The Radioactivity of Natural Waters

Kelly Myers
February 26, 2016

Submitted as coursework for PH241, Stanford University, Winter 2016


Source Radium Content
(10-12 gm per gm)
Continental Rocks
Granites 0.2 - 5.0
Basalts 0.1 - 1.0
Sedimentary 0.05 - 0.5
Ocean-Bottom Sediments
Red Clay 3 - 22
Globogerina Ooze 3 - 7
Blue Mud 1 - 3
Table 1: Radium Content of Rocks and Ocean-Bottom Sediments. [2]

Knowledge of the presence of radioactive substances in water is comparatively recent. Therefore, it has transformed into a hot topic as a new and unique property of may natural waters. [1] In studying this remarkable find further, radioactivity has been recorded as a normal property occurring in many natural waters. Contrary to what would be expected however, seawater does not contain the large amounts of radium detected. Holding only approximately 0.08 × 10-12 grams per liter, seawater only accounts for one seventh of the radium required to achieve equilibrium with the uranium present in the water. [2] It is the ocean-bottom sediments that hold the key and provide the amount of radium required to balance the uranium. Although reported to produce an excess of 500,000 × 10-12 gram per liter, in reality the radioactivity of most spring waters is less than 100,000 × 10-12 . [2] From the information recorded by multiple U.S. geological and foreign surveys, it appear that the radioactivity of normal surface and ground waters not associated with thermal or mineralized spring rangers from about 0.1 × 10-12 gram of radium per liter. [2] These numbers and categorizing are essential in determine the various identities and causes of radioactivity that spring from the oceans surface. It is crucial to understand these details in order to evaluate radioactivity due to pollution from waste materials and possible bomb bursts.

Sources of Natural Radioactivity in Water

Natural radioactivity of water is derived primarily from the radioactive rocks and minerals sitting below the waters surface. The water adapts their traits when it comes in contact with them. In certain areas more than others, radioactive gases emitted from molten magmas have seeped into the natural bodies of water creating larger traces of radioactivity magnitude. [1] In order to break it up, there are three extended series of naturally radioactive elements: the uranium series, the thorium series and the active actinium series. Each series holds a gaseous member, a member with a very long half-life, and the end product: a stable isotope of lead. [1]

Radioactivity of Sea Water

The sea, a natural repository for all dissolved and suspended masses, is continually exposed to relatively large concentrations of uranium, radium and thorium. Under these particular concentrations of uranium and radium, it would be assumed that these two constants would remain in equilibrium with the other. When balanced, the ratio rests at an estimated 3,000,000 to 1. Meaning, for each concentration of uranium present in seawater, 0.3 × 10-6 of radium should be present. [2] Many investigators have studied the radium content of seawater for several decades and found actually only about one seventh the theoretical amount exists at present times. Values as high as 40 × 10-12 of radium per liter have been reported in the past, estimating many times greater than the actual concentrations. [2] If equilibrium is said to be the natural ratio between uranium and radium, something must act to deplete the radium content far below its theoretical concentration. This differentiating factor is a crucial determinant in understanding natural water radioactivity. As of now, explanations for the radium differential include extraction by calcareous animals, marine algae, chemical precipitation, and physical processes such as adsorption or Base Exchange. [2] However, none of these as of late seem to be entirely satisfactory for the conditions observed.

Radium Content of Ocean-Bottom Sediments

As of late, attention has been directed towards the radium content present in ocean-bottom sediments. Home to the largest concentrations of radium, the sediments might be the answer to the disjointed ratio of radium required for equilibrium conditions with uranium. [2] Recently determined by the examination of bottom grab samples, the sediments lying below about 3000 meters are the ones to contain the greatest concentrations of radium. In comparison, these sediments that lay far enough beyond landmasses to be beyond the range of immediate detrital material, have much more radium than those of continental igneous rocks. [2] For example, Fig. 1 shows the radium content, 10-12 gram per gram of continental rocks is 0.2-5.0, while red clay (an ocean-bottom sediment) ranges from 3-22. [2] Several cores of bottom sediments have been examined and studied to large extent with the overwhelming conclusion that the high radium content contained in the sediments does not exist at any appreciable depth below the immediate surface. When depths of a few meters are hit beyond the core, the radium content of sediments drops off to reach values in equilibrium with the uranium present. Therefore, the equilibrium ratio is balanced, with the deficiency of radium in seawater is thus accounted for by the excess of radium in bottom sediments. [2]

Why It Matters

The collection of background data is essential in all areas in order to understand if there is a possibility that radioactive wastes may contaminate natural surface or ground water supplies. The information collected and currently available indicates that the radioactivity of uncontaminated natural waters does not exceed a dangerous amount. [1] Spring waters, however, which tend to produce fairly high contents of radium concentrations, seem to exceed this danger scale quite often. [1] Therefore, it is extremely important to know the radium and uranium contents of all potentially affected bodies of water to properly assess contamination. While a greater focus has been placed on preventing contamination of natural surface and ground waters, natural water supplies may become dangerously concentrated without proper attention, by reason of accident or inadequate precaution. However, if excessive wastes either temporarily or for longer periods penetrate surface waters, the effects, though potentially danger, are hypothesized transitory. [1] The flow of the water, in a relatively short amount of time, will flush out the contamination from a channel for example, but the contamination of ground waters will remain. [1] The ground water constitutes a much more serious hazard as the flow moves at a substantially slower pace. If waste materials, with long half-lives, remain unusable for many years the radioactive build-up in the materials may have the potential to be removed through the mechanism of adsorption or ion exchange.


Through the mechanism of adsorption or ion exchange, harmful wastes may be removed by passing them through the ground. It seems equally possible, however, that under suitable conditions the adsorbed wastes may at a later but unpredictable time be released from the ground to the water in the ground, and thus become potentially hazardous again. Research is proposed to determine under what conditions radioactive waste materials may be removed from solution and subsequently released back into solution by different types of soils and rock materials. The Radioactivity of NaturalWaters is an interesting and intriguing part of earth's natural eco- system, except in further review has no immediate or relevant affect on day to day life. If this radioactivity can be harnessed in a successful and economically efficient way in the future, the discussion of its benefits and overall importance could be re-introduced.

© Kelly Myers. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] W. D. Collins, "The Radioactivity of Natural Waters," Public Health Rep. 41, 1937 (1926).

[2] S. K. Love, "Natural Radioactivity of Water," Ind. Eng. Chem. 43, 1541 (1951).