Iodine-129 in the Snake River Plain Aquifer at and 

Near the Idaho National Laboratory, Idaho, 

2003 and 2007

Scientific Investigations Report 2009–5088

Prepared in cooperation with the U.S. Department of Energy

U.S. Department of the Interior

U.S. Geological Survey

DOE/ID-22208
Cover: Photographs (from left to right) of sample collection equipment and U.S. Geological Survey pumping 

well. (Photograph courtesy of the U.S. Geological Survey, Idaho National Laboratory Project Office.)
Iodine-129 in the Snake River Plain Aquifer at and 

Near the Idaho National Laboratory, Idaho, 

2003 and 2007

By Roy C. Bartholomay

Prepared in cooperation with the U.S. Department of Energy

Scientific Investigations Report 2009-5088

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Bartholomay, R.C., 2009, Iodine-129 in the Snake River Plain aquifer at and near the Idaho National Laboratory, Idaho, 

2003 and 2007: U.S. Geological Survey Scientific Investigations Report 2009–5088, 28 p.
iii

Figures

Figure 1. Map showing location of the Idaho National Laboratory, Idaho ..................... 2

Figure 2. Map showing location of wells at and near the Idaho National Laboratory, 

Idaho .......................................................................................... 3

Figure 3. Map showing location of wells near the Idaho Nuclear Technology and 

Engineering Center, Idaho National Laboratory, Idaho ................................. 5

Figure 4. Map showing areal distribution of iodine-129 in the Snake River Plain 

aquifer at the Idaho National Laboratory, Idaho, 2003 ................................. 13

Figure 5. Map showing change in concentration of iodine-129 between samples 

collected from selected wells, Idaho National Laboratory, Idaho, 

1990–91 and 2003 ........................................................................... 16

Figure 6. Map showing areal distribution of iodine-129 in the Snake River Plain aquifer 

at the Idaho National Laboratory, Idaho, 2007 .......................................... 18

Figure 7. Map showing change in concentration of iodine-129 between water samples 

from wells at the Idaho National Laboratory, Idaho, 2003 and 2007 .................. 19

Figure 8. Graph showing concentrations of iodine-129 in well Middle 2051 in relation to 

piezometric head, Idaho National Laboratory, Idaho, 2007 ........................... 23

Contents

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

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

Purpose and Scope .............................................................................................................................4

Geohydrologic Setting .........................................................................................................................4

Previous Investigations........................................................................................................................6

Methods and Quality Assurance.................................................................................................................7

Sample Collection Methods................................................................................................................7

Analytical Methods...............................................................................................................................8

Guidelines for Interpretation of Analytical Results ........................................................................8

Quality Assurance/Quality Control ....................................................................................................8

Concentrations of Iodine-129 in the Snake River Plain Aquifer...........................................................12

Concentrations of Iodine-129 in 2003 ..............................................................................................12

Concentrations of Iodine-129 in 2007 ..............................................................................................17

Vertical Distribution of Iodine-129 in the Snake River Plain Aquifer..........................................20

Summary........................................................................................................................................................24

Acknowledgments ......................................................................................................................................24

References Cited..........................................................................................................................................24
iv

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Conversion Factors

Multiply By To obtain

acre 4,047 square meter (m2)

acre-foot (acre-ft) 1,233 cubic meter (m3)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

foot (ft) 0.3048 meter (m)

foot per day (ft/d) 0.3048 meter per day (m/d)

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

foot per second (ft/s) 0.3048 meter per second (m/s)

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

gallon (gal) 3.785 liter (L)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

mile (mi) 1.609 kilometer (km)

ounce, avoirdupois (oz avdp) 28.32 gram (g)

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L)

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

square mile (mi2) 2.590 square kilometer (km2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F= (1.8×°C) +32.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Tables

Table 1. Concentrations of iodine-129 in ground water, Idaho National Laboratory, 

Idaho, 2003 and 2007 ........................................................................ 10

Table 2. Concentrations of iodine-129 at multiple aquifer depths, Idaho National 

Laboratory, Idaho, 2007...................................................................... 11

Table 3. Concentrations of iodine-129 in water from selected wells, Idaho National 

Laboratory, Idaho, 1990–91, 2003, and 2007 ............................................. 14

Table 4. Concentrations of chloride, iodine-129, sodium, and strontium-90 in water 

from selected wells, Idaho National Laboratory, Idaho, 2003 and 2007 ............ 20

Table 5. Concentrations of selected chemicals at multiple aquifer depths, Idaho 

National Laboratory, Idaho ............................................................... 22
v

Datums

Vertical coordinate information is referenced to National Geodetic Vertical Datum of 1929 

(NGVD 29).

Horizontal coordinate information is referenced to North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations and Acronyms 

Abbreviation or 

Acronym Definition

129I Iodine-129

AMS accelerator mass spectrometer

CFA Central Facilities Area

Ci curie

DOE U.S. Department of Energy

ESRP eastern Snake River Plain

INL Idaho National Laboratory

INTEC Idaho Nuclear Technology and Engineering Center

MCL maximum contaminant level

RTC Reactor Technology Complex

PRIME Purdue Rare Isotope Measurement

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
vi

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Abstract

From 1953 to 1988, wastewater containing approximately 

0.94 curies of iodine-129 (129I) was generated at the Idaho 

National Laboratory (INL) in southeastern Idaho. Almost all 

of this wastewater was discharged at or near the Idaho Nuclear 

Technology and Engineering Center (INTEC) on the INL 

site. Most of the wastewater was discharged directly into the 

eastern Snake River Plain aquifer through a deep disposal well 

until 1984; however, some wastewater also was discharged 

into unlined infiltration ponds or leaked from distribution 

systems below the INTEC.

In 2003, the U.S. Geological Survey (USGS), in 

cooperation with the U.S. Department of Energy, collected 

samples for 129I from 36 wells used to monitor the Snake 

River Plain aquifer, and from one well used to monitor 

a perched zone at the INTEC. Concentrations of 129I 

in the aquifer ranged from 0.0000066 ± 0.0000002 to 

0.72 ± 0.051 picocuries per liter (pCi/L). Many wells 

within a 3-mile radius of the INTEC showed decreases of 

as much as one order of magnitude in concentration from 

samples collected during 1990–91, and all of the samples 

had concentrations less than the Environmental Protection 

Agency’s Maximum Contaminant Level (MCL) of 1 pCi/L. 

The average concentration of 129I in 19 wells sampled 

during both collection periods decreased from 0.975 pCi/L 

in 1990–91 to 0.249 pCi/L in 2003. These decreases are 

attributed to the discontinuation of disposal of 129I in 

wastewater after 1988 and to dilution and dispersion in the 

aquifer.

Although water from wells sampled in 2003 near the 

INTEC showed decreases in concentrations of 129I compared 

with data collected in 1990–91, some wells south and east of 

the Central Facilities Area, near the site boundary, and south 

of the INL showed slight increases. These slight increases 

may be related to variable discharge rates of wastewater that 

eventually moved to these well locations as a mass of water 

from a particular disposal period.

In 2007, the USGS collected samples for 129I from 

36 wells that are used to monitor the aquifer south of 

INTEC and from 2 wells that are used to monitor perched 

zones at INTEC. Concentrations of 129I in the eastern Snake 

River Plain aquifer ranged from 0.000026 ± 0.000002 to 

1.16 ± 0.04 pCi/L, and the concentration at one well exceeded 

the maximum contaminant level (1 pCi/L) for public drinking 

water supplies. The average concentration of 19 wells sampled 

in 2003 and 2007 did not differ; however, slight increases and 

decreases of concentrations in several areas around the INTEC 

were evident in the aquifer. The decreases are attributed to 

the discontinued disposal and to dilution and dispersion in the 

aquifer. The increases may be due to the movement into the 

aquifer of remnant perched water below the INTEC. 

In 2007, the USGS also collected samples from 31 zones 

in 6 wells equipped with multi-level WestbayTM packer 

sampling systems to help define the vertical distribution of 

129I in the aquifer. Concentrations ranged from 0.000011 

± 0.0000005 to 0.0167 ± 0.0007 pCi/L. For three wells, 

concentrations of 129I between zones varied one to two orders 

of magnitude. For two wells, concentrations varied for one 

zone by more than an order of magnitude from the wells’ 

other zones. Similar concentrations were measured from 

all five zones sampled in one well. All of the 31 zones had 

concentrations two or more magnitudes below the maximum 

contaminant level. 

Introduction

The Idaho National Laboratory (INL), encompassing 

about 890 mi2 of the eastern Snake River Plain (ESRP) in 

southeastern Idaho (fig. 1), is operated by the U.S. Department 

of Energy (DOE). The INL was established in 1949 for 

the development of peacetime atomic energy applications, 

nuclear safety research, defense programs, environmental 

research, and advanced energy concepts. Until 1993, uranium 

from spent nuclear fuel elements from government-owned 

reactors was recovered after reprocessing at the Idaho Nuclear 

Technology and Engineering Center (INTEC, fig. 1). As part 

of the fuel reprocessing activities, several fission products 

were released in wastewater at the INTEC. Iodine-129 (129I), 

produced by the fission of uranium-235 and plutonium-239, 

was one of the products released in wastewater. Prior to 1984, 

most of the wastewater generated at the INTEC was injected 

directly to the ESRP aquifer through a 598-ft-deep disposal 

well. Beginning in February 1984, routine use of the disposal 

well was discontinued, and wastewater was discharged to 

unlined infiltration ponds (fig. 2), which allow the wastewater 

Iodine-129 in the Snake River Plain Aquifer at and Near the 

Idaho National Laboratory, Idaho, 2003 and 2007

By Roy C. Bartholomay 
2 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

Figure 1. Location of the Idaho National Laboratory, Idaho.

tac09-0332_fig01

MFC

CFA

INTEC

NRF

RWMC

TAN

RTC

PBF

Boundary of Idaho National Labororatory

Selected facilities at Idaho National Laboratory

 Materials and Fuels Complex 

 Central Facilities Area

 Idaho Nuclear Technology and Engineering Center

 Naval Reactors Facility

 Power Burst Facility

 Radioactive Waste Management Complex

 Reactor Technology Complex—also known as Test 

 Reactor Area (TRA)

 Test Area North

EXPLANATION

IDAHO

BOISE

EASTERN 

SNAKE RIVER

IDAHO PLAIN

NATIONAL

LABORATORY

Twin

Falls

Pocatello

Idaho

Falls

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

0

0 10

10

20 KILOMETERS

20 MILES

Mud

Lake

Big

Lost

River

Little

Lost

River

Birch

Creek

Mackay

Reservoir

Camas

Creek

Big 

Southern

Butte

East

Butte

Middle Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

PIONEER MOUNTAINS

Location shown 

 in figure 2 

26

9320

26

33

22

28

20

93

TAN

RTC INTEC

CFA

NRF

RWMC

MFC

PBF

Big Lost River

playas and sinks

Birch River

sinks

SPREADING

AREAS

113°00’ 45’

44°00’

45’

43°30’

113°30’ 15’ 30’ 112°15’

Atomic

City

Howe

Arco

Mud

Lake Terreton
 3

Figure 2. Location of wells at and near the Idaho National Laboratory, Idaho.

tac09-0332_fig02

EBR-1

CFA-2

CFA-1

USGS 9

USGS 88

USGS 85

USGS 83

USGS 20

USGS 14

USGS 11

USGS 109 USGS 108

USGS 106

USGS 105

USGS 104

USGS 103

USGS 128

USGS 127

USGS 125

USGS 124

USGS 134 USGS 133

USGS 132

RWMC PROD.

CFA LF2-10

RIFLE RANGE

MIDDLE 2051

Spreading

Areas

Big

Southern

Butte

ICPP 1797

ICPP 1798

ICPP 1796

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

See figure

3 for wells

in this area

113 ̊06’ 113 ̊

43 ̊34’

43 ̊30’

43 ̊26’

43 ̊22’

112 ̊54’

0 4,000 METERS

0 12,000 FEET

Boundary of

Idaho National Laboratory
4 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

to percolate through about 450 ft of basalt and sediment to the 

aquifer. 

Iodine-129 in the ESRP aquifer originates from 

atmospheric deposition, rock weathering, and wastewater 

disposal (Mann and Beasley, 1994a). The amount of 129I in the 

aquifer from atmospheric deposition and rock weathering is 

considered small, and is included in the estimated background 

concentration of 0.0000054 picocuries per liter (pCi/L) in the 

ESRP aquifer in eastern Idaho (Cecil and others, 2003). Mann 

and Beasley (1994a) reported that wastewater discharged to 

the injection well and infiltration ponds at the INTEC between 

1953 and 1990 contained an estimated 0.56–1.18 curies (Ci) 

of 129I. A more detailed estimate of wastewater discharge 

was performed by the DOE Idaho Operations Office (U.S. 

Department of Energy, 2004, appendix D), and results 

indicated that a maximum of 0.86 Ci of 129I was discharged 

to the aquifer through the injection well. In addition, about 

0.08 Ci of 129I were discharged to the infiltration ponds from 

1984 to 1988 (Litteer, 1988; Mann and others, 1988, table 2; 

Litteer and Reagan, 1989), and about 0.001 Ci of iodine-129 

was released at the INTEC Tank Farm (fig. 3) between 1958 

and 1986 (Cahn and others, 2006, table 5-2). Therefore, some 

129I may still be present in perched zones around the INTEC. 

Some 129I also was discharged into the radioactive waste 

ponds at the Reactor Technology Complex (RTC), but annual 

concentrations of the discharge water generally were much 

less than a pCi/L; for example EG&G Idaho, Inc. (1979) 

showed an average annual concentration of 12.8 × 10-6 pCi/L 

in the 1978 discharge water. Because of its 15.7 million-year 

half-life, 129I released to the environment is a permanent 

addition to the global inventory (Mann and others, 1994a). 

Given that 129I is a known carcinogen, there is concern 

that its disposal at the INL might pose a health hazard 

to downgradient communities. The current (1976) U.S. 

Environmental Protection Agency maximum contaminant 

level (MCL) for 129I in drinking water is 1 pCi/L (U.S. 

Environmental Protection Agency, 1976, Appendix B). 

The MCL is based on the average concentration in public 

drinking water supplies that will yield an annual whole-body 

dose equivalent to 4 millirem for man-made beta-particle 

and photon-emitting radiounuclides; the proposed MCL 

based on effective dose equivalent for 129I is 21 pCi/L (U.S. 

Environmental Protection Agency, 2000). To evaluate the 

potential hazards, the U.S. Geological Survey (USGS) in 

cooperation with the DOE, have periodically monitored 

for 129I in ground water from the ESRP aquifer at and 

downgradient of the INTEC since 1977. Monitoring programs 

from 1977, 1981, 1986 and 1990–91 were summarized by 

Mann and others (1988) and by Mann and Beasley (1994b). 

This report summarizes concentrations in the aquifer in 2003 

and 2007. 

Purpose and Scope 

The USGS collected water samples from wells during 

2003 and 2007 to evaluate recent concentrations of 129I in 

the ESRP aquifer. This report describes the results of water 

samples collected for 129I analyses (41 samples in 2003 and 

75 samples in 2007). 

In 2003, samples were collected from 36 wells that are 

used to monitor the ESRP aquifer and from 1 well that is 

used to monitor a perched zone at the INTEC. Four replicate 

samples were collected as a measure of quality assurance/

quality control (QA/QC). 

In 2007, samples were collected from 31 zones in 6 wells 

equipped with multi-level WestbayTM packer sampling 

systems. In addition, samples were submitted for analyses 

from 36 other wells that are used to monitor the aquifer south 

of the INTEC and from 2 wells that are used to monitor 

perched zones at the INTEC. Three samples were collected 

as QA/QC replicates, and three samples were collected 

as QA/QC equipment blanks. The wells were sampled to 

determine the current concentrations in the ESRP aquifer. 

Those concentrations were compared with past data to 

determine concentration changes. 

Geohydrologic Setting

The INL is located above the west-central part of the 

ESRP. The ESRP is a northeast-trending structural basin 

about 200-mi long and 50–70 mi wide (fig. 1). The basin, 

bounded by faults on the northwest and by downwarping and 

faulting on southeast, has been filled with basaltic lava flows 

interbedded with terrestrial sediments. The basaltic rocks 

and sedimentary deposits combine to form the ESRP aquifer, 

which is the main source of ground water on the plain. 

The ESRP aquifer is one of the most productive aquifers 

in the United States (U.S. Geological Survey, 1985, p. 193). 

Movement of water in the aquifer generally is from northeast 

to southwest, and water eventually discharges to springs along 

the Snake River downstream of Twin Falls, Idaho—about 

100 mi southwest of the INL. Water moves horizontally 

through basalt interflow zones and vertically through joints 

and interfingering edges of interflow zones. Infiltration of 

surface water, heavy pumpage, geologic conditions, and 

seasonal fluxes of recharge and discharge locally affect the 

movement of ground water (Garabedian, 1986). Recharge 

to the ESRP aquifer is primarily from infiltration of applied 

irrigation water, infiltration of streamflow, ground-water 

inflow from adjoining mountain drainage basins, and 

infiltration of precipitation. 
Introduction 5

Figure 3. Location of wells near the Idaho Nuclear Technology and Engineering Center, Idaho National Laboratory, Idaho.

tac09-0332_fig03

Reactor Technology Complex

Idaho Nuclear 

Technology and 

Engineering Center

Lincoln Boulevard

ICPP 1795

CPP-1

USGS 82

USGS 77

USGS 67

USGS 65

USGS 59

USGS 57

USGS 52

USGS 51

USGS 50

USGS 48

USGS 47

USGS 46

USGS 45

USGS 44

USGS 43

USGS 42

USGS 40

USGS 39

USGS 38

USGS 37

USGS 36

USGS 35

USGS 34

USGS 123

USGS 121

USGS 116

USGS 115 USGS 114

USGS 113

USGS 112

USGS 111

ICPP 2018

MIDDLE 2050A

Big Lost River

USGS 41

Disposal

Well

Tank 

Farm

0 1,000 METERS

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000 0 3,000 FEET

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

43 ̊34’

43 ̊35’

112 ̊58’ 112 ̊56’
6 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

At the INL, depth to water in wells completed in the 

ESRP aquifer ranges from about 200 ft in the northern part 

of the site to more than 900 ft in its southeastern part. A 

significant proportion of the ground water moves through 

the upper 200–800 ft of basaltic rocks (Mann, 1986, p. 

21). Ackerman (1991, p. 30) and Bartholomay and others 

(1997, table 3) reported a range of transmissivity of basalt 

in the upper part of the aquifer of 1.1 to 760,000 ft2/d. The 

hydraulic gradient at the INL ranges from 2 to 10 ft/mi, with 

an average of about 4 ft/mi (Davis, 2006, figure 9). Horizontal 

flow velocities of 2 to 20 ft/d have been calculated based 

on the movement of various constituents in several areas of 

the aquifer at the INL (Robertson and others, 1974; Mann 

and Beasley, 1994b; Cecil and others, 2000; Busenberg and 

others, 2001). These flow rates equate to a travel time of about 

70–700 years for water beneath the INL to travel to springs 

that discharge at the terminus of the ESRP aquifer. Localized 

tracer tests at the INL have shown vertical and horizontal 

transport rates as high as 60–150 ft/day (Nimmo and others, 

2002; Duke and others, 2007).

Previous Investigations

Many investigations have been done to evaluate the 

geology and hydrology of the ESRP aquifer at the INL. 

A comprehensive listing of publications by the USGS is 

available at http://id.water.usgs.gov/projects/INL/publication.

html.

Previous investigations of 129I in water from the ESRP 

aquifer include those by Barraclough and others (1982), 

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

and Beasley (1994a; 1994b), Cecil and others (2003), U.S. 

Department of Energy (2004, 2007, 2008), Hall (2006), and 

Forbes and others (2007). Results from April 1977 sampling 

for 129I in 14 wells indicated concentrations ranged from 0.9 

to 27 pCi/L for statistically positive values (Barraclough and 

others, 1982, fig. 42), and since discharge began in 1953, 

129I was identified in wells less than 3 mi from the disposal 

well. In October 1981, concentrations of 129I ranged from 

0.05 to 41 pCi/L for statistically positive values (Lewis 

and Jensen, 1985), and since discharge began in 1953, 129I 

was identified in wells about 6.3 mi from the disposal well. 

The major difference between the 1977 and 1981 results 

was that the sample size was increased from 1 to 4 L for a 

four-fold reduction in the reporting level. The increase in 

sensitivity of analyses (Lewis and Jensen, 1985), along with 

a more extensive set of wells sampled (20 in 1977 and 32 

in 1981) were the primary reasons for the increase in the 

size of the 129I plume. In August 1986, 129I concentrations 

ranged from 0.49 ± 0.12 to 3.6 ± 0.4 pCi/L for 20 wells, with 

concentrations greater than the reporting level (Mann and 

others, 1988), and 129I had migrated about the same distance 

from the disposal well as in 1981. The large decrease in 

the maximum concentration between 1981 and 1986 was 

attributed to changes in disposal practices at the INTEC, 

reduction in the mass of 129I in wastewater, and to increased 

dilution in the mid-1980s from a large amount of flow in the 

Big Lost River (Mann and others, 1988).

Prior to the 1990–91 data collection, neutron activation 

methods were used for analyses. During 1990–91, Mann and 

Beasley (1994b) collected samples from 51 wells at and near 

the INL, and they analyzed the samples using an accelerator 

mass spectrometer (AMS) method. The AMS method allowed 

for increased sensitivity of the analyses (2 to 6 times more 

sensitive than neutron activation). The increased sensitivity 

allowed for determining a background concentration of 

0.0000009 ± 0.0000002 pCi/L from a sample located 

upgradiant from the INTEC. The increased sensitivity resulted 

in detectable concentrations of 129I downgradient of the INL 

that were used to calculate ground-water flow velocities of at 

least 6 ft/d. The maximum concentration detected in 1990–91 

samples was 3.82 ± 0.19 pCi/L, which was similar to the 

maximum concentration detected in 1986; however, mean 

concentrations from 18 wells sampled in 1986 and 1990–91 

decreased from 1.30 ± 0.26 to 0.81 ± 0.19 pCi/L (Mann and 

Beasley, 1994b). This decrease was attributed largely to a 

decrease in disposal rates. 

In 1992, Mann and Beasley (1994a) collected ground 

water and surface water samples from 16 sites not likely to 

have been affected by wastewater disposal at the INTEC to 

determine background concentrations of 129I. Concentrations 

of 129I in water from nine wells, four springs, and three 

streams on or tributary to the ESRP ranged from 0.0000001 

± 0.0000001 to 0.0000081 ± 0.0000006 pCi/L (average of 

0.0000033 ± 0.0000021 pCi/L). At the 99-percent confidence 

level, background concentrations of 129I for the 16 sites were 

estimated to be less than or equal to 0.0000082 pCi/L. Cecil 

and others (2003) reevaluated the background concentrations 

by analyzing results of 52 samples collected from ground 

water and surface water in 1992–94 from various locations 

in the ESRP in southeastern Idaho, and includes the samples 

collected by Mann and Beasley (1994a). Cecil and others 

(2003) determined that surface water samples generally 

contained larger 129I concentrations than ground water samples 

because of anthropogenic fallout and evapotranspiration. 

They determined background concentrations using a subset of 

30 water samples from wells analyzed to be 0.0000054 pCi/L, 

and the 95-percent nonparametric confidence interval was 

0.0000052 to 0.00001 pCi/L. 

Hall (2006) collected samples from 13 sites downgradient 

of the INL during 1997 and 1998. Using AMS methods, 

Hall (2006) determined that concentrations in four of the 

sites—USGS 11, 14, 124, and 125 [fig. 2])—were greater than 
Methods and Quality Assurance 7

estimated background concentrations, and he postulated that 

1958 peak 129I in the ESRP aquifer had already passed these 

wells. Concentrations of the four sites ranged from 0.0000061 

± 0.00000018 to 0.00074 ± 0.00003 pCi/L, but they were less 

than concentrations measured in 1991 and 1993. 

U.S. Department of Energy (2004) collected samples 

from four wells south of the INTEC (ICPP 1795-1798, 

figs. 2 and 3) from three zones in the aquifer to determine 

concentrations above and below the H-I interbed. 

Concentrations of 129I in well ICPP-1795 (northernmost well, 

fig. 3) increased from 0.34 ± 0.04 pCi/L at 560 ft below land 

surface to 0.43 ± 0.07 pCi/L at 620 ft below land surface. 

The three wells farther to the south showed a decrease in 

129I concentration with depth, with concentrations in the 

upper zone ranging from 0.58 ± 0.1 to 0.88 ± 0.08 pCi/L and 

concentrations in the lower zone ranging from not detected 

to 0.33 ± 0.05 pCi/L. The U.S. Department of Energy (2004, 

fig. 5-5) also presented results from analyses of 49 wells 

sampled in 2001; concentrations ranged from less than the 

method detection level of approximately 0.1–1.06 pCi/L. 

Analyses were completed using gamma spectroscopy 

methods. Concentrations for 20 sites sampled in 2003 were all 

less than the MCL of 1 pCi/L (DOE/ID 2004, fig. 6-1). 

Forbes and others (2007) presented results for 25 wells 

sampled in 2006 near the INTEC; concentrations ranged 

from less than the reporting level to 0.65 ± 0.097 pCi/L in 

USGS 67. Analyses of data collected from 2004–06 at wells 

around the INTEC indicated no discernable change in the 

concentrations when the uncertainty of the data was taken into 

account.

U.S. Department of Energy (2007) presented results 

for 129I data collected in 2005 and 2006 from 24 wells 

downgradient of the INTEC and the RTC (including several 

wells south of the INL), along with results from five zones 

each from two WestbayTM equipped wells (Middle 2050A 

and 2051). Samples were analyzed using the AMS method at 

the Purdue Rare Isotope Measurement (PRIME) laboratory, 

Purdue University, West Lafayette, Indiana, and some 

comparison was made to the 2003 USGS data presented in 

this report. Results were used to speculate on the source of 129I 

in wells around the RWMC. Concentrations in most southern 

wells were greater than background concentrations. 

U.S. Department of Energy (2008) presented results for 

129I data collected in 2007 from six zones in one WestbayTM

equipped well (USGS 132). Samples were analyzed using the 

AMS method at the PRIME laboratory in Indiana, and the 

results from the six zones ranged from 0.0004 ± 0.000013 to 

0.002 ± 0.00009 pCi/L. Results for all six zones were more 

than two orders of magnitude less than the MCL. 

Methods and Quality Assurance

Sample Collection Methods

Sample collection by the USGS at the INL generally 

followed guidelines established by the USGS and documented 

in the USGS National Field Manual (U.S. Geological Survey, 

variously dated), and in Bartholomay and others (2003). 

Water was collected from wells with dedicated submersible 

pumps and from six wells equipped with dedicated WestbayTM

packer sampling systems. The WestbayTM packer sampling 

systems allow for isolation of particular zones within the 

upper 30–650 ft of the aquifer for sample collection. The 

other monitoring wells sampled for this study consist of 

open boreholes with variable completion depths in the upper 

30–500 ft of the aquifer. The water sampled from these open 

boreholes often is a mixture of old, regional ground water with 

young water that recharged in or near the INL (Busenberg and 

others, 2001). 

Water from wells equipped with dedicated pumps was 

monitored during sampling for temperature, pH, and specific 

conductance using methods described by Wood (1981) and 

Claassen (1982). Water samples in 2003 were collected 

after field measurements stabilized and after one volume of 

water was purged from each well. In 2007, water samples 

were collected after field measurements stabilized and after 

1 volume of water was purged from each well. Samples 

collected prior to October 2003 were collected after 3 volumes 

of water were purged. Bartholomay (1993) and Knobel 

(2006) determined that the difference between purging 1 and 

3 wellbore volumes at selected INL wells had no discernable 

effect on statistical comparability of select water-quality data.

For wells with dedicated WestbayTM packer sampling 

systems, pre-cleaned stainless-steel thief sampling bottles were 

lowered to the zone to be sampled, connected to the sampling 

port, and filled with formation water. The filled stainless-steel 

bottles then were raised to the surface and emptied into a 

pre-cleaned container; the water was then processed to fill 1-L 

glass amber bottles. Field measurements also were taken from 

the pre-cleaned container. 

Field processing of all samples 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 

or 1 L of sample water. Filtration was necessary to remove 

particulate matter that could affect the laboratory preparation 

of the silver-iodide targets used in the AMS measurements of 

129I (Cecil and others, 2003). 
8 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

Analytical Methods

Iodine-129 concentrations in the ground water samples 

were determined using AMS methods described by Sharma 

and others (1997) and quality assurance requirements 

described by Mark Caffee, Purdue University (written 

commun., accessed February 12, 2008, at https://www.physics.

purdue.edu/ams/AMSQAQC/normaq.php). The AMS used 

to analyze the samples is located at the PRIME laboratory, 

Purdue University, West Lafayette, Indiana. Water samples go 

through a chemical process to produce a silver iodide target 

material. The silver iodide is prepared after the addition of 

an iodine carrier, and the target is placed in a holder for AMS 

analyses. Analyses of the target produce a ratio of 129I to stable 

iodine-127 (127I). 

Guidelines for Interpretation of Analytical 

Results 

Concentrations of 129I are reported with an estimated 

sample standard deviation, s, which is obtained by propagating 

sources of analytical uncertainty in measurements. The 

guidelines for interpreting analytical results are based on 

an extension of a method proposed by Currie (1984) and 

are given in Mann and Beasley (1994b). In this report, 

129I concentrations less than 3s are considered less than a 

“reporting level.” The reporting level should not be confused 

with the analytical method detection limit, which is based on 

laboratory procedures.

Laboratory results from 2003 analyses were reported by 

the PRIME laboratory as the ratio of 129I to 127I and converted 

to concentrations in picocuries per liter (pCi/L) by using an 

equation described by Rao (1997): 

129

129 127

[ ( ) ] ,

where

is the I concentration in atoms per liter of water,

is the ratio of I/ I measured by AMS,

is the concentration of iodine in grams per liter 

(g/L) of carr

m c v c v c v r T

v

T

m

c

R C C S S C C C N S

fS

S

R

C

+ − =

129 127

ier solution,

is the volume of carrier solution added in L,

is I/ I ratio for carrier solution,

is the concentration of iodine in g/L for sample, 

is the volume of the sample in L,

is Avogadr

v

r

c

v

C

C

S

S

N 23 o's number (6.023 × 10 atoms/mole), 

and

is the formula weight for iodine (126.9 grams per 

mole).

f

(1)

The mass of the carrier given by the PRIME laboratory 

is the product of carrier volume (Cv) and carrier concentration 

(CC ) in milligrams. The concentration of iodine in most 

samples was less than 0.002 mg/L, which was used for the 

concentration for all samples except those with estimated or 

reported concentrations. 

In 2007, the laboratory reported results in ratios of 

129I /127I and in pCi/L based on the equation:

7

129

127

127

( ) ( ) 1.80 10 ( ) ( ) , L

where

is the measured sample ratio of I atoms to 

I atoms,

is the ratio of the chemistry blank,

is the weight of I in sample (in mg),

is the

s b s c

s

b

s

c

pCi R R W W x pCi sample L Wt

R

R

W

W

− − + = x x

127 weight of I added as carrier (in mg), and

Wt is weight of sample (aliquot).

(2)

The laboratory calculations do not take into account 

the concentration of iodine in the sample because the 

concentration typically is negligible; the concentrations were 

estimated in the equation (Rao, 1997) used for the 2003 data. 

Differences between the reported concentrations are similar 

between both equations when sample ratios are larger than 

carrier blank ratios; however, when 129I/127I ratios of the 

samples are less than the ratio for the carrier blank sample, 

negative results occur for calculations based on the Rao 

(1997) equation. The PRIME lab did not subtract the carrier 

blank ratios for the samples that had carrier blank ratios 

larger than sample ratios (Susan Ma, PRIME lab, written 

commun., September 7, 2008). This difference in calculation 

methods between 2003 and 2007 results does not affect the 

comparability of the data between these analysis periods.

Quality Assurance/Quality Control 

Quality assurance and reproducibility were assessed 

with QA/QC replicate and blank samples submitted for 

AMS analysis along with the standard samples. Four QA/

QC replicate samples were collected in 2003; three QA/

QC replicates and three QA/QC field blanks were collected 

in 2007. The blanks consisted of inorganic-free water 

obtained from the USGS National Water Quality Laboratory 

that was passed through the field equipment used for 

sample collection. Results for two of the blanks were near 

(QAI-4, 0.000071±0.000015 pCi/L) or below background 

concentrations (QAI-6, 0.0000014 ± 0.0000002 pCi/L). 
Methods and Quality Assurance 9

The third blank sample (QAI-1, table 1) had a concentration of 

0.0025 ± 0.0005 pCi/L. This sample was from the same batch 

of inorganic free water as QAI-6. The blanks (QAI-1 and 

QAI-6) were processed in the field during different times of 

year. Samples QAI-1 and QAI-4 were processed as unknowns 

with estimated concentrations of 1 pCi/L, so PRIME used a 

large dilution factor during processing. QAI-6 was processed 

as a blank with a concentration less than background, so no 

dilution in the chemical process was used. A rerun for QAI-4 

(table 2) was done with no dilution, and the concentration 

(0.000001 ± 0.00000005 pCi/L) was less than background. 

The laboratory discarded the QAI-1 sample before a rerun was 

requested, so the sample could not be rerun without dilution. 

The difference between the analytical results was not resolved.

Statistical equivalency of radiochemical-constituent 

concentrations in sample replicate pairs was determined 

following a method defined by Volk (1969) and described in 

more detail by Williams (1996). In this method, statistical 

equivalence is determined within a specified confidence level. 

A value for the standard deviate, Z, is calculated, and then the 

level of significance of the result is evaluated (evaluation of 

the level of significance assumes that the sample population 

is distributed normally). For this report, concentrations of 

individual constituents in sample pairs (constituent pairs) were 

considered equivalent when the results were within 2 standard 

deviations of each other. At this confidence level (95-percent), 

the level of significance, determined from a standard normal 

probability curve, was 0.05 for a two-tailed test, and it 

corresponded to a Z-value of 1.96.

The equation used to determine Z was adapted from Volk 

(1969):

2 2 ,

( ) ( )

where

is the concentration of a constituent in the routine 

sample,

is the concentration of the same constituent in the 

sequential replicate sample, 

is the standard deviation of , 

x y

x

x y Z

s s

x

y

s x

− =

+

and 

is the standard deviation of . y s y

(3)

When the population is not distributed normally, which 

often is the case with radiochemical results (L. DeWayne 

Cecil, U.S. Geological Survey, written commun., January 4, 

2009), or an approximation of the standard deviation is 

used, a Z-value less than 1.96 must be considered as a guide 

when testing for equivalence. Constituent concentrations in 

sample pairs were considered statistically equivalent when the 

calculated Z-value was less than or equal to 1.96. 

The use of equation 3 therefore is considered a guide in 

determining if the results of 129I analyses of a replicate pair 

of samples were equivalent. The results and reported standard 

deviations for the analyses of 129I replicate pairs and the 

Z-values are listed in tables 1 and 2. Results for five of the 

seven replicate pairs had Z-values less than 1.96 and can be 

considered statistically equivalent. 

Both samples with results not considered statistically 

equivalent using equation 3 had uncertainties several orders of 

magnitude smaller than the concentrations that caused them 

to have Z-values greater than 1.96. If the uncertainty of the 

result is not used, the relative percent difference (RPD) can be 

used to compare equivalency of replicate pairs. The RPD is 

calculated based on the formula:

RPD = ((ABS(X1-X2))/((X1+X2)/2))*100,

where

RPD is relative percent difference,

ABS is absolute value, 

X1 is result for primary environmental sample, and

X2 is result for field replicate sample.

(4)

A typical data-quality objective for field replicate samples is 

a maximum relative percent difference of 20 percent (Taylor, 

1987), and results for both pairs with Z-values greater than 

1.96 had RPDs less than 20 percent (USGS 85 and replicate 

was 12 percent and USGS 106 and replicate was 16 percent) 

using equation 4. Results of replicate and blank samples (other 

than the one blank mentioned previously) generally indicated 

that the sample collection and laboratory procedures used were 

appropriate for the data obtained. 
10 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

Table 1. Concentrations of iodine-129 in ground water, Idaho National Laboratory, Idaho, 2003 and 2007. 

[Well locations are shown in figures 2 and 3. Concentrations and analytical uncertainties are in picocuries per liter; uncertainties are 1 standard deviation. 

Z-value: the statistical test used to compare replicate values. Abbreviations: pCi/L, picocurie per liter; NA, not applicable; NS, not sampled; USGS, U.S. 

Geological Survey; QA, quality assurance; QAI, quality assurance iodine] 

Well identifier Date sampled Iodine-129 

2003 (pCi/L) Z-value Date sampled Iodine-129 2007 (pCi/L) Z-value

CFA-1 NA NS NA 04-19-07 0.318±0.015 NA

CFA-2 NA NS NA 10-23-07 0.131±0.006 NA

CPP-1 NA NS NA 04-11-07 0.0237±0.0022 NA

EBR-1 NA NS NA 04-19-07 0.0084±0.0014 NA

ICPP 2018 NA NS NA 05-01-07 0.361±0.026 NA

CFA LF 2-10 09-29-03 0.079±0.003 NA NA NS NA

Rifle Range NA NS NA 10-23-07 0.001±0.004 NA

RWMC Prod. NA NS NA 10-11-07 0.0188±0.0005 NA

USGS 9 10-09-03 0.0000088±0.00000035 NA NA NS NA

USGS 11 10-08-03 0.000018±0.0000005 NA NA NS NA

USGS 14 10-08-03 0.00004±0.000002 NA NA NS NA

USGS 14 10-08-03 0.00004±0.000002 NA NA NS NA

USGS 20 11-03-03 0.026±0.0011 NA 04-03-07 0.0282±0.0009 NA

USGS 34 NA NS NA 04-16-07 0.0098±0.001 NA

USGS 35 10-23-03 0.0018±0.00002 NA NA NS NA

USGS 36 11-13-03 0.162±0.008 0.09 NA NS NA

QA 9/USGS 36 11-13-03 0.164±0.007 NA NA NS NA

USGS 37 11-13-03 0.452±0.025 NA 10-16-07 0.395±0.017 NA

USGS 38 11-13-03 0.556±0.019 NA 04-16-07 0.281±0.008 NA

USGS 39 11-03-03 0.00028±0.000003 NA 10-15-07 0.000532±0.000025 NA

USGS 40 NA NS NA 04-11-07 0.306±0.01 NA

USGS 42 11-10-03 0.216±0.0064 NA 04-11-07 0.325±0.01 NA

USGS 44 11-10-03 0.12±0.0035 NA NA NS NA

USGS 47 11-17-03 0.621±0.022 NA NA NS NA

USGS 48 NA NS NA 10-31-07 0.35±0.013 NA

USGS 50 11-17-03 0.669±0.023 NA 04-10-07 0.779±0.022 1.75

QA-7/USGS 50 NA NS NA 04-10-07 0.727±0.02 NA

USGS 51 11-12-2003 0.164±0.0071 NA 10-25-07 0.231±0.01 NA

USGS 52 NA NS NA 10-25-07 0.284±0.014 NA

USGS 57 11-05-03 0.64±0.023 NA 11-1-07 0.521±0.023 NA

USGS 59 NA NS NA 04-09-07 0.262±0.013 NA

USGS 67 NA NS NA 10-15-07 1.16±0.04 NA

USGS 77 11-03-03 0.586±0.0193 NA 10-18-07 0.71±0.04 NA

USGS 82 11-06-03 0.0112±0.0004 NA 04-04-07 0.011±0.0004 NA

USGS 85 11-04-03 0.283±0.009 2.31 04-10-07 0.173±0.006 NA

QA 8/USGS 85 11-04-03 0.251±0.0105 NA NA NS NA

USGS 88 NA NS NA 10-16-07 0.000026±0.0000022 NA

USGS 103 10-06-03 0.00001±0.0000008 NA NA NS NA

USGS 103 10-06-03 0.000009±0.0000003 NA NA NS NA

USGS 104 10-06-03 0.0049±0.0002 NA 10-24-07 0.005±0.00023 NA

USGS 105 10-02-03 0.0017±0.0001 NA 10-16-07 0.00227±0.00009 NA

USGS 106 10-02-03 0.034±0.001 NA 10-16-07 0.0274±0.0013 2.82

QAI-5/USGS 106 NA NS NA 10-16-07 0.0322±0.0011 NA 

USGS 108 10-06-03 0.0015±0.00005 NA 04-24-07 0.00102±0.00004 NA

USGS 109 10-02-03 0.0004±0.00001 NA 04-17-07 0.000677±0.00002 NA

USGS 111 11-05-03 0.138±0.007 NA 04-11-07 0.192±0.007 NA

USGS 112 11-04-03 0.617±0.067 1.07 NA NS NA
Methods and Quality Assurance 11

Table 1. Concentrations of iodine-129 in ground water, Idaho National Laboratory, Idaho, 2003 and 2007.—Continued

[Well locations are shown in figures 2 and 3. Concentrations and analytical uncertainties are in picocuries per liter; uncertainties are 1 standard deviation.

Z-value: the statistical test used to compare replicate values. Abbreviations: pCi/L, picocurie per liter; NA, not applicable; NS, not sampled; USGS, U.S. 

Geological Survey; QA, quality assurance; QAI, quality assurance iodine] 

Well identifier Date sampled Iodine-129 

2003 (pCi/L) Z-value Date sampled Iodine-129 2007 (pCi/L) Z-value

QA 5/USGS 112 11-04-03 0.696±0.031 NA NA NS NA

USGS 113 11-04-03 0.72±0.051 NA 04-17-07 0.75±0.04 NA

USGS 114 11-04-03 0.153±0.0063 NA 10-15-07 0.173±0.006 NA

USGS 115 11-05-03 0.0172±0.0005 NA NA NS NA

USGS 116 11-10-03 0.069±0.0026 NA 04-09-07 0.144±0.005 NA

USGS 121 09-30-03 0.0000066±0.0000002 NA NA NS NA

USGS 123 11-04-03 0.678±0.034 NA 04-10-07 0.048±0.004 NA

Rerun NA NA NA 04-10-07 0.0382±0.001 NA

USGS 124 10-08-03 0.0023±0.0001 NA 04-17-07 0.00225±0.00007 NA

USGS 125 10-08-03 0.00013±0.0000008 NA NA NS NA

USGS 127 10-29-03 0.001±0.00008 NA 04-05-07 0.00071±0.00003 NA

USGS 128 11-05-03 0.263±0.0085 0.29 10-18-07 0.162±0.008 NA

QA 11/USGS 128 11-05-03 0.267±0.011 NA NA NS NA

QAI-1 (Blank) NA NS NA 04-24-07 0.0025±0.0005 NA

QAI-6 (Blank) NA NS NA 10-25-07 0.0000014±0.0000002 NA

Well identifier Port depth Date sampled Iodine-129 

(pCi/L) Z-value Iodine-1291(pCi/L)

USGS 134 Zone 1 855.5 09-04-07 0.0079±0.0003 NA NS

USGS 134 Zone 2 805.5 09-05-07 0.000218±0.000025 NA NS

Rerun 805.5 09-05-07 0.000018±0.0000009 NA NS

USGS 134 Zone 3 705.5 09-05-07 0.0167±0.0007 NA NS

USGS 134 Zone 4 644.5 09-06-07 0.00034±0.00004 NA NS

USGS 134 Zone 5 577.5 09-10-07 0.00179±0.00011 NA NS

Middle 2051 Zone 1 1,142.5 09-11-07 0.00599±0.00027 NA 0.000084±0.0000031

Middle 2051 Zone 2 1,092.5 09-11-07 0.0066±0.00022 0.33 0.00018±0.0000088

QAI-3 1,092.5 09-11-07 0.00668±0.00022 NA NS

Middle 2051 Zone 3 827.7 09-11-07 0.00091±0.00004 NA 0.00091±0.000028

Middle 2051 Zone 4 750.1 09-12-07 0.00079±0.00005 NA 0.00054±0.000024

QAI-4 NA 09-12-07 0.000071±0.000015 NA NS

Rerun NA 09-12-07 0.000001±0.00000005 NA NS

Middle 2051 Zone 5 604.2 09-12-07 0.000047±0.000007 NA 0.0000087±0.00000033

Rerun 604.2 09-12-07 0.000013±0.0000008 NA NS 

USGS 132 Zone 1 1,172.2 09-17-07 0.0012±0.00006 NA 0.00113±0.00005

USGS 132 Zone 2 1,010.3 09-17-07 0.00173±0.00021 NA 0.00139±0.00004

USGS 132 Zone 3 917.2 09-18-07 0.00142±0.00009 NA 0.00128±0.00006

USGS 132 Zone 4 825.8 09-18-07 0.00127±0.00008 NA 0.00111±0.00003

USGS 132 Zone 5 763.9 09-18-07 0.00111±0.00007 NA 0.000845±0.000023

USGS 132 Zone 6 636.4 09-18-07 0.00024±0.00004 NA 0.0024±0.00009

Duplicate NA NA NA NA 0.000367±0.000013

Table 2. Concentrations of iodine-129 at multiple aquifer depths, Idaho National Laboratory, Idaho, 2007. 

[Well locations are shown in figures 2 and 3. Concentrations and analytical uncertainties are in picocuries per liter (pCi/L); uncertainties are 1 standard deviation. 

Port depth: feet below land surface. Z-value: statistical test used to compare the replicate sample. QAI-3 is a replicate of Middle 2051 Zone 2. QAI-4 is an 

equipment blank of inorganic free water. Abbreviations: USGS, U.S. Geological Survey; NA, not applicable; NS, not sampled; QAI, quality assurance iodine]
12 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

Concentrations of Iodine-129 in the 

Snake River Plain Aquifer

Concentrations of Iodine-129 in 2003

In October 2003, water samples were collected from 

36 wells that obtain water from the ESRP aquifer at or south 

of the INL. Additionally, water was collected from one well 

at the INTEC that obtains water from a perched ground-water 

zone that may eventually recharge to the ESRP aquifer. The 

areal distribution of concentrations greater than 0.1 pCi/L 

is shown in figure 4. Concentrations of 129I in the aquifer 

ranged from 0.0000066 ± 0.0000002 to 0.72 ± 0.051 pCi/L, 

and the concentration of water from the perched zone at 

the INTEC was 0.669 ± 0.023 pCi/L (table 1, fig. 4). Water 

from one aquifer well (USGS 121) had a concentration 

(0.0000066 pCi/L) that was within the uncertainty of the 

estimated background concentration of 0.0000054 pCi/L for 

the ESRP aquifer as calculated by Cecil and others (2003). 

Well USGS 121, upgradient of the INTEC, was considered 

by Mann and Beasley (1994b) as a background well for 

comparative purposes. 

Concentrations of 129I concentrations in the ESRP 

aquifer generally decreased from 1990–91 to 2003 (table 3). 

The average concentration of 19 wells sampled during both 

sample periods decreased from 0.975 pCi/L in 1990–91 to 

0.249 pCi/L in 2003. Many of the wells within a 3-mi radius 

of the INTEC showed decreases of as much as one order of 

magnitude in concentration (table 3). These decreases are 

attributed to discontinuation of disposal of 129I in wastewater 

after 1988 and to dilution and dispersion in the ESRP aquifer. 

Dilution from the Big Lost River probably was quite prevalent 

during 1995–2000 (Davis, 2006, fig. 8), and this additional 

dilution may have contributed to the marked decreases in the 

wells near the INTEC. The decrease in concentrations during 

2003 is a continuance of the decreases described by Mann and 

Beasley (1994b) from earlier sampling periods. 

Well identifier Port depth Date sampled Iodine-129 

(pCi/L) Z-value Iodine-1291(pCi/L)

Middle 2050A Zone 1 1,178.8 09-19-07 0.00027±0.00003 NA 0.000046±0.0000017

Middle 2050A Zone 2 997.6 09-19-07 0.00023±0.00006 NA 0.000019±0.00000058

Middle 2050A Zone 3 790.2 09-20-07 0.0002±0.00003 NA 0.000017±0.00000075

Middle 2050A Zone 4 641.9 09-20-07 0.00019±0.00003 NA 0.000018±0.00000063

Middle 2050A Zone 5 514.9 09-20-07 0.00019±0.00005 NA 0.000018±0.00000076

USGS 133 Zone 1 744.3 09-24-07 0.00017±0.00005 NA NS

Rerun 744.3 09-24-07 0.000017±0.0000008 NA NS

USGS 133 Zone 2 684.9 09-24-07 0.00025±0.00004 NA NS

Rerun 684.9 09-24-07 0.00005±0.0000025 NA NS

USGS 133 Zone 3 568.5 09-24-07 0.000186±0.000029 NA NS

Rerun 568.5 09-24-07 0.000011±0.0000005 NA NS

USGS 133 Zone 4 468 09-24-07 0.0164±0.0007 NA NS

Rerun 468 09-24-07 0.0011±0.00024 NA NS

USGS 103 Zone 1 1,269.4 09-25-07 0.0012±0.00004 NA NS

USGS 103 Zone 2 1,220.2 09-25-07 0.00046±0.00003 NA NS

USGS 103 Zone 3 1,095.1 10-01-07 0.0017±0.0014 NA NS

USGS 103 Zone 5 913.8 10-01-07 0.00079±0.00003 NA NS

USGS 103 Zone 6 805.2 10-02-07 0.000415±0.000018 NA NS

USGS 103 Zone 7 681.9 10-02-07 0.0015±0.0001 NA NS

1

Data from U.S. Department of Energy, 2007, 2008.

Table 2. Concentrations of iodine-129 at multiple aquifer depths, Idaho National Laboratory, Idaho, 2007.—Continued

[Well locations are shown in figures 2 and 3. Concentrations and analytical uncertainties are in picocuries per liter (pCi/L); uncertainties are 1 standard deviation. 

Port depth: feet below land surface. Z-value: statistical test used to compare the replicate sample. QAI-3 is a replicate of Middle 2051 Zone 2. QAI-4 is an 

equipment blank of inorganic free water. Abbreviations: USGS, U.S. Geological Survey; NA, not applicable; NS, not sampled; QAI, quality assurance iodine]
Concentrations of Iodine-129 in the Snake River Plain Aquifer 13

tac09-0332_fig04

Idaho Nuclear Technology 

and Engineering Center

Central Facilities Area Lincoln Boulevard

East Portland Avenue

West Portland Avenue

Big Lost River

0.25

EXPLANATION

Line of Equal Iodine-129 Concentration—Concentration in picocuries per liter. Lines of equal concentrations were interpreted from analyses of samples collected from a 3-dimensional flow 

system. Mapped concentrations represent samples collected from various depths in boreholes with 

differing well completions; for example, single and multiple screened intervals, and open boreholes. 

Location is approximate. Interval is variable.

Well in the USGS Water-Quality Monitoring Network— Samples analyzed for iodine-129. 50P is local well identifier; P indicates well obtains water from a perched qround-water zone. 

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

0.25

0.1

0.12 USGS 44

0.283 USGS 85

0.586 USGS 77

0.164 USGS 51

0.669 USGS 50P

0.621 USGS 47

0.216 USGS 42

0.556 USGS 38

0.452 USGS 37

0.162 USGS 36

0.026 USGS 20

0.069 USGS 116

0.153 USGS 114

0.617 USGS 112

0.138 USGS 111

0.0112 USGS 82

0.0018 USGS 35

0.263 USGS 128

0.001 USGS 127

0.669 USGS 50P

0.079 CFA LF2-10

0.0000066 USGS 121

0.5

0.5

0.5

0.678 USGS 123

0.64 USGS 57

0.00028 USGS 39

0.72 USGS 113

0.0172 USGS 115

0 2,000 METERS

0 6,000 FEET

43 ̊34’

43 ̊32’

112 ̊58’ 112 ̊56’

Figure 4. Areal distribution of iodine-129 in the Snake River Plain aquifer at the Idaho National 

Laboratory, Idaho, 2003.
14 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

Table 3. Concentrations of iodine-129 in water from selected wells, Idaho National Laboratory, Idaho, 1990–91, 2003, and 2007.

[Well locations are shown in figures 2 and 3. Concentrations and analytical uncertainties are in picocuries per liter; uncertainties are 1 standard deviation. 

Average concentration calculated from the 19 wells with concentrations from all three sample periods (USGS 123 excluded because well was modified in 2004). 

Abbreviations: pCi/L, picocurie per liter; NS, not sampled; USGS, U.S. Geological Survey] 

Well identifier

Iodine-129

1990–91

(pCi/L)

2003

(pCi/L)

2007 

(pCi/L)

CFA-1 0.24±0.05 NS 0.318±0.015

CFA-2 0.10±0.03 NS 0.131±0.006

CPP-1 0.033±0.010 NS 0.0237±0.0022

EBR-1 NS NS 0.0084±0.0014

ICPP 2018 NS NS 0.361±0.026

CFA LF 2-10 NS 0.079±0.003 NS

Rifle Range NS NS 0.001±0.004

RWMC Production NS NS 0.0188±0.0005

USGS 9 NS 0.0000088±0.00000035 NS

USGS 11 0.00001±0.000001 0.000018±0.0000005 NS

USGS 14 0.00003±0.000002 0.00004±0.000002 NS

USGS 14 NS 0.00004±0.000002 NS

USGS 20 0.033±0.002 0.026±0.0011 0.0282±0.0009

USGS 34 0.39±0.02 NS 0.0098±0.001

USGS 35 0.57±0.03 0.0018±0.00002 NS

USGS 36 1.19±0.03 0.162±0.008 NS

USGS 37 1.80±0.08 0.452±0.025 0.395±0.017

USGS 38 2.00±0.07 0.556±0.019 0.281±0.008

USGS 39 0.005±0.001 0.00028±0.000003 0.000532±0.000025

USGS 40 0.50±0.02 NS 0.306±0.01

USGS 41 0.58±0.03 NS NS

USGS 42 3.82±0.19 0.216±0.0064 0.325±0.01

USGS 43 0.16±0.01 NS NS

USGS 44 0.20±0.03 0.12±0.0035 NS

USGS 45 0.32±0.01 NS NS

USGS 46 0.35±0.02 NS NS

USGS 47 0.83±0.04 0.621±0.022 NS

USGS 48 0.22±0.02 NS 0.35±0.013

USGS 50 1.20±0.04 0.669±0.023 0.779±0.022

USGS 51 0.28±0.01 0.164±0.0071 0.231±0.01

USGS 52 0.38±0.03 NS 0.284±0.014

USGS 57 1.38±0.07 0.64±0.023 0.521±0.023

USGS 59 0.57±0.02 NS 0.262±0.013

USGS 65 0.0008±0.004 NS NS

USGS 67 1.43±0.04 NS 1.16±0.04

USGS 77 1.37±0.06 0.586±0.0193 0.71±0.04

USGS 82 0.119±0.002 0.0112±0.0004 0.011±0.0004

USGS 83 0.0000006±0.0000002 NS NS

USGS 85 1.64±0.08 0.283±0.009 0.173±0.006

USGS 88 NS NS 0.000026±0.0000022
Concentrations of Iodine-129 in the Snake River Plain Aquifer 15

Although the wells near the INTEC showed decreases 

of 129I, wells south and east of the Central Facilities Area 

(CFA) (USGS 104 and USGS 106), near the site boundary 

(USGS 105 and 108) and south of the INL (USGS 11 and 14) 

showed slight increases in concentrations between 1990–91 

and 2003 (table 3, fig. 5). These slight increases may be 

related to variable discharge rates of wastewater that moved 

to these well locations as a mass of water from a particular 

disposal period. For example, the highest concentrations of 

129I probably were in wastewater discharged in 1957 (U.S. 

Department of Energy 2004, appendix D) and 1978 (Mann 

and Beasley, 1994b, fig. 2; U.S. Department of Energy, 2004, 

appendix D). Therefore, when water reaches a well from 

discharge in 1957 and 1978, the concentration of the sample 

most likely would be greater than concentrations in samples of 

water discharged during other periods. Data from wells USGS 

11 and USGS 14 from Hall (2006) somewhat support this 

idea because 129I concentrations for these two wells were less 

in 1998 than during the 1990–91 and 2003 sample periods. 

Cecil and others (2000) indicated that 1958 peak disposal of 

chlorine-36 probably reached USGS 11 in 1984 and reached 

USGS 14 in 1987. Beasley and others (1998) examined the 

relative mobility of several isotopes disposed at the INTEC, 

and they concluded that chlorine-36 behaves conservatively 

in the basalt; however 129I is attenuated, and probably moves 

more slowly in the system. Data for 1990–91 from some 

southern wells, therefore, could represent the end of high 

concentrations related to the large discharge event in 1957, and 

2003 data could represent the beginning of high concentrations 

related to the 1978 discharge event or possibly some larger 

discharge event between 1957 and 1978. Accurately predicting 

the first arrival of peak concentrations with the limited sample 

periods and the complexity of the basaltic aquifer system is 

difficult because of the uncertainty of 129I concentrations in 

wastewater discharged prior to 1976. 

Well identifier

Iodine-129

1990–91

(pCi/L)

2003

(pCi/L)

2007 

(pCi/L)

USGS 103 0.0001±0.00004 0.00001±0.0000008 NS

USGS 103 0.00008±0.00005 0.000009±0.0000003 NS

USGS 104 0.0036±0.0001 0.0049±0.0002 0.005±0.00023

USGS 105 0.00053±0.00004 0.0017±0.0001 0.00227±0.00009

USGS 106 0.025±0.001 0.034±0.001 0.0274±0.0013

USGS 108 0.00083±0.00002 0.0015±0.00005 0.00102±0.00004

USGS 109 NS 0.0004±0.00001 0.000677±0.00002

USGS 111 0.86±0.09 0.138±0.007 0.192±0.007

USGS 112 2.40±0.25 0.617±0.067 NS

USGS 113 3.25±0.14 0.72±0.051 0.75±0.04

USGS 114 0.28±0.01 0.153±0.0063 0.173±0.006

USGS 115 0.19±0.02 0.0172±0.0005 NS

USGS 116 0.45±0.01 0.069±0.0026 0.144±0.005

USGS 121 0.0000009±0.0000002 0.0000066±0.0000002 NS

USGS 123 1.00±0.05 0.678±0.034 0.048±0.004

USGS 124 NS 0.0023±0.0001 0.00225±0.00007

USGS 125 NS 0.00013±0.0000008 NS

USGS 127 NS 0.001±0.00008 0.00071±0.00003

USGS 128 NS 0.263±0.0085 0.162±0.008

Average concentration 0.975 0.249 0.250

Table 3. Concentrations of iodine-129 in water from selected wells, Idaho National Laboratory, Idaho, 1990–91, 2003, and 2007.—

Continued

[Well locations are shown in figures 2 and 3. Concentrations and analytical uncertainties are in picocuries per liter; uncertainties are 1 standard deviation. 

Average concentration calculated from the 19 wells with concentrations from all three sample periods (USGS 123 excluded because well was modified in 2004). 

Abbreviations: pCi/L, picocurie per liter; NS, not sampled; USGS, U.S. Geological Survey]
16 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

tac09-0332_fig05

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

NS 

USGS 9

-1.36 USGS 85

 NS USGS 83

-0.007 USGS 20

+0.00001 USGS 14

+0.00001 USGS 11

NS USGS 109

+0.0007 USGS 108

+0.009 USGS 106

+0.0012 USGS 105

+0.0013 USGS 104

-0.00009 USGS 103

NS USGS 125

 NS USGS 124

Spreading

Areas

Big

Southern

Butte

Change in Iodine-129 Concentration Between 1991 and 2003—Concentration in picocuries per liter. 

+ indicates increase; - indicates decrease in concentration. NS, indicates no sample collected in 

both years. Data calculated from table 3. Location is approximate. Interval is variable.

Well in the USGS Water-Quality Monitoring Network—Samples analyzed for iodine-129.

EXPLANATION

+0.0012

USGS 105

113 ̊06’ 113 ̊

43 ̊34’

43 ̊30’

43 ̊26’

43 ̊22’

112 ̊54’

0 4,000 METERS

0 12,000 FEET

See figure

3 for wells

in this area

Figure 5. Change in concentration of iodine-129 between samples collected from selected 

wells, Idaho National Laboratory, Idaho, 1990–91 and 2003.
Concentrations of Iodine-129 in the Snake River Plain Aquifer 17

Concentrations of Iodine-129 in 2007

During April and October 2007, water samples were 

collected from 36 wells that obtain water from the ESRP 

aquifer at or south of the INL. Additionally, water was 

collected from two wells at the INTEC that obtain water from 

a perched ground-water zone that may eventually recharge 

to the ESRP aquifer. The areal distribution of concentrations 

greater than 0.1 pCi/L is shown in figure 6. Concentrations of 

129I in the ESRP aquifer ranged from 0.000026 ± 0.000002 

to 1.16 ± 0.04 pCi/L, and the concentrations of water from 

the two perched zone wells (ICPP-2018 and USGS 50) at 

the INTEC were 0.361 ± 0.026 and 0.779 ± 0.022 pCi/L, 

respectively (table 1). Concentrations in water from all aquifer 

wells sampled were greater than the estimated background 

concentration of 0.0000054 pCi/L for the ESRP aquifer as 

calculated by Cecil and others (2003). One concentration 

in well USGS 67 exceeded the MCL of 1 pCi/L for public 

drinking water supplies, but the concentration was much 

less than the proposed MCL of 21 pCi/L. Well USGS 67 

was not sampled by the USGS for 129I in 2003 because of 

pump problems, but a concentration from this well was 

0.68 pCi/L from a 2003 sample collected and analyzed by 

the site contractor using a different analytical method (U.S. 

Department of Energy, 2004, fig. 6-1). 

On average, 129I concentrations in the ESRP aquifer 

remained the same between 2003 and 2007 in 19 wells that 

were sampled during both years. The average concentration of 

129I in water from 19 wells sampled in 2003 was 0.249 pCi/L; 

the average concentration of 129I in water from the same wells 

sampled in 2007 was 0.250 pCi/L (table 3). These average 

concentrations were significantly less than the average 

concentration of 0.975 pCi/L for the same wells sampled in 

1990–91. These decreases are attributed to the discontinuation 

of 129I disposal in wastewater after 1988 and to dilution and 

dispersion in the ESRP aquifer. 

Although samples from several wells southwest of the 

INTEC showed slight decreases between 2003 and 2007 

(USGS 37, 38, 57, and 123; table 3, fig. 7), samples from 

several other wells also showed some slight increases in 

concentration (USGS 42, 50, 51, 77, 111; table 3, fig. 7). 

With the exception of USGS 123, all samples from wells that 

showed the decrease in concentrations are west of the primary 

flow path from the INTEC. The sharp decrease of 129I in water 

from USGS 123 probably can be attributed to the deepening 

of the well in 2004, which resulted in sampling a larger zone 

of the upper part of the aquifer, thereby probably diluting the 

sample (Forbes and others, 2007). Some recharge from the 

Big Lost River probably occurred because of surface water 

flow during 2005 and 2006 (Brennan and others, 2006; U.S. 

Geological Survey, 2006), accessed February 28, 2008, at 

http://wdr.water.usgs.gov/) in the INTEC area. This additional 

recharge could have moved remnant perched water containing 

elevated 129I concentrations from below the INTEC and (or) 

the infiltration ponds into the aquifer. An examination of 

sodium, chloride, and strontium-90 concentrations in the same 

wells that showed slight increases indicated similar slight 

increases in concentrations for some constituents between 

the 2003 and 2007 data; however, some decreases also are 

evident for some of the constituents (table 4). For example, 

USGS 42 showed a slight increase in all four constituents 

between samples collected in 2003 and 2007, and well USGS 

77 showed an increase in sodium and 129I, but a decrease in 

chloride and strontium-90 (table 4). USGS 42 is upgradient 

of the infiltration ponds and is on the west side of the INTEC, 

so increases in concentrations probably are related to leakage 

around the facility reaching the aquifer. The high chloride 

concentration in USGS 77 in 2003 probably was related to 

remnant discharge of chloride in the infiltration ponds, and 

the decrease of the concentration in 2007 was attributed to 

the discontinued use of the infiltration ponds in 2002. The 

variability of constituent concentrations near the INTEC 

has been a continual issue because of the uncertainty of the 

amount of constituents that have leaked below ground at 

the facility and because of the uncertainty of the chemical 

exchange processes occurring in the unsaturated zone. 
18 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

tac09-0332_fig06

Idaho Nuclear Technology 

and Engineering Center

Central Facilities Area East Portland Avenue

West Portland Avenue

Big Lost River

EXPLANATION

Line of Equal Iodine-129 Concentration—Concentration in picocuries per liter. Lines of equal concentrations were 

interpreted from analyses of samples collected from a 3-dimensional flow system. Mapped concentrations represent 

samples collected from various depths in boreholes with differing well completions; for example, single and multiple 

screened intervals, and open boreholes. Location is approximate. Interval is variable.

Well in the USGS Water-Quality Monitoring Network— Samples analyzed for iodine-129. 50P is local well identifier; 

P indicates well obtains water from a perched ground-water zone.

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

0.5

0.5

0.5

0.1

0.25

0.1

0.1

0.5

0.5

0.131 CFA-2

0.318 CFA-1

0.71 USGS 77

0.0084 EBR-1

0.0237 CPP-1

0.173 USGS 85

0.011 USGS 82

0.521 USGS 57

0.284 USGS 52

0.35 USGS 48 0.325 USGS 42

0.306 USGS 40

0.281 USGS 38 0.144 USGS 116

0.192 USGS 111

0.0282 USGS 20

0.0274 USGS 106

0.779 USGS 50P

0.000532 USGS 39

0.00071 USGS 127

0.001 RIFLE RANGE

0.5

0.395 USGS 37

0.75 USGS 113

0.173 USGS 114

0.779 USGS 50P

0.361 ICPP 2018P

0.262 USGS 59

1.16 USGS 67

0.048 USGS 123 0.231 USGS 51

0.162 USGS 128

0.25

113 ̊

43 ̊34’

43 ̊32’

43 ̊30’

112 ̊58’ 112 ̊56’ 112 ̊54’

0 2,000 METERS

0 6,000 FEET

Figure 6. Areal distribution of iodine-129 in the Snake River Plain aquifer at the Idaho National Laboratory, Idaho, 2007.
Concentrations of Iodine-129 in the Snake River Plain Aquifer 19

Figure 7. Change in concentration of iodine-129 between water samples from wells at the Idaho National Laboratory, Idaho, 

2003 and 2007.

tac09-0332_fig07

+0.124 USGS 77

-0.11 USGS 85

NC USGS 82

-0.19 USGS 57

+0.09 USGS 42

-0.275 USGS 38

+0.075 USGS 116

+0.054 USGS 111

+0.002 USGS 20

-0.007 USGS 106

+0.11 USGS 50P

+0.0002 USGS 39

-0.0003 USGS 127

-0.057 USGS 37

NC USGS 113

+0.02 USGS 114

+0.11 USGS 50P

+0.49 USGS 67

-0.63 USGS 123

+0.067 USGS 51

-0.1 USGS 128

+0.49

Idaho Nuclear Technology 

and Engineering Center

Central Facilities Area East Portland Avenue

West Portland Avenue

Big Lost River

EXPLANATION

Change in Iodine-129 Concentration Between 2003 and 2007—Concentration in picocuries per liter. + indicates increase; - indicates 

decrease in concentration. NC, indicates no statistically significant change between sample periods. Data calculated from table 3 

except for USGS 67 which used data from U.S. Department of Energy (2004) for 2003 USGS 67 concentration. Location is 

approximate. Interval is variable.

Well in the USGS Water-Quality Monitoring Network— Samples analyzed for iodine-129. USGS 50P is local well identifier; P 

indicates well obtains water from a perched ground-water zone.

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

113 ̊

43 ̊34’

43 ̊32’

43 ̊30’

112 ̊58’ 112 ̊56’ 112 ̊54’

0 2,000 METERS

0 6,000 FEET
20 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

Vertical Distribution of Iodine-129 in the Snake 

River Plain Aquifer

During September and October 2007, water samples 

were collected from 31 zones from 6 wells equipped with 

multi-level WestbayTM packer sampling systems (Middle 

2050A, 2051, USGS 103, 132, 133, and 134; figs. 2 and 3). 

The WestbayTM systems isolate various zones of the aquifer so 

the vertical distribution of 129I can be determined. Water from 

all but one of the sampled zones contained concentrations that 

were greater than the reporting level of 3s, and concentrations 

ranged from 0.00001 ± 0.0000005 to 0.0167 ± 0.0007 pCi/L 

(table 2). All concentrations were greater than the estimated 

background concentration of 0.0000054 pCi/L for the ESRP 

aquifer as calculated by Cecil and others (2003). Three wells 

(USGS 103, 134 and Middle 2051) showed variability of 

one to two orders of magnitude in 129I concentrations among 

various zones. Two of the wells (USGS 132 and 133) had 

one zone with a concentration that differed by more than 

one order of magnitude from their other zones, and one well 

(Middle 2050A) showed a similar concentration in all five 

zones (table 2). Concentrations were well below the MCL in 

all zones. 

Well 

identifier

Chloride Iodine-129 Sodium Strontium-90

2003 

(mg/L)

2007 

(mg/L)

2003 

(pCi/L)

2007 

(pCi/L)

2003 

(mg/L)

2007 

(mg/L)

2003 

(pCi/L)

2007 

(pCi/L)

USGS 37 31.4 20.3 0.452±0.025 0.395±0.017 24.8 18.3 6.9±0.7 4.2±0.7

USGS 38 45.3 16.9 0.556±0.019 0.281±0.008 27.8 13.8 13.1±0.8 5.9±0.7

USGS 42 18.2 18.8 0.216±0.0064 0.325±0.01 10.8 11.5 7.5±0.7 9.7±0.8

USGS 48 27.5 23.1 NS 0.35±0.013 15.4 13.8 21.7±0.9 14.5±0.9

USGS 50 54.1 52.5 0.669±0.023 0.779±0.022 60 58.2 105±2 101±2

USGS 51 172 171 0.164±0.0071 0.231±0.01 34.7 36.3 0.6±0.5 0.2±0.7

USGS 57 60.3 24.6 0.64±0.023 0.521±0.023 30.4 15.3 18.5±0.9 14±0.9

USGS 67 1160 62.4 NS 1.16±0.04 NS 30.9 19±0.7 15.9±0.9

USGS 77 137.4 95.4 0.586±0.0193 0.71±0.04 37.6 40.4 2.5±0.7 1.1±0.7

USGS 85 21.3 15 0.283±0.009 0.173±0.006 16.2 12.5 4.1±0.6 4±0.7

USGS 111 97.6 104 0.138±0.007 0.192±0.007 25 24.8 0.9±0.5 0.4±0.7

USGS 113 141 61.6 0.72±0.051 0.75±0.04 66.6 36.4 10.2±0.8 10.3±1

USGS 116 104 102 0.069±0.0026 0.144±0.005 30.1 32.8 0.5±0.5 0.6±0.7

1

 Data from April 2003.

Table 4. Concentrations of chloride, iodine-129, sodium, and strontium-90 in water from selected wells, Idaho National Laboratory, 

Idaho, 2003 and 2007. 

[Well locations are shown in figures 2 and 3. Concentrations of chloride and sodium are in milligrams per liter (mg/L). Concentrations and analytical 

uncertainties for iodine-129 and strontium-90 are in picocuries per liter (pCi/L). Uncertainties are 1 standard deviation. Data from 2003 are from October 

sampling unless noted. Abbreviation: NS, not sampled] 
Concentrations of Iodine-129 in the Snake River Plain Aquifer 21

Iodine-129 data collected by the USGS in September 

2007 for three wells (Middle 2050A, Middle 2051, and USGS 

132) also were sampled during 2005 and 2007 by the site 

contractor (table 2). Those samples were analyzed using the 

same laboratory and the same methods (U.S. Department of 

Energy, 2007, 2008). The data collected in 2007 from USGS 

132 indicated similar concentrations; however, data from 

Middle 2050A and Middle 2051 collected by the contractor 

in 2005 were about one order of magnitude lower than 2007 

USGS data in most of the zones. The samples analyzed in 

2005 were processed as low-level samples, and they were not 

diluted in the chemical process (Mike Roddy, CH2M-WG 

Idaho (CWI), oral commun., May 15, 2008). The USGS 

samples were diluted, and rerun results for several of the 

samples in 2007 generally indicated that non-diluted samples 

produce results an order of magnitude smaller than the diluted 

samples at the concentration levels detected in these wells 

(table 2).

The variability with some of the 129I concentrations 

with depth in one well is consistent with the variability of the 

water chemistry. For example, USGS 132, near the RWMC, 

showed relatively consistent concentrations for all five deeper 

zones of water (763.9–1,172.2 ft) for 129I, calcium, sodium, 

bicarbonate, chloride, oxygen and deuterium isotopes, and 

tritium (table 5). The upper zone 6 (636.4 ft), however, 

showed much higher concentrations of sodium and chloride 

and lower concentrations of tritium and 129I than the other five 

zones, which indicated that the upper zone probably is derived 

from another source of recharge water. 

Concentrations of 129I in well Middle 2050A were 

consistently the same in all five zones, but calcium, chloride, 

and tritium concentrations were higher in the upper zone 

(zone 5) than the other samples (table 5). Tritium and chloride 

concentrations in zone 5 probably can be attributed to local 

recharge associated with wastewater disposal. 

An 129I concentration of 0.000013 ± 0.0000008 pCi/L 

was measured in water from well Middle 2051 in the upper 

zone of the aquifer (zone 5) that was one order of magnitude 

lower than the next two zones (zones 3 and 4) and two orders 

of magnitude lower than the two deepest zones (zones 1 

and 2, table 5). The lower 129I concentration in the upper 

zone, along with a tritium concentration that is less than the 

reporting level, indicates that this zone of water probably is 

not influenced by wastewater disposal. The chemical content 

of water from this zone is similar to Big Lost River drainage 

water (Carkeet and others, 2001) with low concentrations of 

sodium and chloride, a higher concentration of calcium, and 

heavier concentrations of oxygen and deuterium than water in 

the other zones. This well is near the Big Lost River, and the 

upper zone probably is influenced by Big Lost River recharge. 

Tritium concentrations in the other four zones are greater 

than the reporting level, so the water chemistry probably is 

influenced by wastewater disposal; however, the variability 

of the 129I between zones also may be related to the same 

hydrologic processes that cause changes in piezometric head. 

Piezometric head decreases dramatically between the upper 

zone 5 and zones 3 and 4 (fig. 8), possibly due to the presence 

of a sedimentary interbed. Another interbed between zones 2 

and 3 may increase head in the lower two zones, which also 

corresponds with an increase in the 129I concentrations. 

A 129I concentration (0.0011 ± 0.00024 pCi/L) in water 

from well USGS 133 in the upper zone of the aquifer (zone 4) 

was more than one order of magnitude larger than the other 

three zones. The water chemistry data for zone 4 is similar 

to zones 2 and 3, and all three upper zones differ from the 

deepest zone (zone 1; table 5). The 129I concentration in the 

upper zone of well USGS 133 is well above background, and 

because the well is upgradient of the INTEC, more research is 

needed to understand the source of this concentration. 

The greatest concentration of 129I (0.0012 ± 

0.00004 pCi/L) in water from well USGS 103 was in 

the deepest zone (tables 2 and 5). The concentration also 

corresponded to the largest tritium concentration in this well 

(table 5), which indicated that water from the deepest zone is 

partially related to wastewater disposal. The concentrations of 

tritium and 129I support the concept introduced in Ackerman 

and others (2006) that wastewater introduced near the INTEC 

and the RTC in the upper part of the aquifer deepens at the 

southern part of the INL.
22 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

Well

 identifier

Port 

depth

Date 

sampled

Iodine-129 

(pCi/L)

Calcium 

(mg/L)

Sodium 

(mg/L)

Bicarbonate 

(mg/L)

Chloride 

(mg/L)

Deuterium 

(permil)

Oxygen-18 

(permil)

Tritium 

(pCi/L)

USGS 134 Zone 1 855.5 09-04-07 0.0079±0.0003 54.1 11 1260 9.25 -138 -18.13 50±60

USGS 134 Zone 2 805.5 09-05-07 0.000018±0.0000009 33.6 7.74 161 9.34 -137.6 -18.14 -130±60

USGS 134 Zone 3 705.5 09-05-07 0.0167±0.0007 27.5 7.77 151 7.34 -138.6 -18.13 -110±60

USGS 134 Zone 4 644.5 09-06-07 0.00034±0.00004 40.8 9.16 205 11.2 -138 -18.1 180±70

QAW-7 644.5 09-06-07 NS 41.2 9.2 205 11.3 -137.3 -18.04 230±70

USGS 134 Zone 5 577.5 09-10-07 0.00179±0.00011 28.0 7.89 156 9.45 -137.8 -18.18 -20±60

Middle 2051 Zone 1 1,142.5 09-11-07 0.00599±0.00027 38.2 7.8 178 11.8 -136.8 -18.09 210±70

Middle 2051 Zone 2 1,092.5 09-11-07 0.0066±0.00022 38 7.69 173 11.6 -137.3 -18.08 230±70

QAI-3 1,092.5 09-11-07 0.00668±0.00022 NS NS NS NS NS NS NS

Middle 2051 Zone 3 827.7 09-11-07 0.00091±0.00004 46.4 8.65 198 10.5 -137.1 -17.97 610±100

Middle 2051 Zone 4 750.1 09-12-07 0.00079±0.00005 44.3 8.21 185 10.6 -136.9 -17.98 450±80

QAW-8 750.1 09-12-07 NS 44.9 8.33 185 11.3 -136.8 -17.96 490±90

QAI-4 NA 09-12-07 0.000001±0.00000005 NS NS NS NS NS NS NS

Middle 2051 Zone 5 604.2 09-12-07 0.000013±0.0000008 47.6 5.82 184 5.68 -133.6 -17.34 -50±70

USGS 132 Zone 1 1,172.2 09-17-07 0.0012±0.00006 39.5 11.8 176 10.3 -135.4 -17.81 310±70

USGS 132 Zone 2 1,010.3 09-17-07 0.00173±0.00021 39.7 9.08 176 10.1 -136.4 -17.9 210±70

USGS 132 Zone 3 917.2 09-18-07 0.00142±0.00009 39.6 8.9 176 9.95 -136 -17.89 150±60

USGS 132 Zone 4 825.8 09-18-07 0.00127±0.00008 39.7 8.87 168 10 -135.8 -17.88 140±60

USGS 132 Zone 5 763.9 09-18-07 0.00111±0.00007 38.6 10.8 173 11 -136.2 -17.85 290±70

USGS 132 Zone 6 636.4 09-18-07 0.00024±0.00004 34.8 28.1 176 23.2 -134.6 -17.69 14.7±59.6

Middle 2050A Zone 1 1,178.8 09-19-07 0.00027±0.00003 41 12.5 202 14.9 -135.9 -17.82 70±60

Middle 2050A Zone 2 997.6 09-19-07 0.00023±0.00006 43 7.82 193 10.2 -136.9 -18.01 -20±60

Middle 2050A Zone 3 790.2 09-20-07 0.0002±0.00003 47.5 7.93 200 10.7 -136.3 -18.04 40±60

Middle 2050A Zone 4 641.9 09-20-07 0.00019±0.00003 43.8 6.7 176 10.9 -137 -17.92 70±60

Middle 2050A Zone 5 514.9 09-20-07 0.00019±0.00005 54.7 8.55 205 16.1 -135.2 -17.77 210±70

USGS 133 Zone 1 744.3 09-24-07 0.000017±0.0000008 57.7 9.45 217 13.95 -136.7 -17.99 -50±60

USGS 133 Zone 2 684.9 09-24-07 0.00005±0.0000025 45.4 7.75 168 12.1 -137.9 -18.12 -30±60

USGS 133 Zone 3 568.5 09-24-07 0.000011±0.0000005 45.6 7.92 161 12.4 -137.3 -18.13 -50±60

USGS 133 Zone 4 468 09-24-07 0.0011±0.00024 43.3 8.53 161 12.5 -137.6 -18.14 50±60

USGS 103 Zone 1 1,269.4 09-25-07 0.0012±0.00004 38.6 8.48 166 14.1 -137 -17.96 400±80

USGS 103 Zone 2 1,220.2 09-25-07 0.00046±0.00003 38.2 8.15 166 13.9 -136.7 -17.92 380±80

USGS 103 Zone 3 1,095.1 10-01-07 0.0017±0.0014 38.7 8.42 168 13.7 -136.7 -17.95 350±80

USGS 103 Zone 5 913.8 10-01-07 0.00079±0.00003 32.1 10 154 11 -137.7 -17.97 160±70

USGS 103 Zone 6 805.2 10-02-07 0.000415±0.000018 33.2 12.2 154 13.9 -136.5 -17.84 -80±60

USGS 103 Zone 7 681.9 10-02-07 0.0015±0.0001 20 13.7 107 20.6 -135.6 -17.7 -70±60

 1 Concentration from laboratory; field measurement not taken.

Table 5. Concentrations of selected chemicals at multiple aquifer depths, Idaho National Laboratory, Idaho. 

[Well locations are shown in figures 2 and 3. Port depth: feet below land surface. QAI-3 is a replicate of Middle 2051 Zone 2. QAI-4 is an equipment blank 

of inorganic free water. QAW-7 is a replicate of USGS 134 zone 4. QAW-8 is a replicate of Middle 2051 zone 4. Abbreviations: pCi/L, picocuries per liter, 

uncertainty is 1 standard deviation; mg/L, milligram per liter; USGS, U.S. Geological Survey; NS, not sampled; QAI, quality assurance iodine; NA, not 

applicable; QAW, quality assurance water. Rerun results used for all applicable iodine-129 concentrations]
Concentrations of Iodine-129 in the Snake River Plain Aquifer 23

tac09-0332_fig08

600

800 

1,000 

0 4,433 4,431 4,429 4,427

DECREASING PIEZOMETRIC HEAD, IN FEET BELOW 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

IODINE-129, IN PICOCURIES PER LITER

ZONE 5

0.000013 

± 0.0000008

ZONE 4

0.00079 

± 0.00005

ZONE 3

0.00091 

± 0.00004

ZONE 2

0.0066 

± 0.00022

ZONE 1

0.00599 

± 0.00027

EXPLANATION

Basalt—Yellow areas indicate sediment.

Port where piezometric head was measured

Water sample collection zone

DEPTH, IN FEET BELOW LAND SURFACE

Figure 8. Concentrations of iodine-129 in well Middle 2051 in relation to piezometric head, 

Idaho National Laboratory, Idaho, 2007.
24 Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007

Summary

From 1953 to 1988, wastewater containing approximately 

0.94 curies of iodine-129 (129I) was generated at the Idaho 

National Laboratory (INL) in southeastern Idaho. Almost all 

this wastewater was discharged at or near the Idaho Nuclear 

Technology and Engineering Center (INTEC) on the INL 

site. Most of the wastewater was discharged directly into the 

eastern Snake River Plain aquifer through a deep disposal well 

until 1984; however, some wastewater also was discharged 

into unlined infiltration ponds or leaked from distribution 

systems below the INTEC.

The U.S. Geological Survey (USGS) conducted 

monitoring programs for 129I in the Snake River Plain aquifer 

at the INL in 1977, 1981, 1986, and 1990–91 prior to sample 

collection in 2003 and 2007. Some additional sampling was 

done in the 1990s to determine an estimated background 

concentration of eastern Snake River Plain aquifer water 

of 0.0000054 picocuries per liter (pCi/L). Current 129I 

concentrations in the Snake River Plain aquifer at the INL 

are compared to the background concentration and to the 

U.S. Environmental Protection Agency’s (USEPA) maximum 

contaminant level (MCL) of 1 pCi/L. 

In 2003, the USGS, in cooperation with the U.S. 

Department of Energy, collected samples for 129I from 

36 wells that are used to monitor the Snake River Plain aquifer 

and from 1 well that is used to monitor a perched zone at the 

INTEC. Four replicate samples were collected as a measure 

of quality assurance. Concentrations of 129I in the aquifer 

ranged from 0.0000066 ± 0.0000002 to 0.72 ± 0.051 pCi/L, 

and concentrations generally decreased in wells near the 

INTEC from samples collected during 1990–91. The average 

concentration of 19 wells sampled in both sample periods 

decreased from 0.975 pCi/L in 1990–91 to 0.249 pCi/L 

in 2003. Many wells within a 3-mile radius of the INTEC 

showed decreases of as much as one order of magnitude in 

concentration, and all samples had concentrations less than 

the USEPA’s MCL. These decreases are attributed to the 

discontinuation of disposal of 129I in wastewater after 1988 

and to dilution and dispersion in the aquifer.

Although wells near the INTEC sampled in 2003 showed 

decreases in concentrations compared with data collected in 

1990–91, some wells south and east of the Central Facilities 

Area, near the site boundary, and south of the INL showed 

slight increases. These slight increases may be related to 

variable discharge rates of wastewater that eventually have 

moved to these well locations as a mass of water from a 

particular disposal period.

In 2007, the USGS collected water samples for 129I 

from 36 wells that are used to monitor the aquifer south 

of the INTEC and from 2 wells that are used to monitor 

perched zones at the INTEC. Concentrations of 129I in 

the ESRP aquifer ranged from 0.000026 ± 0.000002 to 

1.16 ± 0.04 pCi/L, and a concentration that exceeded the MCL 

for public drinking water supplies was measured in one well. 

The average 129I concentration in water from 19 wells sampled 

during 2003 and 2007 stayed about the same, but slight 

increases and decreases in several areas around the INTEC 

were evident in the aquifer. The decreases are attributed to the 

discontinued disposal of 129I and to dilution and dispersion in 

the aquifer. The increases may be due to the movement into 

the aquifer of remnant perched water below the INTEC. 

In 2007, the USGS also collected samples from 31 zones 

in 6 wells equipped with multi-level WestbayTM packer 

sampling systems to help define the vertical distribution of 

129I in the aquifer. Concentrations ranged from 0.000011 ± 

0.0000005 to 0.0167 ± 0.0007 pCi/L. Three wells showed 

variability of one to two orders of magnitude of concentrations 

of 129I among various zones. Two wells included one zone 

with a concentration that differed by more than one order 

of magnitude from the other zones, and one well showed 

a similar concentration of 129I in all five zones sampled. 

Concentrations were well below the MCL in all zones. 

Acknowledgments 

The author gratefully acknowledges efforts by Gordon 

Rattray of the USGS in compiling initial information for 

inclusion in this report, and by the USGS INL hydrologic 

technicians, led by Betty Tucker, for sample collection. 

The author is also grateful to Susan Ma of PRIME Lab 

for assistance provided with the analytical methods and 

calculations used for the results.

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Bartholomay—Iodine-129 in the Snake River Plain Aquifer at and Near the Idaho National Laboratory, Idaho, 2003 and 2007—Scientific Investigations Report 2009–5088