Geological Survey Bulletin 395 Radioactivity of the Thermal Waters of Yellowstone National Park

GENERAL STATEMENT.

The experimental results of this investigation are presented in five tables, showing the radioactivity of gases, waters, water residues, spring deposits, and rock samples. Finally, the results for the activity of gases, waters, and deposits are summarized for better comparison in an additional table.

EXAMPLE SHOWING METHOD.

The data obtained in the test of White Sulphur Spring at the Lower Geyser basin are summarized here to illustrate the procedure followed and to show the method of calculation of the values given for radioactivity in the tables. The test was conducted August 1, 1906. The temperature of the water in the pool at the point where samples were collected was 59° C., and the water was slightly alkaline to litmus paper. The gas escaping from two orifices under the water at the south end of the pool were first tested qualitatively for thorium and radium emanation by the use of the apparatus shown in figure 3. As the gas issued relatively slowly, ten minutes was consumed in passing the accumulating gases along the circuit through electroscope No. 1. About 1,100 cubic centimeters of gas was transmitted, as measured by 15 squeezes of the hand bellows. The first readings of the fall of the charged leaf were taken thirty seconds after the gas delivery was stopped. In this case no decrease in activity was observed during the first two minutes of observation. Hence thorium emanation was absent. In the course of ten minutes the leak increased about 12 per cent, which is characteristic of radium emanation. The gas evidently was relatively rich in emanation, as the leak was fully 1,000 times the usual air leak of the instrument. Our field journal shows that 60 small divisions of the scale were passed over by the leaf in eleven and six-tenths seconds. After twelve minutes of observation the gas was blown out and the electroscope set aside for a time to wait for the decay of the induced activity. Later in the day a quantitative determination of the radium emanation present in the escaping gas was made with the same electroscope. Ninety cubic centimeters of gas were collected over water (temperature 40° C.) and introduced into the partially exhausted electroscope. At maximum activity the leaf fell 23.6 small scale divisions per minute after deducting the normal air leak, which was 0.2 division per minute. The volume of the gas sample reduced to normal temperature and pressure is 56.3 cubic centimeters.^a In this electroscope a leak of one division per minute represented 2.8 X 10^-4 grams uranium or 10.64 X 10^-11 grams radium. The activity of the gas per liter, in terms of the uranium standard, is hence expressed by the following equation:

Expressed in terms of the radium standard the activity equals 117 X 10^-3 X 3.8 X 10^-7 = 44.5 X 10^-9.

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^aIn reducing the observed gas volumes to those for normal conditions the value 600 millimeters was used for the barometric pressure in all the calculations. Correction was of course made for the aqueous tension.

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The value of the ionization current per liter of gas, in electrostatic units, is obtained by substituting in the formula:

For q may be put the potential fall in volts during the interval of time (t) expressed in seconds, and c is the electric capacity of the electroscope. Since each division of the scale represents 1.24 volts, and the capacity is 2.81 centimeters, the numerical equation for the value of the ionization current is:

For the determination of the activity of the water from this spring, the water gas separated from a sample of 2,570 cubic centimeters was introduced into electroscope No. 2. This sample of water was collected in the immediate vicinity of the orifices from which the gas issued. The maximum net leak observed was 10.9 scale divisions per minute. The activity of the water sample, per liter, in terms of the uranium standard, is given by the equation:

This value is equivalent to 1,238 X 10^-12 grams radium. The numerical equation for the ionization current in electrostatic units has the form:

The water sample upon evaporation left a residue weighing less than 2 grams. Its radium content was too small to be determined in so small a quantity of material. It therefore follows that the activity of the water is temporary in nature, and that the dissolved radium emanation did not come directly from the radium salts carried in solution. The greater portion of the emanation doubtless diffused into the water from the radium-bearing rocks with which the water came in contact in the course of its underground flow.

RADIOACTIVITY OF GASES.

The names of the gas sources in Table 2 are those found in the atlas accompanying Monograph 32 of the United States Geological Survey, entitled "Geology of Yellowstone Park." The locality names in the second column of the table are also the official names. Names inclosed in quotation marks are not official. The third column shows whether thorium emanation was detected in the qualitative field test. In some instances the activity due to thorium is compared with that observed for radium. In the last column the activity values are recorded in both the uranium and the radium standards. They represent the radium emanation present per liter of gas. The blank spaces indicate that no test was conducted.

TABLE 2—Radioactivity of gases in Yellowstone Park.

The values recorded for the activities of the water samples in Table 3 are expressed in terms of both the uranium and the radium standards. They represent the radium emanation in a fresh sample of water. In several instances some time elapsed between the collection of the sample and the determination. In such tests correction was made for the decay in the activity of the samples during the interval between the time of collecting and the time of testing by means of the formula I_o = I_te_-rt, expressing the decay of activity with the time. I_o represents the initial activity, I_t the activity observed t hours after the water was collected, and r the radioactive constant of radium emanation. The value of r used in the calculations was 0.0077, which is an approximate mean of the values found by several investigators. The temperatures given were taken with a normal centigrade thermometer graduated in degrees. The instrument was not self-registering. At some of the springs it was difficult to get accurate readings, as the hot water and vapors interfered materially with readings at close range. For the most part the values recorded are correct to within half a degree. As the reaction of the waters to litmus paper was tested, the results are recorded in a separate column. This table, like the preceding one, is subdivided on the basis of the more important localities. Several of the names recorded are not found on the maps of the United States Geological Survey. Most of these, however, are probably the official names, as they appear on the printed signboards. New names are inclosed with quotation marks. We aimed to include the spring waters of the park analyzed by Gooch and Whitfield.a

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^aBull. U. S. Geol. Survey No. 47. 1888.

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TABLE 3.—Radioactivity of waters in Yellowstone Park.

RADIOACTIVITY OF WATER RESIDUES, SPRING DEPOSITS, AND ROCK SAMPLES.

The values given for the activities of solids in Tables 4, 5, and 6, represent the radium present per gram of the air-dry material. The quantity of material taken in each determination is recorded in the third column of the tables. The samples in Table 4 are water residues; those in Table 5 are incrustations deposited by the springs where the water issues. The samples were collected for the most part at the time the water samples were tested, and the radium was determined during the years 1906 and 1907. Several samples of travertine at Mammoth Hot Springs were collected by Capt. William H. Forsythe, U. S. Army. Some of the rock samples from typical formations in the park were obtained from the collection of the United States Geological Survey through the kindness of Dr. Arnold Hague.

TABLE 4.—Radioactivity of water residues, Yellowstone Park.

TABLE 5.—Radioactivity of spring deposits, Yellowstone Park.

TABLE 6—Radioactivity in rock samples, Yellowstone Park.

SUMMARY OF RADIOACTIVITIES.

To facilitate comparisons, most of the results presented in the foregoing tables are summarized in Table 7. The values are expressed in terms of the radium standard.

TABLE 7—Summary of radioactivities of gases, waters, and deposits in Yellowstone Park.