(

(

Ottawa

Chicago

Madison

Lincoln

Ocala

Waco

Salt Lake City

Bismark

Boston

Washington, D.C.

Cape Hatteras

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigation Report 2018–5086

National Water-Quality Program

Tritium Deposition in Precipitation in the United States, 

1953–2012
Cover image: Map showing estimated tritium deposition for the continental United States from 1953 to 1983.
Tritium Deposition in Precipitation in the 

United States, 1953–2012

By Robert L. Michel, Bryant C. Jurgens, and Megan B. Young

National Water-Quality Program

Scientific Investigations Report 2018–5086

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

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Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. 

Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner

Suggested citation:

Michel, R.L., Jurgens, B.C., and Young, M.B., 2018, Tritium deposition in precipitation in the United States,

1953–2012: U.S. Geological Survey Scientific Investigations Report 2018–5086, 11 p., 

https://doi.org/10.3133/sir20185086.

ISSN 2328-0328 (online)
iii

Contents

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

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

Background............................................................................................................................................2

Prior Tritium Source Functions ...........................................................................................................2

Methods...........................................................................................................................................................3

Tritium Correlations .......................................................................................................................................5

Tritium Deposition ..........................................................................................................................................6

References Cited............................................................................................................................................9

Figures

1. Scatterplot showing tritium in precipitation at Ottawa, Canada, 1953–2016,

and Vienna, Austria, 1961–2012...............................................................................................2

2. Graph showing measured and reconstructed tritium concentrations in

precipitation at Lincoln, Nebraska, 1962–86 ........................................................................5

3. Graph showing tritium concentrations in precipitation at Big Meadows, 

Virginia, and National Airport at Washington, D.C. compared to computed

tritium in precipitation for quadrangle lat 39°–37° N., long <80° W. ................................5

4. Graph showing tritium concentrations in precipitation at Cape Hatteras, 

North Carolina compared to computed tritium in precipitation for 

quadrangle lat 37°–35° N., long <80° W. ...............................................................................6

5. Map showing estimated tritium deposition for the continental United States 

from 1953 to 1983........................................................................................................................7

6. Graph showing tritium concentrations in precipitation at at Ocala, Florida, 

and Lincoln, Nebraska, 1962–84..............................................................................................7

7. Graph showing differences in estimated tritium concentration in precipitation

using the correlation model coefficients for 37°–35° N. latitude and three 

longitudes (120°–115° W., 110°–105° W., <80° W.) calculated monthly, 

assuming a concentration of 20 tritium units in Vienna, Austria.......................................8
iv

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg)

Radioactivity

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

Tritium in tritium units (TU) 3.19 Tritium in picocuries per liter

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Supplemental Information

Activities for radioactive constituents in water are given in picocuries per liter (pCi/L).

Abbreviations

Big Meadows station Big Meadows, Virginia, station

IAEA International Atomic Energy Agency

Lincoln station Lincoln, Nebraska, station

Ocala station Ocala, Florida, station

Ottawa Ottawa, Canada, station

Vienna station Vienna, Austria, station

USGS U.S. Geological Survey

Washington station Washington, D.C., station

Acknowledgments

This effort is the result of decades of work by many laboratories throughout the world. 

Particular mention has to be made of the foresight of H.E. Suess (University of California), F. 

Pearson (U.S. Geological Survey), H.G. Ostlund (University of Miami), R.M. Brown (Chalk 

River Nuclear Laboratory), and the International Atomic Energy Agency in recognizing 

the long-term importance of tritium concentrations in precipitation and setting up the early 

monitoring networks to provide this information. The authors also thank the large number 

of individuals who collected and carried out the technical analyses of the samples during 

the past 50 years. Among the agencies contributing to the support of this work during this 

period has been the U.S. Geological Survey, National Science Foundation, National Oceanic 

and Atmospheric Administration, Atomic Energy Commission, International Atomic Energy 

Agency, and World Meteorological Organization. 
Tritium Deposition in Precipitation in the United States, 

1953–2012

By Robert L. Michel, Bryant C. Jurgens, and Megan B. Young

Abstract

Tritium is a radioactive isotope of hydrogen (half-life is 

equal to 12.32 years). Since it is part of the water molecule, 

tritium can be used to track and date groundwater and surface 

water when the history of tritium in precipitation and recharge 

is known. To facilitate that effort, tritium concentrations in 

precipitation were reconstructed from measurements and 

correlations for 10 precipitation stations in the continental 

United States. Using these data, and other data sets and 

correlations, estimates of tritium concentrations in precipitation from 1953 through 2012 and total tritium deposition 

from 1953 through 1983 were derived for 2 degrees latitude 

by 5 degrees longitude quadrangles of the continental United 

States. For August 1953 through December 1987 correlations 

were derived from the data of the Ottawa, Canada, station; for 

estimates after 1987, the Vienna, Austria, station was used. For 

quadrangles where no precipitation station records are available from measurements or correlations, concentrations were 

interpolated. The International Atomic Energy Agency has 

proposed that a correlation with the Vienna, Austria, station, 

which has been in operation since 1960, be used instead of 

the Ottawa, Canada, station for the period after 1987. Linear 

correlations calculated for all stations with the Vienna data 

are reported. Correlations also have been calculated for each 

latitude-longitude quadrangle using the estimated concentrations and the measured Vienna data for 1960–87. Because the 

Vienna, Austria, station is on a different continent, and not 

subject to the same seasonal weather patterns that affect North 

American stations, a correlation was obtained for each month 

individually. This reflects the fact that the North American 

stations were strongly affected by the input of low-tritium 

moisture from the south during the summer. The correlated 

station values were then used with measured Vienna data to 

obtain estimates of tritium concentrations in precipitation 

for the period 1988–2012 at each precipitation station and 

latitude-longitude quadrangles. This approach has two major 

advantages: (1) it blends the Ottawa correlation with the 

Vienna correlation to cover the period 1953 to present, and (2) 

it yields correlation coefficients for each quadrangle that can 

be used in future years when more data become available from 

the Vienna, Austria, station. The data, the estimated tritium 

concentrations derived from the correlations, and the correlation coefficients are provided as spreadsheets and associated 

comma delimited files in a data release that accompanies this 

report.

Introduction

Tritium has been one of the most useful tracers in oceanography and hydrology for more than 50 years (Suess, 1969). 

Being a radioactive isotope of hydrogen (half-life is equal 

to 12.32 years), it is ideal for tracing the movement of water 

through the environment at decadal timescales. It participates 

in the same reactions as water with only slight variations 

caused by fractionation (Bigeleisen, 1962). These variations 

are small and can be ignored in almost all hydrologic systems. 

The radioactive decay constant is known, and concentrations 

are unaffected by mineral exchange, gas loss, oxidationreduction reactions, or processes that frequently affect other 

tracers. In situ tritium production is rare and can generally be 

ignored (Macklin and Banta, 1954; Andrews and Kay, 1982). 

Thus, the concentration of tritium in any system is simply 

a function of the input, mixing with other waters, and the 

fraction lost through decay since it entered the system. Tritium 

has been used to study timescales for surface-water processes 

and interactions between surface water and groundwater. 

Tritium balances can be used as a proxy for water balances 

in many of these studies. One difficulty in using tritium as 

a tracer in hydrology is the variation in its concentration 

in precipitation, temporally and geographically. Accurate 

interpretation of tritium concentrations requires accounting 

for these variations in local input functions. The purpose 

of this report is to provide a methodological framework for 

estimating spatially and temporally varying tritium-input 

functions for the contiguous United States. Estimated tritiuminput functions for precipitation stations and 2 degrees latitude 

by 5 degrees longitude quadrangles for the United States are 

provided in tables in the accompanying data release (Jurgens, 

2018).
2 Tritium Deposition in Precipitation in the United States, 1953–2012

Background

The radioactive nature of tritium was known in the 

1930s, but measurement techniques were inadequate to 

detect its presence in the environment at that time (Alvarez 

and Cornog, 1939). Using electrolysis of water to enrich 

the tritium concentration, the discovery of natural tritium 

in the environment roughly coincided with the beginning 

of the nuclear age (Grosse and others, 1951; Giletti and 

others; 1958). Before nuclear testing, tritium found in 

the environment resulted from accretion from the sun or 

cosmic-ray spallation, the latter of which was thought to be 

the dominant process (Wilson and Fergusson, 1960; Nir and 

others, 1966; Teegarden, 1967). The cosmic-ray production 

probably kept a steady-state burden of tritium of 3–4 

kilograms (kg) on the Earth’s surface (Lal and Peters, 1967). A 

few measurements of tritium were taken before the beginning 

of major atmospheric testing, the natural concentrations of 

tritium in precipitation at that time are not well known. From 

the few measurements available, it is assumed that tritium 

concentrations in precipitation were about 1–2 tritium units 

(TU) near the coast and less than 10 TU in the mid-continental 

areas (Kaufman and Libby, 1954; von Buttlar and Libby, 

1955; Thatcher, 1962). 

Tritium produced by atmospheric testing of nuclear 

weapons resulted in a large increase in the tritium burden of 

the Earth (Carter and Moghissi, 1977). Most of this tritium 

was quickly converted to water and entered the hydrosphere. 

Production by fusion testing was much greater than what 

was produced by fission testing (Miskel, 1973), and several 

hundred kg of tritium were produced by the time of the 

Nuclear Test Ban Treaty in 1963 (Michel, 1976). The earliest 

time series of measurements of tritium concentrations in 

precipitation began in Ottawa, Canada, in mid-1953 (Brown 

and Grummitt, 1956; Brown, 1961). Data from this station 

are available with almost no interruption from August 1953 

to 2016 (fig. 1). Beginning in the late 1950s and early 1960s 

major atmospheric nuclear testing increased, and many other 

monthly stations were established, either by individual laboratories, or through a network established by the International 

Atomic Energy Agency (IAEA) and the World Meteorological 

Organization to monitor isotopes in precipitation. Measurements from these stations demonstrated a large increase in 

tritium concentrations in precipitation following major nuclear 

test series, with highest concentrations following the large 

tests of 1962 (Eriksson, 1965). Concentrations in precipitation reached 10,000 TU in inland areas of Canada during this 

period and were routinely above 1,000 TU for large areas of 

the North American continent (International Atomic Energy 

Agency, 2017). Tritium concentrations in fresh surface waters 

and ocean waters also increased dramatically following these 

tests, reaching concentrations of more than 40 TU in the North 

Pacific Ocean and more than 1,000 TU in some continental 

rivers (Eriksson, 1963; Stewart, 1966; Dockins and others, 

1967; Stewart and Farnsworth, 1968; Ostlund and Fine, 1979).

Prior Tritium Source Functions

Little or no data are available on tritium concentrations 

in precipitation for many areas and periods during the nuclear 

era for the continental United States. Data from the period 

before 1960 are particularly limited. Because the bombtritium pulse is a transient event, it is now not possible to take 

measurements to cover these deficiencies, as it might be with 

steady-state tracers or stable isotopes. Estimates of historical 

tritium concentrations in precipitation have been obtained by 

measuring wine samples or other archived waters (Roether, 

1967; Allison and Hughes, 1977; Michel and Kraemer, 1995). 

There are probably few of these types of water available to 

overcome the gaps in space and time. There are locations 

where historical tritium concentrations can be derived from 

ice cores (Miller and others, 1965; Merlivat and others, 1977; 

Naftz and others, 1996). However, for most of the areas in 

the United States, this approach is not possible. One option 

to derive estimates of tritium concentrations in precipitation 

where data are not available is through correlations obtained 

with stations that were operating for a long time and the use of 

interpolation for regions which were not well covered. 

For the Northern Hemisphere, the Ottawa, Canada, 

station (Ottawa station) can be used for correlations because it 

was in operation almost continuously starting in August 1953 

and the data were of high quality (International Atomic Energy 

Agency, 1981). Correlations among the Northern Hemisphere 

stations and the Ottawa station can be made at monthly and 

sac18-0670_fig 01

1950 1970 1990 2010

Year

Tritium units

1

10

100

1,000

10,000

EXPLANATION

Ottawa, Canada

Vienna, Austria

Figure 1. Tritium in precipitation at Ottawa, Canada, 1953–2016, 

and Vienna, Austria, 1961–2012 (International Atomic Energy 

Agency, 2017).
Methods 3

yearly time steps using linear regression of measured tritium 

concentrations and log, base-10, concentrations: 

 (1a)

 (1b)

where

 is the concentration at the station of interest, i, 

of month j;

is the concentration at the Ottawa station of 

month j;

 is the slope of the correlation regression at the 

station of interest, i, of month j; and

is the intercept of the correlation regression at 

the station of interest, i, of month j.

Correlations are normally done using a log transformation because tritium in the atmosphere changed by four orders 

of magnitude between 1953 and 1983. For the Southern Hemisphere, similar correlations can be done with the Pretoria, 

South Africa, station, which started collection of tritium 

data in precipitation in the mid-1950s. A later publication 

changed the stations used to Vienna, Austria, for the Northern 

Hemisphere and Kaitoke, New Zealand, for the Southern 

Hemisphere (International Atomic Energy Agency, 1992). 

Kaitoke was more suitable because Pretoria has seasonal 

lapses in rainfall and was discontinued for long periods of 

time. Kaitoke has operated continuously since 1960. The 

Ottawa station did not have large gaps in tritium measurements, but those measurements could have been influenced by 

a local nuclear power station that made the station unsuitable 

for correlations starting in the late 1980s. The Vienna, Austria, 

station (Vienna station), which has tritium in precipitation 

from 1961 to 2012, was substituted (fig. 1). 

Other efforts have been made to fill in the tritium data 

gaps in precipitation. Some authors have developed localsource functions for limited areas (Rabinowitz and others, 

1977; Simpkins, 1995). For the United States, Michel (1989) 

used the Ottawa correlation (International Atomic Energy 

Agency, 1981) to estimate tritium deposition in 2- by 5-degree 

quadrangles on a yearly basis for the United States from 

1953 to 1983. The rationale for the correlations and using 

the 2- by 5-degree grid is discussed in Michel (1989). There 

are long-term monitoring programs in some countries that 

include precipitation and surface-water concentrations over 

time (Rank and others, 1998; Schurch and others, 2003). 

Other efforts have provided global input functions for tritium, 

generally focusing on the world oceans (Weiss and Roether, 

1980; Doney and others, 1992). In general, input functions 

for the oceans are smoother than functions over land surfaces. 

The tritium/helium-3 pair also has been used to overcome 

the source-function problem in many cases. It is particularly 

useful in groundwater studies (Solomon and others, 1992; 

Carmi and Gat, 1994; Larsen and others, 2003), but also can 

be of value in studies of subsurface waters in lakes (Peeters 

and others, 2000; Hofer and others, 2002). However, issues 

related to gas retention can raise concerns with the use of this 

type of data, so a source function for tritium is still valuable 

(Schlosser and others, 1988).

Estimates of tritium concentrations in precipitation that 

take into account the temporal, geographical, and seasonal 

variations that influence the concentrations in the continental 

United States are needed in a wide range of hydrologic studies 

(Michel, 2005). Most modeling approaches for groundwater 

and surface water issues depend on an understanding of the 

tritium-input function (Przewlocki and Yurtsever, 1974; Zuber, 

1986; Michel, 2004). Because recharge generally occurs on 

a seasonal basis, it is sometimes necessary to know what the 

variations in tritium concentrations are on a monthly basis 

(Foster and Smith-Carrington, 1980). Correlations with 

existing databases can be used to overcome these deficiencies and establish a basis to estimate tritium concentrations in 

future years when more data become available from the IAEA.

Methods

Tritium data used in this report are primarily from five 

sources: (1) U.S. Geological Survey (USGS) Tritium Laboratory, that was initially in Washington, D.C.; relocated to 

Reston, Va., in 1972; and then relocated to Menlo Park, Calif., 

in 1994; (2) La Jolla Tritium Laboratory, Scripps Institution 

of Oceanography, University of California, San Diego, Calif.; 

(3) Miami Tritium Laboratory, Rosenstiel School of Marine 

and Atmospheric Sciences, University of Miami, Miami, Fla.; 

(4) Chalk River Nuclear Laboratory, Ontario, Canada; and 

(5) International Atomic Energy Agency Tritium Laboratory, 

Vienna, Austria. All of these laboratories perform high-quality 

tritium analyses and have participated in the inter-comparison 

studies conducted by the IAEA over the past several decades. 

The results of these analyses of tritium in precipitation done 

on a monthly basis, along with analytical uncertainties, are 

available from the IAEA Global Network of Isotopes in 

Precipitation (GNIP) website (International Atomic Energy 

Agency, 2017). 

Two approaches were used to construct source functions for the continental United States. One approach used the 

long-term collection stations in the continental United States, 

and used correlations to fill in months and years when no data 

were collected. The stations chosen for this approach are listed 

in Jurgens (2018, table 1) and are USGS stations where tritium 

in precipitation was analyzed and reported to the IAEA (International Atomic Energy Agency, 2017). Some USGS stations 

that reported tritium in precipitation values to the IAEA were 

not used; data from Albuquerque, N. Mex., and St. Louis, Mo., 

were not used because samples were composites of 3 months 

of precipitation; data from Portland, Oreg., were not used 

because samples were composites of non-monthly periods; 

and data were omitted from Menlo Park, Calif., which suffered 

from occasional tritium contamination from a local source.

Ci j ,

Cott j ,

ai j ,

bi j ,

Log C a Log C b i j i j ott j i j 10( ) 10( ) , ,   , ,

C a C b i j , ,   i j ott j , ,i j
4 Tritium Deposition in Precipitation in the United States, 1953–2012

Correlations of the form y = ax + b were made using 

the Ottawa station for the period before 1987, and the Vienna 

station for the period 1987–2012 as recommended by the 

International Atomic Energy Agency (1992). For the period 

before 1987, the Ottawa station was used for correlations 

with United States stations because of the close proximity to 

the United States and the availability of monthly concentrations throughout the 1950s (International Atomic Energy 

Agency, 1992). Tritium concentrations were estimated for 

every month except February 1954 because no measurements 

were available for the Ottawa station. Concentrations were 

changing rapidly during this period and it was not possible 

to make an estimate. After 1987, the Vienna station was used 

and there were data gaps for some months between 2003 and 

2012. Correlations with the Vienna station were done for each 

month spanning the entire tritium record at each Northern 

Hemisphere station to account for the differences in weather 

patterns in the United States and Europe. Thus, separate correlation coefficients of a and b were calculated for each month 

for each station when using the Vienna correlation.

A second effort was done to develop tritium source functions for locations where long-term stations are not available. 

In this effort, the reconstructed station data done in the first 

approach was spatially interpolated to a grid covering the 

continental United States. Estimates of tritium concentrations in the continental United States from August 1953 to 

December 2012 were computed for quadrangles using the 

same latitude-longitude pattern as in Michel (1989). The 

48 contiguous states are divided into 96, 2- by 5-degree grid 

cells. Long-term tritium precipitation stations operated in 

about 25 percent of these boxes during the 1960s, which is the 

period when concentrations changed most rapidly. There also 

are a series of stations operated by the Canadian government 

on the United States–Canada border that were used to help 

estimate concentrations in the northern United States. Monthly 

correlations with Ottawa data were used to estimate tritium 

concentrations for periods when data were not available at the 

long-term precipitation stations. This was primarily the period 

prior to 1961, before most stations were established, and after 

about 1980 when many stations ceased. 

For estimates after 1987, the Vienna station was used for 

correlations. For a given 2- by 5-degree quadrangle, the correlation was carried out by using a regression of the estimated 

concentrations from the quadrangle with data from the Vienna 

station for 1961–87. Similar to previous correlations (International Atomic Energy Agency, 1981, 1992; Michel, 1989), a 

linear regression was used with the natural log:

 

(2)

where

is the concentration at the station of interest, i, 

of month j;

 is the concentration at the Vienna station of 

month j;

 is the slope of the correlation regression at the 

station of interest i, of month j; and

 is the intercept of the correlation regression at 

the station of interest, i, of month j.

The regression coefficients, a and b, were determined 

with Vienna data measured between 1987 and 2002 to estimate 

tritium concentrations for each quadrangle. These correlations 

were extended to estimate concentrations in precipitation for 

the 2002–12 period. The last data available from the IAEA 

website for Vienna are from December 2012, but these correlations can be extended further if more data become available.

The estimated concentrations for tritium in precipitation in tritium units (TU) have been compiled in three tables 

for ease of use. These tables are available as spreadsheet files 

(.xlsx format) and text files (.csv format) in the data release 

that accompanies this report (Jurgens, 2018). Reconstructed 

tritium concentrations in precipitation for ten monitoring 

stations in the United States for the period 1953–2012 are 

reported in Jurgens (2018, table 1). Estimated tritium values 

were determined from correlations with Ottawa, Canada 

(before 1987), or Vienna, Austria (after 1987), and are identified by italic text in the table. Estimated tritium concentrations 

in precipitation for ninety-six, 2- by 5-degree quadrangles 

covering the continental United States in monthly increments for the period 1953–2012 are reported in Jurgens 

(2018, table 2). The slope, intercept, and R2 values for 

monthly correlations between the tritium concentration in 

precipitation at the Vienna station and the 10 precipitation 

stations and 96 quadrangles in the continental United States 

are reported in Jurgens (2018, table 3). These correlation 

coefficients can be used with equation 2 to extend the estimated monthly tritium concentrations for the stations and 

quadrangles from 2013 to future years as new tritium concentrations from the Vienna station become available. There were 

a few months when data were not collected at Vienna, and an 

estimate was derived from concentrations the month prior to 

and following the missing month. All tritium concentrations in 

the three tables are given in TU. 

Tritium deposition for the continental United States was 

calculated by multiplying the estimated tritium concentration 

with the average precipitation value (in meters [m]; PRISM 

Climate Group, 2018) for every month from 1953 to 1983 

in each 2- by 5-degree quadrangle. The sum of these values 

in a quadrangle is the total tritium deposition that primarily 

resulted from atmospheric nuclear weapons testing that 

occurred from 1953 to 1980.

Cvie j ,

Ci j ,

Ln C a Ln C b t j i j vie j i j ( ) ( ) , ,   , ,

ai j ,

bi j ,
Tritium Correlations 5

Tritium Correlations

The estimated tritium concentrations are only meant to 

be used for periods and locations when no measured data are 

available (Jurgens, 2018, table 2). They should not be considered a substitute for periods when historical data are available 

or as a replacement for measurements of tritium in precipitation. Figure 2 is a plot of actual measured data (International 

Atomic Energy Agency, 2017) versus concentrations reconstructed from the monthly correlations with the Vienna data 

for the Lincoln, Nebr., station (Lincoln station; Jurgens, 2018, 

table 1). The reconstruction of the Lincoln station data from 

Vienna station data showed that estimated tritium agreed 

well (R2=0.87) with long-term and short-term tritium trends 

overall, but correlations also can tend to overestimate or 

underestimate measured values at a particular month or year.

During the period of high-tritium concentrations, and 

particularly during periods of nuclear testing in the 1960s, 

tritium concentrations in precipitation were primarily 

controlled by the changing global burden of tritium and the 

location and timing of nuclear tests. Thus, tritium concentrations in precipitation were impacted in all parts of the country 

at the same time. This global control of concentrations has 

resulted in high values (more than 0.8) for the R2 of the 

correlation for all stations and latitude-longitude quadrangles 

(Jurgens, 2018, table 3). There has been no major atmospheric testing since the 1980s, and the atmospheric burden 

has decreased through decay and deposition with most of the 

tritium now in the Northern Hemisphere oceans. 

The 2- by 5-degree grid is coarse, and there can be large 

variations in tritium concentrations in precipitation within a 

quadrangle. This is particularly true for coastal areas and areas 

where mountain ranges are crossed. Stable isotopes of water 

can vary over small distances in such areas, and many of the 

same variations are in tritium concentrations (Yurtsever and 

Gat, 1981).

Few stations with tritium data are available that can be 

compared to concentration estimates from the correlation 

model. In the quadrangle formed at lat 39°–37° N. and long 

<80° W. to the eastern border of United States, two stations 

are available–Big Meadows, Va. (Big Meadows station; lat 

38°52’ N., long 78°21’ W.), and Washington, D.C. (Washington station; Jurgens, 2018, table 1; lat 38°51’ N., long 77°2’ 

W.). The Washington station (International Atomic Energy 

Agency, 2017) had data from 1963 to 2001, whereas the Big 

Meadows station had data beginning in 1999, so only a 2-year 

period was available for comparison (fig. 3). The measurements at the two stations compare well to each other, with 

maxima in the spring of each year. The estimated tritium from 

the correlation model also compares well to the data from 

Figure 2. Measured and reconstructed tritium concentrations in 

precipitation at Lincoln, Nebraska, 1962–86 (International Atomic 

Energy Agency, 2017).

Figure 3. Tritium concentrations in precipitation at Big Meadows, Virginia, and National Airport at Washington, D.C. (International 

Atomic Energy Agency, 2017), compared to computed tritium in precipitation for quadrangle lat 39°–37° N., long <80° W. (data are from 

Jurgens, 2018, table 2).

Tritium units

16

14

12

10

8

6

4

2

1999 2000

Year

2001 2002

EXPLANATION

Big Meadows, Virginia

Washington, D.C.

Correlation at 39°–37° N.,

 less than 80° W.

Tritium units

Year

1960 1965 1970 1975 1980 1985

1

10

100

1,000

Measured tritium in Lincoln, Nebraska

Correlation with Vienna, Austria

EXPLANATION
6 Tritium Deposition in Precipitation in the United States, 1953–2012

the two stations, although it underestimates the differences 

between the highest and lowest measured concentrations. 

However, the average concentration appears to lie well within 

the variations of the measured data, so the tritium estimates 

give a good representation of the concentration over time for 

these stations in recent years. 

Another station available for comparison is near Cape 

Hatteras, N.C., where the most recent years of data are 

plotted versus the correlation model estimate for quadrangle 

lat- 37°–35° N., long <80° W. (fig. 4). The correlation model 

estimate generally lies near, but above the measured data. 

This discrepancy is not surprising because Cape Hatteras is 

a coastal location and can receive precipitation that has been 

affected by water vapor from the ocean, which has a lowtritium concentration—see the “Tritium Deposition” section 

in this report about the “continental effect”. A more inland 

location would likely lie closer to the estimated data. Consequently, the correlation model can often reproduce seasonal 

and yearly tritium patterns but may not capture the full extent 

of variability of tritium concentrations within a quadrangle, 

particularly near shorelines where water vapor from the 

oceans can influence local tritium concentrations. In these 

cases, tritium values could need adjustment for local climatic 

and geographic factors. Comparisons of the estimated tritium 

source functions to measured tritium and other environmental 

tracers collected from local groundwater and surface water can 

often reveal these deviations and be used to adjust the correlation model values to the local conditions.

Tritium Deposition

Estimated tritium deposition for the continental United 

States is shown in figure 5. In general, tritium concentrations reported in precipitation for mid-continental areas tend 

to be higher than in coastal areas for the same latitude. This 

“continental effect” was seen in figure 4 and can be seen 

in figure 6, which is a plot of the tritium concentrations in 

precipitation at Ocala, Fla. (Ocala station), and Lincoln, Nebr., 

during the height of tritium releases to the atmosphere. The 

Ocala station is a low-latitude station with a strong oceanic 

influence. The Lincoln station is a mid-continental station, 

which had some of the highest tritium concentrations in North 

America. The low-tritium concentrations reported in coastal 

precipitation are the result of exchange of atmospheric water 

vapor with ocean surface water, which generally has a much 

lower tritium concentration (Schell and others, 1970). As a 

weather system leaves the coast and continues inland, tritium 

concentrations in precipitation tend to rise. This is caused by 

the injection of stratospheric air with high-tritium concentrations in water vapor into the troposphere and the exchange 

of atmospheric water vapor with continental surface-water 

bodies that can have higher tritium concentrations than those 

in oceanic waters. As weather systems traverse the continent, tritium concentrations in precipitation tend to increase. 

Highest concentrations are reported in the mid-continental 

areas at high northern latitudes (fig. 5).

There also is a strong latitudinal gradient in concentrations caused by the location of nuclear weapons tests and 

atmospheric circulation patterns (fig. 5). Most of the tests were 

in the Northern Hemisphere and the largest tests by the Union 

of Soviet Socialist Republics (USSR) were at high northern 

latitudes. These tests deposited large amounts of tritium 

into the troposphere and the stratosphere (Libby, 1961). The 

tritium in the water in the troposphere tended to be removed 

rapidly, as water vapor in the atmosphere has a residence time 

of 10–30 days. Most of this tritium is removed by molecular 

exchange, primarily to the ocean (Ostlund and Berry, 1970; 

Simpson, 1970; Suess, 1970). The tritium in the stratosphere 

can be mixed into the troposphere by two processes in the 

Northern Hemisphere. During thunderstorms, thunderheads 

can penetrate the tropopause, resulting in rain bringing down 

Figure 4. Tritium concentrations in precipitation at Cape Hatteras, North Carolina (International Atomic Energy Agency, 2017),

compared to computed tritium in precipitation for quadrangle lat 37°–35° N., long <80° W. (data are from Jurgens, 2018, table 2).

Tritium units

10

8

6

4

2

0

1999 2000

Year

2001 2002

EXPLANATION

Cape Hatteras, North Carolina

Correlation at 37°–35° N.,

 less than 80° W.
Tritium Deposition 7

Figure 5. Estimated tritium deposition for the continental United States from 1953 to 1983.

Figure 6. Tritium concentrations in precipitation at at Ocala, Florida, and Lincoln, Nebraska, 1962–84 (International Atomic Energy 

Agency, 2017).

tritium or other nuclear debris from the stratosphere. The other 

mechanism is the “spring leak,” which occurs in the Northern 

Hemisphere in the spring of each year. During the spring, 

the tropopause breaks down between lat 30° N. and 60° N., 

resulting in the mixing of stratospheric air with higher tritium 

concentrations and air in the troposphere. As a result, tritium 

concentrations in precipitation tend to increase in the spring of 

each year. This break up only occurs in the Northern Hemisphere because it is brought about by the heating up of the 

land masses at those latitudes in the spring. This results in the 

seasonal effect seen in tritium and cosmogenic radioisotope 

data in the Northern Hemisphere. In the Southern Hemisphere, 

Tritium units

3,500

3,000

2,500

1,500

500

0

1,000

2,000

1960 1965 1970

Year

1975 1980 1985

EXPLANATION

Ocala, Florida

Lincoln, Nebraska

(

(

0 400 800 MILES

0 400 800 KILOMETERS

120°

110° 100°

90°

80°

70° 47°

43°

39°

35°

31°

27°

Ottawa

Chicago

Madison

Lincoln

Ocala

Waco

Salt Lake City

Bismark

Boston

Washington, D.C.

Cape Hatteras

EXPLANATION

<3,000

3,000 to 6,000

>6,000

Precipitation stations

0 to 2,000

>2,000 to 4,000

>4,000 to 6,000

>6,000 to 8,000

>8,000

Quadrangles

Tritium deposition 1853–83, tritium units, in meters; <, less than; ≥, greater than or equal to

Base modified from U.S. Geological Survey and other Federal and State digital data, 

various scales; Albers Equal-Area Conic projection, standard parallels are 29°30’ N. 

and 45°30’ N.; North American Datum of 1983
8 Tritium Deposition in Precipitation in the United States, 1953–2012

no equivalent land masses are at similar southern latitudes, so 

the breakup of the tropopause does not occur. Penetration of 

thunderheads into the stratosphere does occur in the Southern 

Hemisphere, and the stratosphere can come close to the land 

surface in the high plateaus of Antarctica, but these processes 

introduce much lower amounts of nuclear debris into the 

troposphere. Therefore, most nuclear debris introduced into 

the stratosphere will ultimately be deposited in the 30°–60° N. 

latitude band. As a result, the largest fraction of nuclear fallout 

occurred in the Northern Hemisphere, producing a strong 

north-south gradient across the continental United States 

(fig. 5).

Local weather patterns also can have a major impact on 

tritium concentrations (Kigoshi and Yoneda, 1970). Superimposed on the general circulation patterns are seasonal effects 

caused by changes in weather patterns. In the continental 

United States, winter storms tend to track from west to east, 

bringing moisture with high-tritium concentrations across 

much of the United States. In the summer, a large amount 

of water vapor is transported north from the Gulf of Mexico 

and the equatorial Atlantic Ocean. Accordingly, the tritium 

concentration in water vapor during the summer is much lower 

(fig. 7). 

Figure 7. Differences in estimated tritium concentration in precipitation using the correlation model coefficients for 37°–35° N. latitude

and three longitudes (120°–115° W., 110°–105° W., <80° W.) calculated monthly, assuming a concentration of 20 tritium units in Vienna, 

Austria. All three longitudes show a relative decrease in tritium concentrations in summer, because of the changes in weather patterns 

during the year. Highest concentrations are reported at the most inland longitude (110°–105° W.).

One of the major factors now controlling tritium concentrations in precipitation at any station is the local weather 

pattern (Kigoshi and Yoneda, 1970). Although local weather 

patterns have always had some influence on concentrations, 

their effect was over-ridden during the height of tritium 

releases into the atmosphere by global factors such as the 

location and timing of atmospheric tests and large scale atmospheric patterns like the spring release of stratospheric air into 

the troposphere. The differences caused by local phenomena 

have become relatively more important compared to the years 

during the peak of the bomb transient. This problem is partly 

offset by using independent monthly correlations, which adjust 

for general weather patterns in the United States on a longterm basis. The yearly average tritium concentrations at most 

stations are likely close to the estimates in this paper, and over 

the long term, the estimated tritium concentrations from the 

correlation model will probably be close to those estimates. 

However, the variations caused by local storm tracks can be 

important on a monthly basis, and errors can be greater for 

estimated tritium concentrations using Vienna measurements. 

Thus, it is still important to measure concentrations when 

possible, and to utilize local historical data when available.

Tritium units10

12

14

16

8

6

4

2

0

Jan. Feb. Mar. Apr. May June July

Month

Aug. Sept. Oct. Nov. Dec.

EXPLANATION

 35°–37° N., 120°–115° W.

Location (latitude and longitude)

 37°–35° N., 110°–105° W.

 37°–35° N., less than 80° W.
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5.50006-1. 
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Michel and others—Tritium Deposition in Precipitation in the United States, 1953–2012 —Scientific Investigations Report 2018–5086

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20185086