Continental water balance, ground water inventory and storage times, surface ocean mixing rates and world-wide water circulation patterns from cosmic-ray and bomb tritium

Abstract

The tritium produced during the Castle operation in the spring of 1954 has been used to study the circulatory rates for waters in the northern hemisphere, particularly in the northern Mississippi Valley. It is observed that northern hemisphere rains 6 months to 18 months following Castle had about half the tritium content that the surface water had, and this difference has revealed that about one-third of the rain in the upper Mississippi Valley is ocean water and about two-thirds re-evaporated surface water. This allows the total inward transport of ocean water for the Upper Mississippi Valley to be calculated. It is about 1.0 m per year. It has been found that on the average tritium has a residence half-life of about 6 years on the continental land mass of North America (radioactive decay taken into account). This probably will be applicable to most other continental land masses. Water from the oceans stays in the northern Mississippi Valley 15 years on the average. The principal mechanisms by which ocean water deposited on the continents returns to the sea are in the rivers, and as moisture in air masses moving from the land to the sea where precipitation occurs. In the northern Mississippi Valley it is shown that on the average about 8 m of ground water or equivalent including bound hydroxyl groups in clays are available for mixing with the rainfall, about 0.28 m of water runs off annually in rivers, and 0.24 m per year are carried back to the oceans by the winds. The atmospheric residence time of Castle tritium has been found to be about 40 days. So far no mixing of any “excess” tritium across the equator into the southern hemisphere has been noticed. The short stratospheric residence time of bomb-tritium as compared to bomb-fission products probably is due to the large amount of water taken into the fireball which condenses in the cold of the stratosphere to form the familiar cloud of relatively large water-drops or ice-crystals. These are large enough to descend more quickly than the finer fission product particles. The cosmic-ray tritium production rate previously calculated to be 0.14 T-atoms/cm 2/sec on the average now is thought to be too low, since the decay of tritium in groundwater and the amount by which the outward tritium vapour transport exceeds the inward flow from the seas are both now known to be important. The new value for Q̄ is 1 T-atom/cm 2/sec, as measured at the earth's surface, and 2 T-atoms/ cm 2/sec total, the difference being due to an assumed stratospheric residence time of 10 years on the basis of fission product fallout information. The direct measurement of tritium production cross-sections and calculation of Q̄ from cosmic-ray intensities predicts that the average cosmic-ray production rate for tritium is about 0·14 T-atoms/cm 2/sec averaged over the whole world, the intensity varying strongly with latitude about as the cosine of the latitude with a factor of 4 in intensity between the equator and the poles. The discrepancy may be due to an appreciable accretion of tritium directly from the sun, as suggested by J.R. Arnold and B. Feld. The new higher value for the production rate means that the world-wide inventory of cosmic-ray tritium is raised to about 30 kg from 1.8 kg, the upper limit to the escape time for 3He from the atmosphere of the earth is about 2.5 million years. Studies of the circulatory pattern of hot springs have shown that the waters of the several hot springs studied are rainwater that has been stored for relatively brief periods. Study of groundwaters has shown that in large areas the water issuing from wells dug for normal use is older than 50 years. It appears that the technique of studying the tritium content of well-water is quite likely to prove to be of real value in studying underground water supplies and in the prediction of their susceptibility to drought as well as depletion by pumping and the possibility of replenishment from rain or snow.