REEVALUATION OF BACKGROUND IODINE-129

CONCENTRATIONS INWATER FROMTHE

SNAKE RIVER PLAIN AQUIFER, IDAHO, 2003

DOE/ID-22186

Prepared in cooperation with the U.S. DEPARTMENT OF ENERGY

U.S. GEOLOGICAL SURVEY WATER-RESOURCES INVESTIGATIONS REPORT 03-4106
REEVALUATION OF BACKGROUND IODINE-129 

CONCENTRATIONS IN WATER FROM THE SNAKE 

RIVER PLAIN AQUIFER, IDAHO, 2003

By L. DeWayne Cecil (a), L. Flint Hall (b) and Jaromy R. Green (c)

(a) U.S. Geological Survey, Idaho Falls, Idaho

(b) State of Idaho INEEL Oversight Program, Idaho Falls, Idaho

(c) Garden City Community College, Garden City, Kansas

________________________________________________________________________

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 03-4106

Prepared in cooperation with the

U.S. DEPARTMENT OF ENERGY

Idaho Falls, Idaho

May 2003
ii

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director

Any use of trade, product, or firm names in this publication is for descriptive

purposes only and does not constitute endorsement by the U.S. Government.

For additional information write to: Copies of this report can be

purchased from:

U.S. Geological Survey U.S. Geological Survey

INEEL, MS 1160 Information Services

P.O. Box 2230 Box 25286, Federal Center

Idaho Falls, ID 83403 Denver, CO 80225
iii

CONTENTS

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Description of the study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Previous investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Sources of iodine-129 in the eastern Snake River Plain aquifer . . . . . . . . . . . . . . . . . .4

Natural sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Above-ground testing of nuclear devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Nuclear-fuel reprocessing facilities releases . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Methods and quality assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Results of iodine-129 analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

FIGURES

Figure 1. Map showing location of sampling sites, eastern Snake River Plain, Idaho. 2

Figure 2. Histogram of the natural log transform (Ln) of iodine-129 concentrations in 

spring water, ground water, and surface water, Snake River Plain, Idaho,

1992–94 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 3. Iodine-129 concentrations in spring water, ground water, and surface water, 

eastern Snake River Plain, Idaho, 1992-94 . . . . . . . . . . . . . . . . . . . . . . . . . .10

TABLES

Table 1. Concentrations of iodine-129 in water from selected sites on or tributary to the 

eastern Snake River Plain, Idaho (recalculated and modified from Mann and 

Beasley, 1994a). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 2. Field measurements and concentrations of iodine-129 in surface water and springs 

at selected sites on or tributary to the eastern Snake River Plain, Idaho (in order of 

downstream flow) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table 3. Field measurements and concentrations of iodine-129 in water from selected wells 

downgradient from the Idaho National Engineering and Environmental 

Laboratory, Idaho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Table 4. Summary statistics for selected iodine-129 data sets from sites on or tributary to 

the eastern Snake River Plain, Idaho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
iv

CONVERSION FACTORS AND OTHER ABBREVIATED UNITS

Multiply By To Obtain

centimeter per year (cm/yr) 0.3937 inch per year (in/yr)

cubic meter (m3) 35.31 cubic foot (ft3)

curie (Ci) 3.7 x 1010 becquerel (Bq)

attocurie per liter (aCi/L) 3.7 x 10-8 becquerel per liter (Bq/L)

gram per cubic centimeter (g/cm3) 0.5780 ounce per cubic inch (oz/in3)

kilometer (km) 0.6214 mile (mi)

liter (L) 0.2642 gallon (gal)

meter (m) 3.281 foot (ft)

meter per kilometer (m/km) 5.280 foot per mile (ft/mi)

square kilometer (km2) 0.3861 square mile (mi2)

square meter per day (m2/d) 10.76 square foot per day (ft2/day)

degree Celsius (oC) [C (9/5) + 32] degree Fahrenheit (oF)

atoms/L iodine-129 (8.4 x 10-14 min-1)/2.22 picocuries/L iodine-129

Other abbreviated units used in this report:

atoms/cm2 (atoms per square centimeter )

atoms/g (atoms per gram)

atoms/L (atoms per liter)

aCi/g (attocuries per gram )

m3/s (cubic meters per second)

μg/L (micrograms per liter)

mg/L (milligrams per liter)

pCi/L (picocuries per liter)

ppm (parts per million )
1

REEVALUATION OF BACKGROUND IODINE-129 

CONCENTRATIONS IN WATER FROM THE 

EASTERN SNAKE RIVER PLAIN AQUIFER, IDAHO, 

2003

By L. DeWayne Cecil, L. Flint Hall, and Jaromy R. Green 

Abstract

Background concentrations of iodine-129 

(129I, half-life = 15.7 million years) resulting from 

natural production in the earth’s atmosphere, in 

situ production in the earth by spontaneous fission 

of uranium-238 (238U), and fallout from nuclearweapons tests conducted in the 1950s and 1960s 

were reevaluated on the basis of 52 analyses of 

ground- and surface-water samples collected from 

the eastern Snake River Plain in southeastern 

Idaho. The background concentration estimated 

using the results of a subset of 30 ground-water 

samples analyzed in this reevaluation is 5.4 

attocuries per liter (aCi/L; 1 aCi = 10-18 curies) and 

the 95-percent nonparametric confidence interval 

is 5.2 to 10.0 aCi/L. In a previous study, a 

background 129I concentration was estimated on 

the basis of analyses of water samples from 16 

sites on or tributary to the eastern Snake River 

Plain. At the 99-percent confidence level, 

background concentrations of 129I in that study 

were less than or equal to 8.2 aCi/L. 

During 1993–94, 34 water samples from 32 

additional sites were analyzed for 129I to better 

establish the background concentrations in surface 

and ground water from the eastern Snake River 

Plain that is presumed to be unaffected by wastedisposal practices at the Idaho National Engineering and Environmental Laboratory (INEEL). Surface water contained larger 129I concentrations 

than water from springs and wells contained. 

Because surface water is more likely to be affected 

by anthropogenic fallout and evapotranspiration, 

background 129I concentrations were estimated in 

the current research using the laboratory results of 

ground-water samples that were assumed to be 

unaffected by INEEL disposal practices.

INTRODUCTION

Background concentrations of iodine-129 

(129I) in water from the eastern Snake River Plain 

(SRP) aquifer need to be established to evaluate 

the long-term effects of past practices for 129I 

disposal in wastewater at the Idaho National 

Engineering and Environmental Laboratory 

(INEEL, fig. 1). Until 1993, spent nuclear-fuel 

elements from government-owned reactors were 

processed at the Idaho Nuclear Technology and 

Engineering Center (INTEC, fig. 1) at the INEEL 

to recover uranium (U). Between 1953 and 1990, 

wastewater discharged to injection wells and 

infiltration ponds at the INTEC contained an 

estimated 0.56 to 1.18 curies (Ci) of 129I (Mann 

and Beasley, 1994a). 

Monitoring for 129I began in 1976 for wastewater and in 1977 for ground water (Barraclough and 

others, 1982). Work by the U.S. Geological Survey (USGS) on the INEEL from 1977 through 

1988 identified 129I in water from wells at the 

INEEL and demonstrated that the isotope was 

present in measurable concentrations and had traveled with water in the aquifer. With the availability of an accelerator mass spectroscopy-based 

(AMS) method for 129I measurements providing a 

much lower level of detection, background levels 

of the isotope could be determined. In 1992, 16 

water samples from selected sites on or tributary to 

the eastern SRP (table 1 and fig. 1) were analyzed 

to determine background concentrations of 129I 

(Mann and Beasley, 1994a). For this report, background 129I concentrations are defined as those 

levels in water resulting from natural production, 

as well as fallout from atmospheric weapons tests 

in the 1950s and 1960s. The present study 

strengthens the foundation laid by previous work 

by reporting 129I analyses for 34 additional water 
2

Figure 1. Map showing location of sampling sites, eastern Snake River Plain, Idaho.
3

samples from 32 sites on the SRP to reevaluate 

background concentrations (tables 2 and 3). These 

34 water samples were collected during 1993–94 

and included water samples selected from USGS 

archives (1992–93) and two sites sampled in both 

1992 and 1994. Additionally, blind replicate samples were collected from two locations and analyzed for 129I as a quality assurance/quality control 

(QA/QC) measure. 

This study was conducted by the USGS, the 

State of Idaho’s INEEL Oversight Program, the 

U.S. Department of Energy (DOE), U.S. Environmental Protection Agency’s (EPA) Region 10, and 

the Idaho Department of Health and Welfare, 

Division of Environmental Quality.

Purpose and Scope

This report describes analytical results for 34 

water samples collected from 32 sites to reevaluate background concentrations of 129I in the eastern SRP aquifer. Some samples were collected 

during 1993–94 and others were selected from the 

USGS archive sample library at the INEEL. In 

addition, data analysis in this report includes 16 

analytical results for samples collected in 1992 and 

reported by Mann and Beasley (1994a). 

Description of the Study Area

The eastern SRP (fig. 1) is a structural downwarp filled with basalt that is generally within 3 m 

of the land surface (Whitehead, 1992). Unconsolidated sediments overlie the margins of the plain 

and are interbeded with basalt and sediments at 

depth. The basalt is several hundred to as much as 

1,500 m thick and underlies most of the plain. 

Fractures and vesicular zones near the surface of 

the basalt flows may be highly transmissive of 

ground water. Reported transmissivities for the 

eastern SRP aquifer range from 0.1 to more than 

70,000 m2/d, a range of nearly six orders of magnitude (Ackerman, 1991). Depth to ground water 

in the basalt aquifer at the INEEL varies from 60 

m below land surface in the northern part to more 

than 275 m in the southern part. The hydraulic gradient is about 1 m/km and horizontal flow velocities range from 1 to 6 m/d. This range is based on 

the distribution of chlorine-36 (36Cl) through time 

as determined from analysis of archived samples 

(Cecil, 2000) and equates to a travel time of 100 to 

200 years for water to travel from beneath the 

INEEL to springs that discharge at the terminus of 

the eastern SRP aquifer, and about 300 years or 

more for water to travel from all places within the 

aquifer to the terminus of the eastern SRP aquifer 

(Ackerman, 1995). 

Long-term (1950–88) average precipitation in 

the vicinity of the INEEL is 22 cm/yr (Clawson 

and others, 1989). About 40 percent of the longterm precipitation on the eastern SRP is rainfall 

between April and September. However, as a 

result of evapotranspiriation, generally less than 5 

percent of that precipitation infiltrates the surface 

locally on the SRP (Cecil and others, 1992). 

Recharge to the SRP aquifer is from precipitation 

in the mountains to the west, north, and east and 

from irrigation return flow and surface water. Five 

watersheds that recharge the aquifer are the Big 

Lost River, Little Lost River, Birch Creek, Camas 

Creek/Mud Lake, and the Snake River drainage 

(fig. 1).

The INEEL comprises about 2,300 km2 of the 

eastern SRP. The INEEL was established in 1949 

and is used by the DOE to construct and test 

nuclear reactors and to participate in various 

defense programs. Radiochemical and chemical 

wastes generated at the INEEL and other DOE 

facilities have been buried at the site since 1952. 

Additionally, from 1952 to 1984, low-level radioactive and chemical wastes were discharged into 

the SRP aquifer at the INTEC through a 182-mdeep disposal well. Since 1984 at the INTEC, and 

from 1952 to 1993 at the Test Reactors Area (fig. 

1), these wastes also have been discharged to infiltration ponds. 

Previous Investigations

Previous investigations of 129I in water from 

the eastern SRP at the INEEL include those by 

Barraclough and others (1982), Lewis and Jensen 

(1985), Mann and others (1988), and Mann and 

Beasley (1994a, 1994b). Barraclough and others 

(1982) and Lewis and Jensen (1985) discussed 

results of sampling for 129I at 20 sites in 1977 and 

32 sites in 1981, respectively. Analytical results 

for samples collected in 1977 included concentra-
4

tions as large as 27±1 pCi/L (1 pCi = 10-12 Ci); 

concentrations were detectable (about 0.9 pCi/L) 

as far as 5 km from the point of disposal at the 

INTEC. Concentrations as large as 41±2 pCi/L 

and detectable as far as 10 km from the point of 

disposal were reported for the 1981 sampling 

(Lewis and Jensen, 1985). The major difference 

between the 1977 and 1981 results was the reduction in the reporting level from approximately 0.9 

to 0.05 pCi/L. The increase in analytical sensitivity was a result of an increase in sample size used 

for the neutron-activation analysis method from 1 

to 4 L. Mann and others (1988) again analyzed 

water samples for 129I by neutron activation, collecting from sites visited in 1977 and 1981, as well 

as from additional sites. Although the portion of 

the aquifer where concentrations exceeded the 

reporting level had changed little from 1981 to 

1986, the maximum concentration had decreased 

by an order of magnitude to 3.6±0.4 pCi/L from 

the largest concentration reported in 1981. 

During 1990–91, Mann and Beasley (1994b) 

collected samples from 51 wells at and near the 

INEEL and utilized AMS analysis methods for 

increased sensitivity (two to six times more sensitive than neutron activation). Included were sites 

sampled in 1977, 1981, and 1986. Although the 

maximum concentration measured during 1990–91 

(3.81±0.19 pCi/L) was indistinguishable from the 

maximum concentration measured in 1986 

(3.6±0.4 pCi/L), the average concentration for 

sites sampled previously decreased, continuing the 

downward trend observed for the 1986 sampling. 

Sampling during 1990–91 included one site upgradient from the point of disposal at the INTEC. The 

result from this site established a preliminary 

background concentration of 0.9±0.2 aCi/L. Also 

included were two wells 6 and 13 km south of the 

INEEL southern boundary. Water from these wells 

contained concentrations of 10±1 and 30±2 aCi/L, 

which documented migration of 129I as much as 26 

km south of the point of disposal and 13 km 

beyond the INEEL boundary.

In 1992, Mann and Beasley (1994a) sampled 

16 sites not likely to have been impacted by 

INEEL waste disposal to better define background 

levels of 129I. Concentrations ranged from 0.1±0.1 

to 8.1±0.6 aCi/L and averaged 3.3±2.1 aCi/L. At 

the 99-percent confidence level, background concentrations of 129I were estimated to be less than or 

equal to 8.2 aCi/L. For comparison, the Maximum 

Contaminant Level (MCL) established by the EPA 

for 129I in drinking water is 1 pCi/L (U.S. Environmental Protection Agency, 1991), or more than 

five orders of magnitude larger than the estimated 

background concentration of 8.2 aCi/L.

SOURCES OF IODINE-129 IN THE 

EASTERN SNAKE RIVER PLAIN 

AQUIFER

The sources of 129I in the ground and surface 

water of the eastern SRP are (1) natural atmospheric production and in situ spontaneous fission 

of 238U in the lithosphere (Fabryka-Martin and 

others, 1985), (2) above-ground testing of nuclear 

devices, and (3) releases from nuclear-power and 

nuclear-fuel reprocessing facilities (National 

Council on Radiation Protection and Measurements, 1983). These sources are reviewed in this 

report to determine their relative importance as 

contributors of 129I to SRP water and to provide a 

basis for determining their relation to measured 

129I concentrations. 

Natural Sources

The principal natural sources of 129I to water of 

the SRP are (1) production in the atmosphere by 

cosmic-ray spallation of xenon isotopes, and (2) 

production from the spontaneous fission of 238U 

isotopes in situ in the lithosphere. Global model 

calculations by Fabryka-Martin and others (1985) 

indicate an equilibrium 129I areal inventory in surface soils to 1-m in depth to be about 2×105

atoms/cm2. On the basis of data from 30-cm-deep 

soil cores collected worldwide by the DOE’s Environmental Measurements Laboratory (EML) (Beasley and others, 1998), average soil density is 1.3 

g/cm3 and the natural 129I concentration in surface 

soil is on the order of 1.5×103 atoms/g of soil. 

Because of the biophilic nature of iodine, which 

results in sequestration in organic materials (Wagner and others, 1996), and because most organic 

materials in soils are in the upper few centimeters 
5

of the soil column, 129I concentration typically are 

on the order of 103 to 105 atoms/g in soils (Rao, 

1997). 

Natural 129I concentrations in ground water 

have been measured across the globe. For example, concentrations from 2×10-3 to 1×10-1 aCi/L 

(5×104 to 2.5×106 atoms/L) were reported for 

ground water from the Great Artesian Basin, and 

from 1×10-2 to 4×10-1 aCi/L (2.5×105 to 9.8×106

atoms/L) for ground water from the Milk River 

aquifer in southern Saskatchewan, Canada (Fabryka-Martin and others, 1987). These background 

129I concentrations are less than that estimated in 

this report by one to three orders of magnitude. 

This suggests that water collected from the SRP 

aquifer system has a component of anthropogenically, as well as naturally, produced 129I and that 

processes such as evapotranspiration probably 

have some effect on the concentrations of all isotopic tracers used to delineate contaminant plumes 

and to describe the hydrogeology at and near the 

INEEL (Cecil and others, 1999; Cecil, 2000).

 Fabryka-Martin and others (1987) calculated 

the secular equilibrium 129I concentration in rocks 

containing differing U concentrations and concluded that 129I in sandstone and basalt (1 ppm U 

on average) reaches maximum concentrations of 

1,500 atoms/g of rock at secular equilibrium. Shale 

(5 ppm U on average) and granite (3 ppm U on 

average) contained calculated concentrations of 

6,800 and 4,200 atoms/g of rock, respectively. 

Given that age dates for SRP basalt have ranged 

from 2,000 to 5,000,000 years before present 

(Kuntz and others, 1992), 129I concentrations in 

SRP basalt should be smaller than the concentrations reported by Fabryka-Martin and others 

(1987). 

In contrast, if elevated concentrations of U 

exist in areas surrounding the plain, they would 

serve as additional sources of 129I to surface and 

ground water. By analogy, reference is made to 

measurements of 129I in the Alligator River Uranium Ore Province, Northern Territory, Australia, 

and in the Stripa granite formation in Sweden 

(Fabryka-Martin and others, 1987, 1988). In the 

Alligator River deposits (12,000 ppm U) the largest ground-water concentration of 129I measured in 

the ore body was 0.5 aCi/L, although residence 

times of the ground water pumped from the vicinity of the ore deposits were believed to be short, on 

the order of years to decades. Also, as a result of 

spatial variations in hydraulic conductivities, 

waters that had not contacted the ore deposits in 

the subsurface may have diluted the pumped 

ground water samples. In the case of the Stripa 

granite (which contained 44 ppm U, but with concentrations in fracture-lining minerals of as much 

as 5 percent U), water collected from 370 to 1,230 

m below land surface contained progressively 

increasing 129I concentrations ranging from 0.05 

aCi/L (at 370 m) to 9.0 aCi/L (at 970 to 1,230 m). 

At the greater depths, the 129I was ascribed entirely 

to in situ production. The residence times of water 

collected at 815 m (0.80 aCi/L) and that collected 

at 970 to 1,230 m were calculated to be 30,000 and 

300,000 years, respectively, which would permit 

accumulation of 129I in the water-bearing fracture 

zones. 

For U to be a significant contributor of 129I to 

water in wells, springs, and streams on and tributary to the eastern SRP, the U content of these 

bodies would need to equal or exceed that measured in the Alligator River ore deposits, and (or) 

water residence times in these host bodies would 

need to approximate (or exceed) those estimated 

for the Stripa granite. Even then, it is unlikely that 

the areal extent of the ore deposits would be sufficient to label enough water flowing through them 

to affect the nearly 4×1011 m3 of water estimated 

to be in the upper 170 m of the eastern SRP basalts 

(Lindholm, 1988). Additional factors that could 

influence the concentration of 129I from in situ production include the distribution of the U in the 

basalt relative to the effective porosity of the 

basalt, the escape rate of 129I into the flow system 

from the production sites within the aquifer 

matrix, and the water/rock ratio within the flow 

system. These factors were not determined for this 

study. 

For analogy, Cecil and others (2000) calculated the in situ production of 36Cl in the major 

water-bearing rock units of the eastern SRP aquifer system. The in situ production of 36Cl was 

determined to be insignificant compared with concentrations measured in ground water near buried 

and injected nuclear waste sites at the INEEL. It 
6

was concluded that this is a result of the relatively 

small concentration of U in the basalt of the SRP 

aquifer system and the relatively short residence 

time of water in this system. Therefore, because 

the residence time of the ground water is relatively short (300 years or less in most cases), compared with the half-life of 129I (15.7 million years), 

concentrations of 129I in ground water that result 

from in situ production by spontaneous fission of 

238U are expected to be insignificant. 

Above-Ground Testing of Nuclear 

Devices

Haury and Schikarski (1977) estimated that 

approximately 12 Ci (3.2×1026 atoms) of 129I were 

introduced into the atmosphere from above-ground 

testing of nuclear devices through 1976. If the deposition pattern of the 129I followed that of plutonium, about 70 percent of the total atoms of 129I 

would have been deposited in the Northern Hemisphere (Hardy and others, 1973). Of this 70 percent, 27 percent would have been deposited in the 

40° to 50° north latitude band (area = 3.17×107

km2; Baumgartner and Reichel, 1975). Thus, the 

areal inventory of 129I in soils of the SRP from 

global fallout would be about 2×108 atoms/cm2. 

Gove and others (1994) have shown that, in southeastern Idaho soils, 90 percent of anthropogenic 

129I is retained in the upper 10 cm of the soil column. Data from 123 soil cores from nonagricultural lands on the SRP give an average volumetric 

soil density of 1.2 g/cm3 for the top 10 cm (Beasley and others, 1998). Thus, the calculated concentration of 129I in SRP soils contaminated by 

integrated global fallout is approximately 0.8 aCi/g 

(2×107 atoms/g). This is about two orders of magnitude larger than the concentration estimated to 

result from the steady-state deposition of naturally 

produced 129I in the atmosphere to near-surface 

SRP soils, even when considering that this naturally produced 129I is likely to be concentrated in 

the upper few centimeters of undisturbed soils. 

This calculated concentration does include regionally derived 129I input from testing at the Nevada 

Test Site (NTS), however, at present this input 

cannot be quantified precisely. 

Nuclear-Fuel Reprocessing Facilities 

Releases

Over the operating history of the INTEC, an 

estimated 1.1×1026 atoms (4 Ci) of 129I have been 

released to the atmosphere (Mann and others,

1988). A comparable release has been estimated 

for nuclear-fuel reprocessing facilities at the 

Savannah River site near Aiken, South Carolina 

(4.7 Ci, or 1.2×1026 atoms; Kantelo and others, 

1982). Approximately 51 Ci (1.4×1027 atoms) of 

129I was present in nuclear fuels from the Hanford 

plutonium production reactors that were processed from 1944 to 1986 (Robkin and Shleien, 

1995). Similarly, large amounts of 129I have been 

released from the nuclear-fuel reprocessing facilities at Sellafield, United Kingdom, and Cap de la 

Hague, France (180 Ci, or 4.8×1027 atoms) to the 

ocean alone; (Raisbeck and others, 1994). Atmospheric releases of 129I from these two European 

facilities are thought to have been transported into 

the Arctic Ocean Basin as far north as Mould Bay 

on Prince Patrick Island, Canada (Kilius and others, 1995), and, hence, also could contribute to 

fallout at the INEEL. 

Depth of sampling is important in the comparisons discussed in the preceding paragraphs; therefore, the comparison made here is qualitative at 

best. Concentrations of 129I in the upper 5 cm of 

soil (where 129I is often efficiently retained) are 

expected to be higher than concentrations in the 

upper 10 cm of the soil column because of greater 

dilution with 129I-free soil as depth of sampling 

increases. However, the pronounced differences in 

soil 129I concentrations measured near and at great 

distances from nuclear-fuel reprocessing facilities 

confirm their importance in influencing environmental inventories of 129I over regional, continental, and global scales. The concentration of 129I in 

soil within 2 km of the INTEC is as large as 

10,400 aCi/g (2.8×1011 atoms/g); at Island Park, 

Idaho (200 km northeast of the INEEL; fig. 1), the 

concentration of 129I in soil is 48 aCi/g (1.3×109

atoms/g). This concentration is more than two 

orders of magnitude smaller than that in soils close 

to the INTEC but nearly 2 orders of magnitude 

larger than that predicted as a result of global 

weapons-tests fallout (about 1 aCi/g, or 2×107

atoms/g, Gove and others, 1994). Brauer (1974) 
7

recorded 129I concentrations in surface soil in 

Washington, Idaho, and Montana ranging from 2 

aCi/g to 8 aCi/g (5×107 and 2.8×108 atoms/g), 

respectively, concentrations similar to those 

expected from global fallout. However, concentrations of 129I in soil within 200 km of the Hanford site were larger than those reported by Brauer 

(1974) by as much as two orders of magnitude as a 

result of nuclear-fuel reprocessing releases. At 

Richland, Washington, near Hanford, concentrations of 129I in surface soil were as large as 1,600 

aCi/g (4×1010 atoms/g) (Brauer and Strebin, 

1982).

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the work 

on an early draft of this research by Larry J. Mann 

(USGS, retired) and Tom M. Beasley (DOE, 

retired). Additionally, thanks are due to David 

Frederick, State of Idaho INEEL Oversight Program and F.P. Brauer, formerly with the Pacific 

Northwest National Laboratory, Richland Washington for their review of an early draft of this 

work. We also acknowledge the earlier studies 

undertaken to establish a background 129I concentration in water from the eastern SRP and the 

extensive sample collection and sample processing efforts (Mann and Beasley, 1994a, 1994b). 

The present report was reviewed by Dr. June 

Fabryka-Martin with Los Alamos National Laboratory, New Mexico, and Ms. Kristi Moser-McIntire, State of Idaho INEEL Oversight Program. 

We thank them for their constructive comments 

that improved the technical content and clarity of 

this report.

METHODS AND QUALITY 

ASSURANCE

The 34 samples from the 32 sites and the 2 

QA/QC samples included in this study were collected, processed, and analyzed in the same manner as the 16 samples previously described by 

Mann and Beasley (1994a). Water samples from 

streams were collected downstream from riffles to 

ensure that the surface water was well homogenized. At sites where the stream was shallow and 

could be waded, samples were obtained from at 

least three locations across the stream and composited into a single sample for field processing. 

For deeper streams, water was collected as far as 

possible from each bank where the water was at 

least 1 m deep and well mixed. Following collection, the two samples were composited into a single sample.

Field processing consisted of filtering the water 

through a disposable, 0.45- micrometer filter cartridge that had been pre-rinsed with at least 1 L of 

deionized water and 1 L of water from the field 

site. Filtration was necessary to remove particulate matter which could affect the laboratory 

preparation of the silver-iodide targets used in the 

AMS measurements of 129I. In addition to the 34 

primary samples, 2 blind replicate samples were 

submitted to the laboratory for analysis (tables 2 

and 3).

Blind replicate QA/QC samples were analyzed 

from the Portneuf River near Tyhee and from the 

Unnamed Spring No. 2 above Shoshone Powerplant near Twin Falls (fig. 1 and table 2). Relative 

percent differences for these replicate pairs were 

15 and 40, respectively. Six samples taken from 

USGS archives are included in table 2. All the 

archived samples were collected in 1992, and 

among them were samples from Box Canyon 

Springs near Wendell and Briggs Creek near Buhl, 

which were also sampled in 1994. Results for the 

archived 1992 samples were compared with results 

from the samples collected in 1994 at Box Canyon Springs near Wendell and Briggs Creek near 

Buhl. Relative percent differences were 27 and 29; 

129I concentrations in both 1992 samples were 

smaller than in the 1994 samples. In addition to 

natural heterogeneity between replicate water samples, factors contributing to differences in analytical results include incomplete equilibration of 

iodide carrier with 129I in the sample and cross 

contamination of the prepared sample during processing or in the ion source of the accelerator during the sputtering process (Mann and Beasley, 

1994a; David Elmore, Purdue University, written 

commun., 2002). Impact to the final analytical 

results from incomplete equilibration and cross 

contamination are lessened by the standard proto-
8

col of repeating measurements that are averaged to 

calculate the final ratio of 129I/127I (David Elmore, 

Purdue University, written commun., 2002).

RESULTS OF IODINE-129 

ANALYSES

The present study combines 52 results from 48 

sites: 20 samples and 2 QA/QC samples collected 

in 1993-94, 6 samples from USGS archives that 

were collected in 1992, 8 samples collected in 

1993, and 16 samples collected in 1992. Subsets of 

this analytical data set were identified for calculation of summary statistics to evaluate possible 

trends with respect to sample type and geographic 

location and to quantify uncertainties for subpopulations. Site locations are shown in figure 1. Concentrations of 129I for these samples are listed in 

tables 1-3. Sample sites from the study by Mann 

and Beasley (1994a) are listed in table 1 with 

recalculated results for this study. Results for 129I 

in water samples from streams and springs that 

were collected for this reevaluation, or that were 

selected from the USGS sample archives are 

shown in table 2. Results for 129I in ground-water 

samples from selected sites in the USGS sample 

archives or samples collected in 1993 are shown in 

table 3. Archived ground-water samples listed in 

table 3 are from five U.S. Bureau of Land Management-owned stock wells sampled in 1993 for 

selected radiochemical and water chemistry to 

address public concerns about the impacts of 

INEEL activities on ground water (Bartholomay 

and others, 2001). The results for the blind replicates (QA/QC samples) from the Portneuf River 

near Tyhee and for the Unnamed Spring No. 2 

above Shoshone Power Plant near Twin Falls 

(table 2) were not included in the calculation of the 

background 129I concentration. The results for the 

1994 primary and 1992 archived samples for Box 

Canyon Springs near Wendell and Briggs Creek 

near Buhl (table 2) were included in the summary 

statistics.

Sample sites were categorized by a combination of their relative location (eastern SRP or tributary valleys) and by the type of site (surface water, 

or water from springs and wells). Eastern SRP 

sites are wells, springs, and surface-water sites 

within the boundaries of the plain (fig. 1). The 

Snake River at King Hill (the outflow point for the 

eastern SRP and tributary basins) was included in 

this subset. Wells that were not completed in the 

eastern SRP aquifer, and springs or surface water 

sites whose sources or drainage basins lie outside 

the eastern SRP, were categorized as tributary 

sites. Although the sampling sites Henrys Fork at 

St. Anthony and the Portneuf River near Tyhee are 

located within the boundaries of the eastern SRP, 

most of their drainage basins lie outside of the 

plain, so these two sites were categorized as tributary sites. The category of ground-water sites 

includes wells completed in, and springs that discharge from, the eastern SRP aquifer. Excluded 

from this category are samples from Lidy Hot 

Springs and the City of Arco well as these sites are 

tributary to the SRP.

Iodine-129 concentrations for these 50 samples 

(minus the 2 replicate samples) ranged from 

0.29±0.18 aCi/L to 30.3±6.2 aCi/L. The largest 

129I concentrations were measured in streams tributary to the SRP: Big Wood River near Bellevue 

(30.3±6.2 aCi/L), Silver Creek near Picabo 

(24.6±0.7 aCi/L), Beaver Creek at Spencer 

(18.0±1.3 aCi/L), Henrys Fork near Island Park 

(12.7±0.5 aCi/L), and Medicine Lodge Creek near 

Small (10.0±0.3 aCi/L). Coffee Point (CCC-3), a 

well completed in the SRP aquifer, contained the 

same concentration as Medicine Lodge Creek near 

Small (10.0±0.4 aCi/L) (table 2). The smallest 129I 

concentrations were measured in a sample from 

the Teton River near St. Anthony (0.49±0.10 

aCi/L) and water samples from wells MV-59 

(0.29±0.18 aCi/L) and MV-04 (0.44±0.19 aCi/L) 

(tables 2 and 3, respectively). The Teton River 

near St. Anthony is tributary to the plain, and wells 

MV-59 and MV-04 are completed in the eastern 

SRP aquifer south of the INEEL (fig. 1).

The log-normal distribution is common with 

variables having a natural lower or upper limit and 

is prevalent in geochemical data (Rock, 1988). A 

log-normal distribution can be identified by testing for normality on the natural logs of the data. A 

natural log transform was applied to the data to 

test for normality using the Shapiro-Wilk W test 

(Royston, 1992). Because the W test (W = 0.9322, 

p = 0.0067) shows that the data were not normally
9

Figure 2. Histogram of the natural log transform (Ln) of iodine-129 concentrations in spring water, 

ground water, and surface water, eastern Snake River Plain, Idaho, 1992–94.

distributed (and more nearly resembled a lognormal distribution, fig. 2) nonparametric statistics 

were used to analyze the data.The median (the 

50th-percentile value) is a resistant statistical 

measure only minimally affected by outliers 

(Helsel and Hirsch, 1992) and best reflects the 

central tendency of these data. A corresponding 

measure of the spread of the data is the 

interquartile range (the difference in value 

between the 25th and 75th percentiles).

Summary statistics that include maximum, 

minimum, median and associated 95-percent confidence interval, the interquartile range, and the 

95th-percentile values for subpopulations based on 

sample locations are presented in table 4. The 95thpercentile values for each subset and associated 

95-percent confidence intervals were calculated 

using nonparametric statistical methods (Helsel 

and Hirsch, 1992). The median concentration for 

sites tributary to the plain (8.4 aCi/L and the 95-

percent confidence interval is 2.6 to 10.0 aCi/L) is 

larger than the median concentration for sites on 

the plain (3.4 aCi/L and the 95-percent confidence 

interval is 3.0 to 4.4 aCi/L). The median concentration for surface-water sites (8.2 aCi/L and the 

confidence interval is 5.7 to 10.0 aCi/L) is larger 

than the median for the ground-water sites (3.3 

aCi/L and the confidence interval is 2.5 to 4.1 

aCi/L, table 4). The differences between groundwater sites and surface-water sites are probably a 

result of evapotranspiration which concentrates the 

129I in surface water and in recharge to ground 

water from irrigation compared to the groundwater concentrations.

The box-whisker plots in figure 3 illustrate the 

absolute and relative values of the median, the 

confidence interval about the median, the interquartile range, and the potential outliers for the 

subsets of data evaluated. Little variation is evident in 129I concentrations from the spring sites 

(fig. 3). Although the range of concentrations is 

larger for wells, the medians do not differ statistically for these data sets. Three possible outliers are 

evident from the diagram for surface-water sites. 

The larger median concentration for surface-water 

sites is also clearly apparent.
10

Figure 3. Iodine-129 concentrations in spring water, ground water, and surface water, eastern Snake 

River Plain, Idaho, 1992-94.

(a) All samples. (b) Samples distinguished by source; spring water, ground water (water from wells 

completed in the Snake River Plain aquifer), and surface water. (c) Ground water (includes spring water 

and water from wells completed in the Snake River Plain aquifer) and surface-water samples. Samples 

from the City of Arco well and Lidy Hot Springs are excluded from (b) and (c), see text for explanation.
11

In this report, review of the 129I results from all 

50 samples (minus the 2 replicate samples) suggests that the background concentration for the 

SRP should be estimated using only data for samples collected from wells completed in the aquifer 

and springs discharging from the aquifer that are 

unaffected by disposal practices at the INEEL, and 

that surface-water samples should be excluded. 

This concentration at the 95th-percentile value is 

5.4 aCi/L, and the 95-percent confidence interval 

about that value ranges from 5.2 aCi/L to 10.0 

aCi/L (table 4). Mann and Beasley (1994a) determined that results for 16 ground-water, surfacewater and spring-water sites sampled in 1992 were 

normally distributed, and constructed a parametric, 99-percent, one-tailed confidence interval to 

establish an upper bound of 8.2 aCi/L for the combined contributions of naturally produced 129I and 

129I from global fallout from atmospheric weapons tests. 

One ground-water sample was identified as an 

outlier in the statistical analysis. The result from 

1993 for water from the Coffee Point well (CCC3; 10.0±0.4 aCi/L) defines the upper bound to the 

95-percent confidence interval for the 95-percent 

value for ground water from the SRP aquifer in 

figure 3. Followup sampling in 1998 by the State 

of Idaho INEEL Oversight Program at this same 

site yielded a result of 0.16±0.013 aCi/L (L. F. 

Hall, unpubl. data, 2002), which suggests that the 

1993 Coffee Point result may represent a transient 

event and, thus, may not be representative of background concentrations or may reflect sample contamination. Because nonparametric statistical 

methods were utilized in this reevaluation, the 

median and the 95-percent value for SRP aquifer 

wells and springs would not be changed significantly (from 3.3 to 3.2 aCi/L for the median and 

5.4 to 5.3 for the 95th-percentile value) by excluding the 1993 Coffee Point result from the calculations. However, excluding this result from the 

calculations would change the confidence interval 

about the median (from 3.1 to 4.4 aCi/L, to 2.5 to 

3.9 aCi/L), and the confidence interval about the 

95th-percentile value (from 5.3 to 10.1 aCi/L, to 

4.6 to 5.3 aCi/L). 

The principal sources of 129I in wells and 

springs on the eastern SRP for those locations that 

can be reasonably assumed to be uncompromised 

by disposal of low-level radioactive waste from 

the INEEL are infiltration of recharge water containing 129I from anthropogenic sources other than 

the INEEL (global fallout) and natural production 

in the atmosphere and the lithosphere. In situ production in the aquifer matrix is considered to be a 

nonexistent or minor source of 129I. 

Cecil and others (2000) presented the results of 

calculations for SRP rock samples and showed that 

the in situ 36Cl production within the rock matrix 

is negligible and is on the same order of magnitude as natural atmospheric production, which suggests that the 129I in situ production would be 

insignificant as well. In situ production is considered small, not only because of the relatively small 

U content and young age of the SRP basalt, as discussed earlier, but also because the residence time 

of water in the eastern SRP aquifer is estimated to 

be generally less than 300 years (Ackerman, 1995; 

Cecil, 2000), a time far too short to allow significant 129I buildup from the spontaneous fission of U 

isotopes. Lindholm (1988) estimated that in 1980, 

the eastern SRP received 8.3×109 m3 of recharge 

and, of this total, 60 percent (5×109 m3) was attributable to irrigation return and 20 percent (1.7×109

m3) was a result of tributary flow. If these volumes are representative of recharge over the past 

two decades, and given that nearly one-third of the 

entire SRP area is presently under irrigation (Lindholm, 1988), additional 129I could move from the 

landscape to surface and ground water. The efficiency of this removal process, however, has not 

been determined.

On the basis of reevaluation of the sample 

results presented in this report, it was concluded 

that the maximum background concentration for 

129I in ground water from the eastern SRP at the 

95th-percentile value is 5.4 aCi/L, and the nonparametric confidence interval is 5.2 to 10.0 aCi/L. 

Concentrations above this level may be a result of 

evapotranspiration processes, resuspension and 

concentration of fallout from anthropogenic 

sources, or contamination from wastewater disposal at the INEEL.
12

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13

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15

Table 1. Concentrations of iodine-129 in water from selected sites on or tributary to the eastern 

Snake River Plain, Idaho (recalculated and modified from Mann and Beasley, 1994a). 

Note: iodide concentrations were not measured at the time of sampling; a concentration of 0.001 mg/L was assumed for the purpose of calculating 129I results.

Site name (figure 1)

Site identification

number

Date sampled

(m/d/y)a

a. Date format: month/day/year.

129Ib

b. Uncertainties at 1 sample standard deviation.

Sample type (aCi/L)c

c. Attocuries per liter.

atoms/Ld

 (x 106)

d. Atoms per liter.

Springs and streamse

e. Springs and streams listed in order of downstream ground-water flow.

Birch Creek at Blue Dome 13117020 4/6/92 2.1±0.4 56.8±9.44 Surface water

Little Lost River near Howe 13119000 4/6/92 7.9±0.8 209±21.2 Surface water

Big Lost River at Mackay 13127700 4/6/92 2.6±0.4 69.8±10.6 Surface water

Crystal Spring near Buhl 13093396 4/6/92 3.0±0.4 78.6±10.9 Spring water

Banbury Springs near Buhl 13095300 3/2/92 3.0±0.5 78.1±12.0 Spring water

Thousand Springs near Hagerman 13132800 3/3/92 1.5±0.4 39.7±10.6 Spring water

Birch Creek Spring near Hagerman 13135100 3/3/92 5.2±0.4 137±11.5 Spring water

Wells 

Atomic City well 432638112484101 1/22/92 1.1±0.4 28.4±11.6 Ground water

City of Arco well 433758113181701 6/2/92 1.0±0.4 26.1±9.73 Ground water

P&W 2 435419112453101 4/8/92 5.3±0.5 141±12.5 Ground water

Well 4 (USGS-004) 434657112282201 4/24/92 5.3±0.6 141±15.6 Ground water

Well 19 (USGS-019) 434426112575701 4/8/92 1.6±0.4 42.9±10.3 Ground water

Well 26 (USGS-026) 435212112394001 4/21/92 3.9±0.5 102±12.5 Ground water

Well 97 (USGS-097) 433807112551501 3/16/92 4.3±0.5 114±13.5 Ground water

Well 100 (USGS-100) 433503112400701 3/27/92 2.0±0.4 52.1±10.3 Ground water

Well 110 (USGS-110) 432717112501501 1/22/92 3.1±0.4 82.6±11.5 Ground water
16

Table 2. Field measurements and concentrations of iodine-129 in surface water and springs at selected sites on or tri

Snake River Plain, Idaho (in order of downstream flow). 

Site Site identification number Date sampled (m/d/y)a

a. Date format: month/day/year.

Discharge

(m3/s)b

b. Cubic meters per second.

Water

tempc

c. Degrees Celsius.

pH Specific conductanced

d. Microsiemens per centimeter at 25°C.

Dissolved

iodidee

e. Milligrams per liter.

129I

f. Uncertainties at 1 sample standard deviation.

(aCi/L)g

g. Attocuries per liter.

a

h. Atoms per liter.

Snake River near Irwin 13032500 3/2/94 41.6 2.5 7.9 370 0.002 7.0±0.3 Snake River at Heise Bridge 433847111420000 3/2/94 57.2 6.0 7.6 482 0.003 7.1±0.4 Henrys Fork near Island Park 13042500 3/3/94 12.5 5.0 7.8 159 0.001 12.7±0.5 Henrys Fork at St. Anthony 13050500 3/3/94 52.1 5.5 8.5 170 0.001 8.6±0.4 Teton River near St. Anthony 13055000 3/2/94 12.1 6.5 7.8 340 0.001 0.49±0.10 Snake River near Shelley 13060000 3/1/94 89.8 4.5 8.1 319 0.002 9.2±0.5 Blackfoot River near Shelley 13066000 3/1/94 2.3 5.5 8.3 450 0.003 14.3±0.6 Portneuf River near Tyhee 13075910 3/4/94 15.2 8.5 7.7 650 0.004 8.4±0.3 

9.8±0.7 

Snake River at Neeley 13077000 3/4/94 64.6 8.0 8.5 590 0.003 6.2±0.3 Snake River at Milner 13088000 3/4/94 6.7 6.5 8.5 590 0.004 7.5±0.4 Devils Washbowl Spring near Kimberly 13089600 3/4/92i

i. Archived sample.

0.4 14.0 8.3 638 j

j. Not measured, assumed to be 0.001 mg/L for calculation of the iodine-129 concentration.

3.4±0.2 

Devils Corral Spring near Kimberly 13090100 3/4/92i 1.1 15.0 8.0 630 j 2.5±0.2 Unnamed Spring No 2 above Shoshone 13090350 3/4/92i 0.1 14.0 8.5 425 j 4.1±0.3 

Powerplant near Twin Falls 2.7±0.2 

Blue Lakes outlet near Twin Falls 13091500 3/9/94 4.1 16.0 8.0 630 0.002 3.4±0.4 Warm Creek near Twin Falls 13091700 3/9/94 0.6 15.0 7.8 650 0.002 4.6±0.5 Briggs Creek near Buhl 13095200 3/7/94 3.0 14.5 7.8 490 0.002 4.4±0.2 

3/2/92i 2.8 14.0 7.9 451 j 3.3±0.5 

Box Canyon Springs near Wendell 13095500 3/8/94 9.5 14.5 7.6 400 0.002 4.6±0.7 

3/2/92i 9.0 14.0 7.9 408 j 3.5±0.4 

Beaver Creek at Spencer 13113000 3/3/94 0.4 0.5 8.2 410 0.002 18.0±1.3 Medicine Lodge Creek near Small 13116500 3/3/94 1.5 6.0 8.4 430 0.001 10.0±0.3 Lidy Hot Springs 440832112331001 3/14/94 j 47.0 7.3 680 0.004 3.7±0.3 Sand Springs near Hagerman 13132600 3/2/92i 2.1 14.0 7.8 380 j 3.2±0.4 Big Wood River near Bellevue 13141000 3/5/94 1.4 5.0 7.2 380 0.003 30.3±6.2 Silver Creek near Picabo 13150430 3/5/94 6.3 7.0 7.3 420 0.002 24.6±0.7 Snake River at King Hill 13154500 3/5/94 248.7 11.0 7.9 510 0.003 5.7±0.3 
17

Table 3. Field measurements and concentrations of iodine-129 in water from selected wells downgradient from the I

Engineering and Environmental Laboratory, Idaho.

Well Site identification number Date sampled (m/d/y)a

a. Date format: month/day/year;

Water

tempb

b. Degrees Celsius.

pH Specific conductancec

c. Microsiemens per centimeter at 25°C.

129Id

d. Uncertainties at 1 sample standard deviation.

Sample 

(aCi/L)e

e. Attocuries per liter.

atoms/Lf

(x 106)

f. Atoms per liter.

Coffee Point (CCC-3) 430911112585401 6/21/93g

g. Archived sample.Note: iodide concentrations were not measured at the time of sampling; a concentration of 0.001 milligrams per liter (mg/L) was assumed for the purpo

culating 129I results.

15.0 8.1 310 10.0±0.4 265±10.2 Ground 

Crossroads 432128113092701 6/22/93g 11.0 8.2 337 3.2±0.2 84.2±4.91 Ground Fingers Butte 432424113165301 6/22/93g 15.0 8.3 306 5.5±0.2 146±6.04 Ground Grazing-2 431553112492001 6/21/93g 15.0 8.1 290 1.5±0.1 40.7±3.31 Ground Houghland 431439113071401 6/22/93g 16.0 8.1 327 4.5±0.2 120±5.70 Ground MV-04 424658113414601 8/2/93 12.5 7.8 661 0.44±0.19 11.7±4.90 Ground MV-33 425455114160101 8/2/93 16.0 7.9 297 1.7±0.2 44.6±5.63 Ground MV-59 430805113102401 8/10/93 15.0 8.0 306 0.29±0.18 7.72±4.63 Ground 
18

Table 4. Summary statistics for selected iodine-129 data sets from sites on or tributary to the eastern Snake River Pl

 Data set Number of samples

Concentration (attocuries per liter)a

a. Uncertainties at 1 sample standard deviation.

Maximum Minimum Median

95-percent 

confidence 

interval about 

the median

Interquartile 

range

Comparison of data sets from different studiesBackground samples, 1992b

b. Data from Mann and Beasley (1994a).

16 7.9±0.8 1.0±0.4 3.0 1.6–3.9 2.7 

Background samples, 1993-94, and archived samples 34 30.3±6.2 0.29±0.18 4.6 3.4–7.0 5.3 Combined data sets, 1992 and 1993-94 with archived samples 50 30.3±6.2 0.29±0.18 4.2 3.2–5.2 4.4 Comparison of data from sites on the SRP with data from sites tributary to the SRPSites on the eastern Snake River Plain 33 10.0±0.4 0.29±0.18 3.4 3.0–4.4 2.5 Sites in areas tributary to eastern Snake River Plain 17 30.3±6.2 0.49±0.10 8.4 2.6–10.0 9.1 Comparison of surface-water data with data from wells and springsSurface-water data 18 30.3±6.2 0.49±0.10 8.2 5.7–10.0 5.4 Ground water data 30c

c. Samples from Lidy Hot Springs and the City of Arco well are excluded.This summary does not include the two replicate samples discussed in the text and listed in table 2.

10.0±0.4 0.29±0.18 3.3 2.5–4.1 2.6 

(wells and springs from the Snake River Plain aquifer)