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Natural Radioactivity in Ground Water- a Review, USGS, Reston VA

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Natural Radioactivity in Ground Water- a Review, USGS, Reston VA 50 National Water Summary 1986—Ground-Water Quality: HYDROLOGIC CONDITIONS AND EVENTS NATURAL RADIOACTIVITY IN GROUND WATER — A REVIEW By Otto S. Zapecza and Zoltan Szabo INTRODUCTION GEOCHEMISTRY OF RADIONUCLIDES Natural radioactivity and its effects on human health Radionuclides are found as trace elements in most rocks recently have become a major environmental concern because of and soils and are formed principally by the radioactive decay of the discovery of widespread occurrence of levels of radon in the uranium-238 and thorium-232, which are the long-lived parent air of homes at concentrations that exceed the U.S. Environmental elements of the decay series that bear their names (fig. 18). The Protection Agency’s (EPA) recommended maximum levels, par- parent elements produce intermediate radioactive daughter ele- ticularly in the eastern United States. Radon-222 in air, even in ments with shorter half-lives (half-life is the time required for half small concentrations, contributes to the high incidence of lung of the initial amount of the radionuclide to decay). Decay occurs cancer among uranium miners in the Western United States by the emission of an alpha particle (a nucleus of the helium atom) (Archer and others, 1962). Recent estimates indicate that radon in or a beta particle (an electron) and gamma rays from the nucleus indoor air may cause 5,000 to 20,000 lung-cancer fatalities annu- of the radioactive element. The geochemical behavior of a daugh- ally in the United States (U.S. Environmental Protection Agency, ter element in ground water may be quite different from that of the 1986a). parent element. However, the parent may govern the occurrence A less publicized but also important health hazard is the and distribution of the daughter element. presence in ground water of naturally occurring radioactive sub- The most common radionuclides in ground water are stances, which along with radon are known as radionuclides. In radon-222, radium-226, uranium-238, and uranium-234 of the addition to radon, large concentrations of dissolved radium and uranium-238 decay series, and radium-228 of the thorium-232 uranium radionuclides have been detected in may ground-water decay series. Other radionuclides of these two decay series, and all supplies across the United States. All radionuclides dissolved in isotopes of the uranium-235 decay series, generally are not water are colorless, odorless, and tasteless and, thus, cannot be present in significant amounts in ground water, because most are detected by our senses, unlike many water pollutants that may highly immobile and many have very short half-lives that preclude impart undesirable colors, odors, and tastes to water. the buildup of large concentrations. Although much is known about the theoretical geochemis- The occurrence and distribution of radionuclides in ground try of radionuclides in ground water, it still is very difficult to water is controlled primarily by the local geology and geochemis- forecast the amount of radionuclide activity in a particular ground- try. For daughter radionuclides to be present in large concentra- water supply because of the strong influence of local geologic, tions, the parent radionuclide must be present in the rock material geochemical, and hydrogeologic conditions. Much of what is composing the aquifer. Each radioactive decay product has its known about the distribution of radionuclides in water has been own unique chemical characteristics, solubility, mobility, and half- derived from analysis of water from public water-supply systems, life, which can be very different from those of the parent. For this which supply slightly more than 80 percent of the population in reason, parent and daughter radionuclides in ground water are not the United States (Solley and others, 1983). However, it is diffi- usually found together in similar amounts (Gilkeson and others, cult to develop site-specific information about the occurrence and 1983, p. 22): nor do they decay at similar rates or produce the activities of radionuclides in specific aquifers because public same level of radioactivity. Therefore, a high concentration of one water supplies commonly are a blend of water from numerous radionuclide in ground water at a specific site does not necessarily ground- and surface-water sources. Relatively little is known of imply that similar concentrations of other radionuclides in the the concentrations of radionuclides in private water supplies, same decay series are present. for example, the parent/daughter which rely heavily on ground water and supply more than 20 per- radionuclide pairs uranium-238 or uranium-234/radium-226 or cent of the total ground water used for human consumption. radium-226/radon-222 usually are not present in high concentra- In recent years, estimates have appeared in the scientific tions in the same ground water. literature about the effects of radionuclides in ground water on The movement of many radionuclides is very dependent human health. these estimates and the growing body of scientific upon the radionuclide’s solubility in water. Uranium, which is knowledge of the distribution and levels of radionuclides in most soluble in bicarbonate-rich oxidizing (oxygen-rich) ground ground water, have stimulated a review of the adequacy of stan- water with low total dissolved-solids content, is easily dissolved dards and regulations for radionuclides in drinking water. Fortu- and transported by oxidizing ground water; thus, it can be trans- nately, conventional methods for treating raw water for some ported to areas far from its original emplacement. Solubility of other contaminants also are effective in removing radionuclides uranium tends to be enhanced by association with carbonate, found in ground water. (See table 2.) phosphate, and fluoride ions, or with organic compounds, espe- cially humic substances (Langmuir, 1978, p. 556; Turner-Peter- son, 1980, p. 163). Uranium is less mobile in reducing ground water, and it tends to be adsorbed very strongly onto humic sub- stances in the aquifer. Conversely, radium is most mobile in chlo- ride-rich reducing ground water with high total dissolved-solids content (Tanner, 1964, p. 261). National Water Summary 1986—Ground-Water Quality: SELECTED EVENTS 51 URANIUM 238 THORIUM 232 U-238 U-234 Th-232 Th-228 Uranium Thorium 9 10 4.49x10 y 2.48x10 5y 1.41x10 y 1.91 y Pa-234 Ac-228 Protactinium Actinium 1.18 m 6.13 h Th-234 Th-230 Ra-228 Ra-224 Thorium 4 Radium 24.1 d 7.52x10 y 5.75 y 3.64 d Ra-226 Radium 1622 y Other Radon Rn-222 radionuclides 3.825 d with short half-lives Other Alpha decay radionuclides with short half-lives Beta decay Pb-206 Pb-208 Lead (stable) Lead (stable) Figure 18. Uranium-238 and thorium-232 radioactive decay series. Colors indicate those radionuclides found most fre- quently in ground water. Times shown are half-lives – y=years, d=days, h=hours, m=minutes. Field measurements or dissolved-oxygen concentration DRINKING-WATER STANDARDS AND MONITOR- and oxidation-reduction potential (Eh) in parts of New Jersey have ING RQUIREMENTS shown that these are important controls on radium-226 and ura- nium concentrations in ground water (Szabo and Zapecza, 1987). Because of the health hazards of radium-226 and Where the ground water is reducing, elevated levels of radium- -228 in drinking water the EPA has established maximum contam- 226 are associated with elevated gross alpha activities. Where the inant levels (MCL) to regulate total radium concentration in pub- ground water is oxidizing, only small concentrations of radium- lic water supplies. According to the EPA’S National Interim 226 are associated with high levels of gross alpha activity. The Primary Drinking-Water Regulations (U.S. Environmental Protec- high levels of gross alpha activity in oxidizing water are caused by tion Agency, 1986b) the maximum contaminant levels for radio- dissolved uranium. nuclides are: Radium-226 and radium-228 combined . 5 pCi/L Gross alpha-particle activity (including radium-226 but excluding uranium and radon . 15 pCi/L Gross beta-particle activity. 4 millirems per year 52 National Water Summary 1986—Ground-Water Quality: HYDROLOGIC CONDITIONS AND EVENTS Radioactivity in ground water commonly is measured in picocu- by a number. Information in figure 19 is based on a study of pub- ries per liter (pCi/L)—1 pCi/L is equal to 0.037 disintegrations of lished reports; no field investigations were performed by the the radionuclide per second per liter of fluid. In the fall of 1986, authors. Most of the data are from work by Cothern and Lappen- the EPA announced its intention to expand its regulations to con- busch (1984) and Hess and others (1985), who examined the trol radionuclides—radium-226 and -228, natural uranium, radon, results of compliance data from a nationwide monitoring of more and gross alpha, gross beta, and gamma emitters—in public water than 50,000 public water supplies for radioactivity in drinking supplies (U.S. Environmental Protection Agency, 1986c.) water, and from data provided in published reports on individual The EPA requires that all public-water suppliers analyze States as indicated in the following discussions. composite water samples from their distribution systems for gross alpha-particle, gross beta-particle, and radium-226 activity every RADON 4 years. Samples also are analyzed for radium-228 when the radium-226 activity exceeds 3 pCi/L. Gross alpha- and beta-parti- Radon in ground water is most prevalent in the Northeast- cle activity are used to determine if further radiochemical analyses ern United states, especially, in the New England area. High con- are necessary. Alpha emitters, such as radium-226, radon-222, and centrations of radon have been detected in ground water uranium add to the total gross alpha-particle activity. Radium-228 associated with granitic and metamorphic rocks of Maine and is beta-particle emitter.
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