Radium, thorium and radioactive lead isotopes in groundwaters: Application to the in situ determination of adsorption‐desorption rate constants and retardation factors

Abstract

Five groundwater samples taken from different hydrogeologic settings in Connecticut were analyzed for major cation chemistry and the concentration of U and Th decay series nuclides 238U, 234Th, 226Ra, 222Rn, 2•øpb, 2•øpo, 232Th, 228Ra, 228Th, and 224Ra. The concentration of 222Rn in the waters ranged between 103 and 104 dpm 1-• and was three to four orders of magnitude greater than that of the shortlived alpha daughters 224Ra, 228Ra, and 234Th, even though the rates of supply of these four nuclides to solution are expected to be similar. We infer that sorption removes radium and thorium from these groundwaters on a time scale of 3 minutes or less. The (224Ra/228Ra)•.and (234Th/228Th) activity ratios in these waters indicate that desorption of these nuclides occurs on a time scale of a week or less and that equilibrium between solution and surface phases is established. In situ retardation factors for radium, thorium, and lead may therefore be calculated directly from the isotopic data; values range from 4,500 to 200,000. Neither sorption time scales nor retardation factors are, strongly dependent on the nuclide or on hydrogeology of the aquifer. Since our study includes nuclides with diverse chemical properties, we suggest that other uncomplexed heavy metals and transuranic elements will also behave in a manner similar to those measured here. The approach presented here should therefore find application in developing site-specific models of the transport of radioactive or stable elemental waste through water-saturated media. 1. geochemical properties, their behaviors may serve as indica-1On leave from the Physical Research Laboratory, Ahmadabad, India. tors of the in situ chemical behavior of other nuclides injected into the system. With this aim we have analyzed several groundwaters from Connecticut for virtually every long-lived (half-life > 1 day) member of the 238U and 232Thseries in order to understand the processes controlling their concentration in solution and thereby to estimate the rates of those processes. The U and Th series nuclides are uniquely suited to this study because several isotopes of the same element are continuously introduced into groundwaters and because the supply rates of many of these nuclides can be estimated with adequate accuracy. Measurements of the distribution of these natural decay series nuclides yield site-specific empirical data on the in situ sorption processes. Parameters such as distribution coefficients or retardation factors derived from these measurements may be used in models of transport of both radioactive and stable nuclides that show analogous chemical behaviors. METHODSFive groundwater samples from the major aquifer types of Connecticut were collected for this study. The sampling locations, aquifer descriptions, and water chemistries are given in Table 1. Samples GW-3 and GW-7 were both taken from municipal water supply production wells in glacial drift aquifers. The large volumes of water pumped from these wells (Table 1) suggests that recharge is induced from n...

Uranium-Series Constraints on Radionuclide Transport and Groundwater Flow at the Nopal I Uranium Deposit, Sierra Peña Blanca, Mexico

“…where q is the groundwater specific discharge, V is the saturated zone well volume (163-208 L), h is the casing perforation height (9.15-12.20 m), C 1 is the U concentration at time t 1 and C 2 that at t 2 , and r is the well casing internal radius (4.825 cm). The model is only approximate, and additional factors are often included to account for flow distortion or skin effects from drilling, which can modify flow through the borehole relative to the surrounding formation (e.g.…”

“…Uranium-series techniques are an important method for determining in-situ retardation factors in groundwater (e.g. [1][2][3]. In a geochemical system that is closed for greater than five daughter half-lives, the activity of each U-series daughter will be equivalent to its parent nuclide.…”

Factors controlling 226Ra, 228Ra and their activity ratio in groundwater – an application in Polish Carpathian mineral waters

“…According to Krishnaswami et al (1982), the equation describing the rate of radium isotopes per time unit in the groundwater of a water-rock system can be written as:…”

“…where: N -number of radium nuclei in a unit volume of water in a time moment t, Krishnaswami et al (1982), the relation between the C os and the concentration of a radium isotope in a host rock can be written as:…”

“…2007, Nowak et al 2012. Krishnaswami et al (1982) investigated radium isotopes in Connecticut groundwater and stated that the desorption coefficient of radium isotopes (k ds ) is much lower than the adsorption coefficient (k ad ), and the k ds varies in a range of decay constant of 228 Ra (λ Ra228 -2.2⋅10 −9 s) to that of 224 Ra (λ Ra224 -3.8⋅10 −6 s). Davidson & Dickson (1986) and Dickson (1990) identified the alpha recoil as the main mechanism responsible for the migration of radium nuclei from the aquifer rocks into water.…”