)

15

20 0

200 5000 10000 15000 20000 25000 30000 35000

137Cs Concentration (mBq/g)

Figure 6. Typical 137Cs depth profiles in undisturbed soil. Negativedepths represent surface litter or vegetation.

A. Cape Cod, Mass., 1972; annual precipitation: 107 cm;source: Hardy (1974a).

B. Central Norway, 1988 (2 years after Chernobyl);source: Rfise et al. (1990).

42

-B-NSFS I

1.1 1 1

20 40 60 800 100

1111111111111111111111111111111

0.0

0.4

1.6

2.0

43

A

2

4 -

I

20 40 60 80

6 7

8 =

10 7

12 =

14

16 -0 100

137Cs Concentration (mBq/g)

13 7Cs Concentration (mBq/g)

-B-- Kenally --111- Little Valley --A- Rapid-0- USGS - -X- Hwy55 NSFS

Figure 7. I37Cs depth distribution in the Gold Fork River Basin, Idaho,1996; annual precipitation: 95.5 cm; source: Bonniwell et al. (1999).(A) Profile of top 17 cm, plotted at the midpoints of the intervals.(B) Profile of top 2 cm at six locations in the basin.

44

Figure 6 shows depth distribution curves from the eastern U.S. and central Norway that

demonstrate the typical exponential decline in 137Cs concentration with depth. Figure 6A

represents a sandy loam soil at Cape Cod, Massachusetts, in 1972, where 91% of 137Cs is

in the top 8 cm of soil, including surface vegetation (Hardy, 1974a). In Figure 6B, 137Cs

concentrations are much higher in two 1988 profiles from central Norway due to fallout

from the Chernobyl accident, but the exponential shape is still present.

Figure 7 contains depth profiles from Idaho sampled in 1996. Figure 7A shows a profile

sampled to a depth of 17 cm where an exponential decline occurs in the 5-cm sampling

intervals below 2 cm of depth. But unlike the other profiles, the top 2 cm was sampled in

2 and 4 mm intervals and shows a peak at 4 to 6 mm below the surface. At this site, the

exponential pattern does not continue upwards through the top 2 cm of soil. Figure 7B

shows 137Cs concentrations in the top 2 cm of soil at six sites in the same Idaho

watershed, including the "NSFS" site plotted in Figure 7A. The two sites with the

highest 137Cs activities show distinct peaks within the top 2 cm, while activities in the

other four profiles are fairly constant over that depth range. None of these depth profiles

demonstrate an exponential decline in the top 2 cm. Sampling intervals as fine as these

are rare in published studies, most likely due to the difficulty in collecting such samples

cleanly and accurately. It would be useful to know if this pattern in the top 2 cm of soil is

common. Walling and He (1997) describe the initial distribution of 137Cs concentration

(Bq/g) with cumulative mass z (g/cm2) from the soil surface at time t with the following

equation:

45

Ia(t) -(z- RyaCo(Z ,t) —

H

where Ia(t) = fallout rate (Bq/cm2/yr),

H = relaxation depth (g/cm2 cumulative mass), and

R = sediment accumulation rate (g/creyr).

The relaxation depth affects the predicted depth of penetration and the rate of decline

with depth. Figure 8 graphs this equation with three relaxation depths, dividing by an

assumed bulk density of 1.5 g/cm3 to convert cumulative mass to depth, with R = 0, and Ia

= 0.125 Bq/cm2/yr, approximately the 1963 rate at a fallout monitoring station in the

region used by Walling et al. (1986). These calculated curves are overlain on a depth

profile from Devon, U.K. measured by Walling et al. (1986). This measured depth

profile is somewhat unusual in having the peak concentration in the 4 to 6 cm depth

interval. Below that, an exponential shape similar to the calculated curves is evident.

Filipovic-Vincekovic et al. (1991) used a power function of the form Y = aLb to describe

soil profiles in Croatia dominated by Chernobyl fallout, where Y = 137Cs concentration

(Bq/Kg) and L = depth (mm) For their profiles, ln a = 5.4 to 10, and b = -0.6 to -1.35.

They sampled the same four locations three different times, at 2.5, 6.5, and 34 months

after the Chernobyl accident. Also, instead of analyzing every interval in the profile, they

only analyzed 4 separate 1.2-cm-wide zones. There was no consistent relationship

between the 6.5-month profile and the other two, but between the 2.5- and 34-month

Yendacott Basin, Devon, U

Relaxation Depth=1 g/dn

Relaxation Depth=2 g/dn-

2- - - - - Relaxation Depth=4 g/cm

11.111111111111111111 1111111111111111111111111111111111 111111111111111111111111 I -

-

-

-

46

profiles, there was a consistent decrease in 'Cs at the top of the profile and an increase

in 137Cs in the lower part of the profile over time. Figure 9 shows this pattern at one

location, both as ` 37Cs activity and as percent of the total 'Cs in the profile.

0 10 20 30 40 50 60 70 80137

C S Concentration (mBq/g)

Figure 8. 137Cs depth profile by Walling et al. (1986) in YendacottBasin, Devon, U.K., (annual precipitation: 80 cm) with depth equationof Walling and He (1997) at 3 relaxation depths.

fitted curve, 2.5 months after Chernobyl• data, 2.5 months after Chernobyl

— — fitted curve, 34 months after Chernobyl0 data, 34 months after Chernobyl

1 1 1

III 1111,1 Il iti1I11111111111111111111111111111111111111111 T

0 10 20 30 40 50

60

30

A

47

10

J: fitted curve, 2.5 months after Chernobyl • data, 2.5 months after Chernobyl— — fitted curve, 34 months after Chernobyl0. data, 34 months after Chernobyl 25

30 1 1 1 1I I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 .1 t 1 1 1 -

0 200 400 600 800 1000 1200 1400 1600

37C S Concentration (mBq/g)

% 137Cs per 1.2 cm depth interval

Figure 9. Change in 137Cs depth distribution over time in Gracac, Croatia(annual precipitation: 159 cm) after the Chernobyl accident.(A) shows concentration per gram; (B) shows percentage distribution.Source: Filipovic-Vincekovic et al. (1991)

48

It should be noted that the decrease in the total 137Cs content of the profile is greater than

can be accounted for by radioactive decay. The two possible means for this loss of 137Cs

are it being eroded off the surface, or it migrating downward out of the profile. The two

possible means for the changes in the profile are for 137Cs to desorb, be washed

downward, and then resorb at a lower level, or for clay particles with sorbed 137Cs to

migrate downward. These data cannot distinguish between the two, although

experiments discussed in the preceeding section indicate that desorption is very limited.

It should also be noted that the two locations with the least precipitation (81 cm/yr)

during the time period studied had the smallest changes over time.

All of the depth profiles presented thus far have been from relatively humid areas.

Loughran et al. (1993) sampled soil profiles from arid western Australia. Six of their

profiles are plotted in Figure 10. A characteristic to note is the shallow depth of 137Cs

penetration, as low as 2 cm at site 42B. The apparent penetration to 19 cm in profile 35B

may be just a function of sampling in that the depth interval from 10 to 20 cm was

collected in one sample. A low average 137Cs activity in the sample means that 137Cs

penetration could have halted anywhere below 10 cm. Also, 137Cs activity at site 35B

peaks below the surface, at 2 to 4 cm, similar to the measured profile in Figure 8 in which

137Cs activity peaks in the 4 to 6 cm interval. Looking at the profiles presented in Figures

6 - 10, the depth of 137Cs penetration appears to be influenced by annual precipitation,

with more precipitation carrying 137Cs deeper into the soil.

0 100 200 300 400 500

49

•

Site Label

El 42B

59C

-*— 61C

—4( 19B

--* 35B

---• 38B

00'

T

2)137Cs Activity (Bq/m

Figure 10. 137Cs depth profiles measured by Loughran et al. (1993)in a semi-arid region (annual precipitation: 21 cm) in western Australia.

0

10

15

20

50

SECTION 4. USE OF RADIONUCLIDES TO STUDY SOIL MOVEMENT

4.1 HISTORY

Studies of the movement of fallout radionuclides and their relation to soil erosion began

in the 1960's. The first isotopes measured for this purpose were "Sr, 85S, and 'I (Ritchie

and McHenry, 1990). 137Cs in soil was first seen as an environmental pollutant

discharged from nuclear processing facilities (e.g. Tamura and Jacobs, 1960). But then

Rogowski and Tamura (1965) sprayed "Cs on test plots and found a logarithmic

relationship between measured soil loss and 137Cs loss. Other studies using 137Cs soon

followed.

In the 1970's, Ritchie and McHenry and colleagues began a series of studies in the U.S.

on the usefulness of 137Cs for erosion and sedimentation studies (e.g. Ritchie et al., 1970;

Ritchie et al., 1973; McHenry et al., 1973; Ritchie et al., 1974; Ritchie et al., 1975; and

McHenry and Ritchie, 1977). In the 1980's, a number of researchers around the world

took up the study of "Cs to track soil movement. One group collaborating on such work

included Loughran, Campbell, and Elliott in Australia (e.g. Campbell et al., 1986;

Loughran et al., 1993; Trzcinka, et. al., 1993; Loughran et al., 1995). Another group

included Walling and his associates in England. They began work in the late 1980's on

the use of 'Cs as a soil tracer, with an emphasis on watershed sediment budgets (e.g.

Walling et al., 1986; Walling and He, 1997). Ritchie and McHenry (1990) and Loughran

(1994) wrote thorough literature reviews with extensive bibliographies on applications of

the 137Cs method.

51

4.2 ASSUMPTIONS

The ' 37Cs method for the measurement of erosion employs a number of assumptions. As

stated by Loughran (1994), these are:

1. Fallout of "Cs commenced in 1954 [in Australia], and erosional losses

have occurred since that time;

2. 'Cs is irreversibly fixed to soil materials;

3. 137Cs fallout has been evenly distributed on the landscape;

4. no preferential sorting of sand, silt, and clay (and therefore 'Cs) has

taken place whilst in transport;

5. removal of 137Cs by harvesting of crops has been minimal;

6. there has been minimal loss of 'Cs in runoff, before it could be fixed to

the soil; and

7. a stable reference site can be identified for the establishment of a

reference value.

Assumption 1 was discussed in section 2.5.1. Assumption 2 was discussed in section 3.1.

Assumption 3 is necessary for the method to be used, unless ' 37Cs deposition can be

related to another measurable environmental variable such as average precipitation. To

measure erosion or deposition with 137Cs, one must know what the 137Cs activity would be

in the absence of either process at any location under study.

Assumption 4 is clearly not true. Preferential sorting by grain size takes place during the

entire sediment detachment and transport process. Loughran (1994) proposes that, "In

some instances, at least, 137Cs distribution may be unaffected by these processes because

of the low levels of wCs involved, and the presence of sufficient adsorption sites on all

52

grain sizes." Later on, 'Cs distribution by grain size will be examined to test this

hypothesis.

Assumption 5 could also state that removal of 137Cs from the soil by rangeland vegetation

is minimal. Studies cited in section 3.1 show that "Cs uptake by plants is low, especially

where artificial fertilizers are not added to the soil. If crop roots are harvested with soil

clinging to them, then assumption 5 could cause concern. Regarding assumption 6,

experiments of Comins and Hockley (1991) described in section 3.1 indicate that any

"Cs lost in runoff will be quickly fixed to suspended sediment in the runoff and still

track soil movement. Assumption 7 is crucial for establishing a baseline value against

which "Cs loss and gain can be calculated.

4.3 QUANTIFYING SOIL LOSS

Once one knows the quantity of 137Cs lost out of the total reference amount, the question

becomes, what quantity of soil loss does this represent? Ritchie et al. (1974) related

average percent 'Cs loss to erosion estimated with the USLE (metric tons/ha/yr) for each

land use type. They then combined their data with data from three previous studies on

test plots, two of which involved 90Sr and two of which used radionuclides sprayed on in

quantities much greater than those in fallout, to produce the relationship

Y = 0.87 X' (R? = 0.88)

where Y = soil loss and X = percent radionuclide loss (Ritchie and McHenry, 1990).

Campbell et al. (1986) measured percent 'Cs loss by collecting core samples from

53

within 26 100-m 2 erosion plots and adjacent to four 2-m2 erosion plots. Their plot of

percent "Cs loss versus measured soil loss (kg/ha/yr) from the plots was somewhat "S"-

shaped on a log-log graph. They fitted two equations to the data. For 'Cs loss below

60% and above 60%, respectively, the equations are: S = 4.54 C I 45 (R2 = 0.77)

S = 0.04 C2-74 (R2 = 0.36),

where S is soil loss (kg/ha/yr) and C is percent 'Cs loss.

Loughran (1994) lists separate equations for grazed and cultivated conditions developed

from Australian soil-loss plots. These are, respectively:

Y = 7.74 * 1.09' (R2 = 0.67)

Y = 80.6 * 1.07' (R2 = 0.72),

where Y is net soil loss (kg/ha/yr) and x is percent 'Cs loss. With the same 'Cs loss,

the soil loss is much greater from cultivated soils because 'Cs is distributed much

deeper in the soil by the plow.

Loughran and Campbell (1995) graph 3 different soil loss versus 'Cs loss relationships

for 3 different uncultivated soil types. These were developed using data from 2-m2 , bare-

soil plots monitored for 2.2 years, during which eroded soil was collected and measured 5

times. Again, soils with deeper "Cs penetration had less soil loss for a given amount of

'Cs loss. Also, soils where the initial 'Cs activity was a smaller percentage of the

reference value had the greatest soil loss during the monitoring period, indicating that a

more rapid erosion rate since the start of 'Cs deposition continued through the study

54

period.

Since i37Cs is approximately evenly distributed in the plough layer of cultivated soils in

most cases, a proportional model for estimating net soil loss has been proposed for such

soils. In this model, soil loss is assumed to be directly proportional to 137Cs loss by the

equation:

(0.95R, — R5 )Ene, = Dpx Pb x 0.95R,

where Ene, is net soil erosion (kg/m2), Dp is thickness of the plough layer (m), Pb is the

average bulk density (kg/m3), Itc is the 137Cs fallout reference value (Bq/m2), and R, is the

i37Cs activity at the sampling site (Bq/m2) (Loughran, 1994). The factor of 0.95 is used

with Rc to account for 137Cs losses in drifted snow (in Canada) and harvested grain.

A model to predict the temporal relationship between soil ' 37Cs and soil loss has been

proposed, again applied to cultivated soils. The model predicts the amount of 137Cs

remaining in the soil as a function of time and the erosion rate, taking into account the

fallout rate, radioactive decay, tillage dilution, and seasonal variations in 'Cs deposition

and erosion rates (Loughran, 1994). This model is presumably a more sophisticated

version of the cumulative curves with an erosion function in Figures 3 and 4. The erosion

rate could be determined by matching the observed soil ' 37Cs activity at a particular time

with the erosion rate that predicts that same 137Cs activity. The tillage dilution would

55

depend on the depth of the plough layer. Deeper ploughing mixes a given quantity of

137Cs with a larger volume of soil. This model may be easier to apply to a situation of

continuous erosion than to the very episodic erosion that one expects in an arid desert

environment.

Another potential method of quantifying soil erosion is to compare depth profiles from

eroded and un-eroded locations. If the eroded profile can be recognized as a truncated

version of the un-eroded profile, then the depth interval truncated off the top can be said

to be eroded off. This method assumes that the properties of the two soils are similar

enough for the 137Cs depth profiles to have originally been similar, which can be hard to

prove, and it does not take into account the occurrence of additional 137Cs fallout after

erosion occurs (Loughran, 1994).

4.4 DATING SEDIMENT PROFILES WITH 137Cs

It has become fairly common in the analysis of recent sediment profiles to measure'Cs

activity and assign dates to the layers with the first appearance of 137Cs and the peak ' 37Cs

activity. Sedimentation rates are calculated from the thicknesses of the sediment deposits

between those two dated horizons and between the second horizon and the depositional

surface at the time of collection. The common assumption is that the peak 'Cs activity

in a sediment profile results from the maximum fallout rate in 1963, with possibly a half-

to one-year delay to allow for sediment transport time. This assumption is used by

Ritchie et al. (1973), McHenry et al. (1973), Ritchie et al. (1975), Ritchie and McHenry

56

(1990), Krey et al. (1990), Stihler et al. (1992), Van Metre et al. (1997), Walling and He

(1997), and possibly others.

However, logic dictates that, since "Cs concentrations in soils reflect the cumulative

137Cs fallout with radioactive decay as shown in Figure 5, sediments eroded from those

soils would also carry the cumulative amount of "Cs, not just the 'Cs fallout from the

previous year. Walling et al. (1986) use cumulative 'Cs activity in soil to estimate the

'Cs concentration of suspended sediment over time, and produce a curve of that

parameter shaped very similarly to the curves in Figure 5, with a peak in 1966. They

admit that they slightly overestimate "Cs concentrations because their calculations do

not account for the slight (in England) depletion of soil "Cs due to erosion. Figures 3

and 4 show that erosion, in addition to decreasing the 'Cs concentration, also causes the

peak concentration to occur at a slightly earlier time. Walling does not take into account

his previous work when Walling and He (1997) state that a sharp 'Cs peak in a sediment

core "was assumed to correspond to sediment deposited in 1963/4."

One argument against using 1966 for the 137Cs peak in a sediment profile is that the

accumulation peaks in Figure 5 are broad, and the 'Cs peaks in many sediment cores

appear sharp. There are three possible explanations for this. One is that coarse sampling

intervals relative to the duration of the peak will sharpen the appearance of the peak. A

second is that erosion sharpens the shape of the peak, as demonstrated in Figures 3 and 4.

57

A third is that because 'Cs preferentially sorbs to finer sediment particles, and those

finer particles are preferentially eroded, 'Cs is concentrated in sediments. Walling and

He (1997) use an enrichment ration of 1.6 to account for the increase in ' 37Cs activity per

unit mass in sediments. Van Metre et al. (1997) calculate what they refer to as focusing

factors that range from 2 to 9 that represent the increase in 'Cs activity per unit area

over that of uneroded soil. This enrichment of 'Cs stretches the curves of Figure 5 in

the vertical direction, making the peaks appear sharper.

Another argument against a 1966 date for the 'Cs peak in a sediment profile is that

Ritchie et al. (1973) matched a 1963 date for the peak 137Cs horizon with elevations from

physical reservoir surveys. Here, the difference between a 1966 (or possible 1965 with

erosion) date and a 1963 date for the peak 137Cs horizon may be within the error inherent

in the measurements.

Both Ritchie and McHenry (1990) and Walling and He (1997) warn that the first

appearance of 'Cs in a sediment profile may not be a reliable date horizon because any

reworking of the sediments will spread 137Cs into lower layers.

4.5 OTHER RADIOACTIVE TRACERS

In addition to I37CS, I34cs, 210pb , 7Be, 226.-•x and 232Th have been employed as tracers of

sediment to indicate its source. 134Cs was studied in Europe following the Chernobyl

disaster (Loughran, 1994). 210,“ro with a half-life of 20.4 years, is a decay product of 226Ra

in the 238U decay series, and arrives on the surface via atmospheric fallout, as well as

58

being produced within the soil. The portion that originated as atmospheric fallout is

referred to as unsupported 210Pb, since its quantity is not supported by production within

the soil. 'Be is a cosmogenic isotope with a half-life of 53 days. It typically does not

penetrate more than 1 cm into the soil, so it can identify sediment recently eroded from

the surface.

Bonniwell et al. (1999) used the ratio of 'Be / 21°Pb to indicate the "newness" of sediment

in a mountain stream and calculate the average residence time in a stream of suspended

sediment. 011ey et al. (1993) used relative amounts of 'Be and 137Cs in sediment to

distinguish between sheet, rill, and gully erosion. High concentrations of both isotopes

indicates sheet erosion; high 137Cs with low 'Be indicates deeper sheet or rill erosion.

Low concentrations of both indicates sediment derived from gully wall collapse, while

high 'Be with low 137Cs indicates sediment from the gully floor. They also identified

sediment sources based on differences in the 226R 232a: Th ratio. Unsupported 210Pb, 'Be,

and 137Cs were utilized in Devon, UK, to identify the relative contributions of cultivated

fields, permanent pasture, woodland, and channel banks (cited in Loughran and

Campbell, 1995).

59

SECTION 5. SAMPLING METHODS OF OTHER RESEARCHERS

5.1 DEPTH PROFILES

The HASL started sampling soils for fallout radionuclides in the late 1950's, and

documented their techniques in periodically updated procedures manuals (e.g. Harley,

1972). Their procedure for depth sampling involved digging a trench about 0.6 by 1 m by

0.6 m deep, smoothing one side of the trench so it is vertical, and pushing a three-sided

square pan (15 x 15 x 5 cm deep) with cutting edges on the open side into the smooth side

of the trench. The back edge of the sample was cut with a flat-bladed knife, and the

sample removed and sealed in a plastic bag. Before the next depth increment was

sampled, the shelf in the side of the trench was widened another 15 cm to either side, and

brushed clean to prevent contamination by soil particles from higher layers. Hardy (1974),

also of HASL, used a 20 x 20 x 2 cm pan to sample the top 16 cm, and the 15 x 15 x 5 cm

pan to sample lower layers. A ruler measured depths, and a lumber two by four across the

top of the trench marked the surface. Harley (1972) admits that this technique only

works where there are "no rocks and stones, and very few pebbles," and that such a

condition is fairly rare.

Loughran et al. (1993), working in semi-arid western Australia, placed a 20 x 50 cm steel

frame on the ground and used a scraper plate to remove soil in 2 cm layers to a depth of

10 cm. Below that, two core samples were taken to 20 cm and bulked. Bonniwell et al.

(1999) also sampled the side of a trench, but instead of pushing in a pan to define a fixed

area, they marked a known surface area (41 x 10 cm) on top of the ground adjacent to the

60

trench, and scraped off samples in 0.2 and 0.4 cm layers down to 2 cm with a dry wall

jointing blade. Intervals that fine would also require a soil with no particles larger than

sand size. Below 2 cm, they reduced the sampled surface area to the area of the jointing

blade and carved out 5 cm layers. McHenry and Ritchie (1977), Walling et al. (1986), and

Walling and He (1997) sectioned increments from soil cores to construct depth profiles.

5.2 REFERENCE SAMPLES

5.2.1 Site Selection Criteria

To measure the total inventory of airborn fallout deposited in an area, a sampling site must

be chosen where fallout deposition has not been blocked, and material has not been added

or removed from the site since fallout deposition, i.e. no erosion or sediment deposition.

The HASL/EML criteria (Harley, 1972) call for sampling far enough away from buildings

or trees so that the sites are not sheltered during blowing rain, and sampling nearly level

sites with moderate to good permeability, little or no runoff during heavy rains, and no

over-wash. Areas with a low vegetative cover such as grass are preferred, since this helps

to hold the soil in place. They found such sites on smooth ridge crests, level virgin land,

and in lawns around institutional buildings.

5.2.2 Reference Sampling Methods

HASL/EML researchers found that a sample with a total surface area between 460 and

930 cm' made up of ten or more individual samples will be representative of an area

(Harley, 1972). Their normal procedure is to lay out a tape in a straight line about 5 m

61

long, and collect ten core samples along the line, spaced about 30 cm apart. They first use

an 8.9-cm-diameter top soil cutter to remove the top 5 cm, and then use an auger that cuts

an 8.9-cm-diameter sample to collect the rest of the sample to the desired depth. These

cores are all composited, resulting in a combined surface area of 621 cm'.

In rocky areas where coring is not possible, the template method is substituted. This

consists of removing a 930 cm2 sample with chisels and scoops using a template for

guidance. Large rocks were initially included with the sample, but were later removed

after weighing and brushing off loose dirt. Loughran (1994) described a technique he had

used that is essentially the same as the template method. An apparently stable site is

selected and sampled within a 20 x 50 cm frame at 2 cm depth increments.

Some researchers have performed statistical analyses to determine the variability of 127Cs

and the number of samples necessary to obtain an accurate measurement of the reference

value. Loughran (1994) quotes a 1991 study by Southerland in Saskatchewan that

determined that between 12 and 20 samples are necessary to estimate the reference value

at the 95% confidence level, with an allowable error of ±10%. Each sample consisted of 3

bulked cores with a combined surface area of 68.7 cm2, so the area of 12 to 20 samples is

824 to 1374 cm2 . Loughan's (1994) sampling area of 1000 cm2 is equivalent to 14.5 of

Southerland's bulked samples. Loughran (1994) also cited a 1987 study by Bacchuber

that determined that a minimum of 14 samples were necessary to get the mean areal 127Cs

activity within 10% at a 95% confidence interval for a cultivated area in Germany.

62

5.3 SAMPLE PREPARATION

The ease of sample preparation is a major advantage of the wCs method. The

HASL/EML procedure from Harley (1972) is as follows:

• Spread out the sample on a plastic sheet and allow to air dry for 3 days ormore.

• Break up soil aggregates and cut up root mats and vegetation so it can be laterdistributed homogeneously.

• When dry, weigh the entire sample to within 50 g.

• Remove large rocks, weigh separately, and discard.

• Crush and blend the entire sample.

• Spread out the sample, mark off quarters, and take scoops consecutively fromeach quarter until about 3 kg have been collected.

• Pass this sub-sample through a grinder or pulverizer and then through a"coarse" sieve.

• Shake and mix the sample, and weigh out aliquots for analysis.

Walling et al. (1986) and McHenry and Ritchie (1977) also air-dried their samples, while

Campbell et al. (1986) and Loughran et al. (1993) oven dried their samples at 100 or

105°C. Most researchers sieved their samples before analysis. Ritchie et al. (1975) and

McHenry and Ritchie (1977) passed samples through a 12 mm mesh. Campbell et al.

(1986) used a 2.36 mm sieve, while Walling et al. (1986) and Loughran et al. (1993) only

analyzed soil that passed through a 2 mm sieve.

63

5.4 SAMPLING PA1 IERNS AND METHODS FOR EROSION MEASUREMENT

Ritchie et al.