Survey of tritium radioactivity in Lake Huron surface water

Environment International, Vol. 9, pp. 221-224, 1983

0160-4120/83 $3.00 + .00 Copyright©1983Pergamon Press Ltd.

Printed in the USA. All rights reserved.

SURVEY OF TRITIUM RADIOACTIVITY IN LAKE HURON SURFACE WATER T. D. Lickly, F. A. Blanchard, and I. T. Takahashi Dow Chemical USA, Environmental Sciences Research, 1701 Building, Midland, Michigan 48640, USA (Received 17 December 1982; Accepted 7 March 1983)

The tritium concentration in Lake Huron water has declined from a high measured in the late 1960sto a 1981 levelof - 200 pCi/L, similar to literature values for the Great Lakes Waterwaysand other U.S. surface waters. All current levels for U.S. surface waters including our local water are well within the limits set by the EPA as a standard for drinking water.

Introduction

Experimental

The Midland location of the Dow Chemical Company uses approximately 107 gal (38,000 m 3) of raw Lake H u r o n water per day, some of which enters chemical processes directly or via steam. For many years the waters used at the Dow Midland location have been sampled and analyzed to verify that they conformed with United States Public Health Service standards (1962) and, more recently, with E P A standards (U.S. EPA, 1976a) for drinking water, and manufacturing requirements on FDA-regulated products. Application of " G o o d Manufacturing Practices" regulations along with the approaching operating date (1985) of the adjacent Consumers Power Company (CPCo) Nuclear Cogeneration Plant make our knowledge of levels, trends, and variability o f radioactivity in this data important. Process steam will be produced for Dow by C P C o using supplementary heat exchangers. The heat source will be secondary steam which has given up much of its energy for electricity production by passing through a high pressure turbine. Sampling and analyzing for radioactivity in process waters used at the Midland location of Dow was started in 1967-1968 to develop a baseline. Shortly after that time, samples retained for other purposes, dating back to the early 1960s, were also analyzed. The measurements have continued to the present date. These data are here reported and compared to literature data.

Sampling

Lake H u r o n water has been analyzed over the last 15 yr and results for gross beta have been previously reported (Blanchard and Lickly, 1983). The following information and procedures pertain to tritium data. Prior to 1976, samples analyzed for H T O were taken as grab samples from various sources derived from Lake H u r o n water, including untreated water, deionized water, and condensate from steam. Since 1976 samples from the various Lake H u r o n water sources have been collected daily and individually composited into weekly, monthly, or quarterly composites. In addition, a few grab samples were collected annually or semiannually. Sample preparation

From 1967 to 1977 about 175 mL of each water sample were treated with potassium permanganate and potassium (or sodium) hydroxide and distilled. Since 1978, water samples were first passed through a Dow Chemical D O W E X MR-3 Ion Exchange Resin Bed prior to the alkaline-permanganate distillation. This method is similar to the method outlined in the A P H A Standard Methods ( A P H A , 1981) and by E P A (Krieger, 1975). Prior to 1972 liquid scintillation samples were prepared by emulsifying 10 m L aliquots of the distillate with 15 mL of scintillator solution in polyethylene counting vials. Five replicate samples were prepared. The scin221

222

T. D. Lickly, F. A. Blanchard, and I. T. Takahashi Table 1. Minimum detection levels for tritium in water for this study. Instrument

Sample Size (mL)

Background (counts/min)

Efficiency

LLD95 (counts/min)

LLD95 (pCi/L)

Packard 3380 Packard 3330

10 5

15 7.5

0.21 0.245

1.47 1.04

316 383

tillator solution contained 62.5 o70toluene, 37.5 o/0 Triton X-100, with 7 g PPO and 1.5 g p-bis-(o-methylstyryl)benzene per liter. Since 1972, liquid scintillation samples were prepared by emulsifying 5 mL aliquots of the distillate with 15 mL of New England Nuclear Aquasol or Aquasol II Scintillation Cocktail. Essentially tritium-free background and standard samples were prepared from water from a groundwater well - 100 ft ( - 30 m) deep. This water was compared to water processed from an extremely deep brine well (Monroe formation) prior to use as background; the backgrounds were equivalent within the sensitivity of the measurement technique. Standards were prepared by spiking a known amount of tritium either as HTO or as tritiated toluene into background samples.

Counting methods Liquid scintillation counting instruments used were: for samples prior to 1972, a Packard Tri-Carb ® Model 3380 was used; for samples from 1976 to 1981, a Packard Tri-Carb ® Model 3330 was used. Five replicate aliquots were analyzed for each sample and blank. Each aliquot was counted a minimum of 30 min, for a total counting time of at least 150 min per sample. Minimum detection levels, LLD95, as defined in the APHA Standard Methods (APHA, 1981) are shown in Table 1. This table shows that for any given sample, a detection limit of about 300-400 pCi/L was achieved, based on radioactive counting statistics.

Results The data generated from the same Lake Huron water source with different treatments (untreated, deminer-

alized, steam condensate) did not show a significant variation by water treatment (analysis of variance, ct = 0.05), which is to be expected with tritium as HTO. Therefore, all Lake Huron water sources, regardless of treatment, have been averaged by year (Table 2). This data shows that the peak activity of the 1965-1981 time period for water from Lake Huron was in the 1960s, with a slow but steady decline since that time.

Discussion With the advent of the nuclear age and the large amount of aboveground nuclear weapons testing in the early 1960s, the amount of tritium in the world inventory went from the natural, equilibrium level of about 70 MCi to a level of about 3100 MCi by 1963 (NCRP, 1979). After the nuclear Test Ban Treaty was signed in 1963, the major nuclear powers have conducted tests underground (with the exception of France and China). The impact of the tests in the early 1960s on tritium concentrations in surface waters around the United States is presented in Fig. l (data from U.S. Bur. Radiol. Health, 1970, 1971; U.S. EPA, 1972, 1973, 1974, 1975a, 1975b, 1976b, 1976c, 1976d, 1976e, 1977a, 1977b, 1977c, 1977d, 1977e, 1978a, 1978b, 1978c, 1979a, 1979b, 1979c, 1979d, 1980a, 1980b). The level of tritium in the major rivers increased from three- to sevenfold almost immediately. In contrast, the level in the Great Lakes Waterways (Lake Michigan, Lake Erie, the St. Lawrence River) showed an increase of twofold or less, indicating the volume-buffering capacity of the Great Lakes. Our data, obtained from a water intake at Whitestone Point on Saginaw Bay of Lake Huron, shows levels similar to the other Great Lakes data, in contrast to results of samples taken at the mouth of the

Table 2. Concentration of tritium in Lake Huron Water, 1965-1981. Tritium Concentration (pCi/L)

4000

Time Period

Number of Analysesa

Mean

Sm

Range b

3500

1965 1966 1967-1968 1970 1971 1976 1977 1978 1979 1980 1981

6 5 3 12 36 53 43 17 8 12 21

480 570 570 400 490 350 430 350 160 230 200

55 47 89 60 20 20 27 34

320-680 420-710 440-740 ND-850 ND-770 ND-907 ND-911 ND-711 ND ND-417 ND-335

24

24 19

aEach analysis was the mean value of five replicate samples. bND = not detected, detection limited -300-400 pCi/L.

3000

\

A --I u

v

"z-

..... Great Lakes Waterways ~---Mississippi River ".-'.-~'---='--~"C°l°~bd'° Riiverr

-. ~ ~ ~

,

Ohio River

2500 2000 1500

I1000

J

/

.'~.

~

"".

• "~ ~N~

"....~ "..4...... ~. ~_~

500

o 1960

,i,, 1963

, 1966

~

,

1969 1972 41975 Year

1978

Fig. 1. Tritium in United States surface waters.

1981

Tritium radioactivity in surface water

223

4000

ing in the late 1960s and continuing through to the early 1970s. The current tritium level in Lake Huron ( - 2 0 0 pCi/L) is similar to values reported in the literature for the Great Lakes Waterways and other surface waters around the United States (200-500 pCi/L). The tritium levels observed in the Lake Huron water and U.S. surface waters are well within the limits set by EPA as a standard for drinking water (20,000 pCi/L). No difference was observed between levels in raw lake water and processed lake water.

- - - - - - Great Lakes Waterways ........... Lake Huron (this report) Mississippi River

3500 30O0 2500 E

2O00 1500

F-

1000 500 0 I I It 1960 1963

t

[

ii

t

1966

i=

1969

IJ

i

1972

I

= i

1975

Ii

i

1978

I

1981

Year

Fig. 2. Comparison of tritium in the Great Lakes surface waters to Mississippi River water.

Mississippi River, an example of large rivers (Fig. 2). The major impact of the bomb tests in the early 1960s appears to have passed by about 1970-1974, and the level of tritium in U.S. surface waters in the late 1970s tends to be stable or slightly decreasing. A plot of tritium concentration in water from Lake Huron and the global atmospheric HT concentration (Ostlund and Mason, 1981), indicate the trend of the tritium concentration in Lake Huron is similar to the global HT concentration (Fig. 3). While some conclusions on several-year trends can be drawn from this data because of the large number of samples taken, no conclusions at these levels could be made from one, two, or even several samples by this method unless considerably more HTO were present (levels greater than 1000 pCi/L). The range data presented in Table 2 emphasizes this point. The amount of HTO currently found in surface waters around the United States (200-500 pCi/L) is well within the limits set by the EPA (U.S. EPA, 1976) as a standard for drinking water (20,000 pCi/L).

Conclusions There has been a slow but steady decline in the tritium concentration in water from Lake Huron, start800 1.25 0

600

0

0

0

0

A

.o_

0

0

1.0

O •

0

O

400

o C.)

O

-0.75

co t.o

O

E

0.5 F-

200

o Global Atmospheric HT Inventory • Concentration of Tritium in Lake Huron Water, PCI/L (this report) L

1964

I i 1967

t

I 1 1970

t

L I 1973 Year

I

I = 1976

0.25

t~ I--/-

I

I l 1979

I

0 1982

Fig. 3. Comparison of trend of tritium concentration in water from Lake Huron with global atmospheric HT inventory.

Acknowledgements-The authors would like to thank Lori Lickly, Laura Heikel, Sarah Lovett, Sue Farran, Greg Roe, and Pat Murphy of Environmental Sciences Research for analytical assistance. The authors would also like to thank Dr. Tetko of the U.S. EPA Eastern Environmental Radiation Facility for supplying needed reference material.

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T . D . Lickly, F. A. Blanchard, and I. T. Takahashi U.S. Environmental Protection Agency (1979a) Environmental radiation data. Report Number 15, pp. 11-13. U.S. EPA Office of Radiation Programs, Montgomery, AL. U.S. Environmental Protection Agency (1979b) Environmental radiation data. Report Number 16, pp. 10-12. U.S. EPA Office of Radiation Programs, Montgomery, AL. U.S. Environmental Protection Agency (1979c) Environmental radiation data. Report Number 17, pp. 21-22. U.S. EPA Office of Radiation Programs, Montgomery, AL. U.S. Environmental Protection Agency (1979d) Environmental radiation data. Report Number 18, pp. 10-12. U.S. EPA Office of Radiation Programs, Montgomery, AL. U.S. Environmental Protection Agency (1980a) Environmental radiation data. Report Number 19-20, pp. 17-19. U.S. EPA Office of Radiation Programs, Montgomery, AL. U.S. Environmental Protection Agency (1980b) Environmental radiation data. Report Number 21-22, pp. 17-19. U.S. EPA Office of Radiation Programs, Montgomery, AL. U.S. Public Health Service (1962) Public health service drinking water standards. Public Health Service Publication No. 956, U.S. Gov't. Printing Office, Washington, DC.