Duke University, Durham, NC, USA.
Duke University, Durham, NC, USA.
Sci Total Environ. 2022 Feb 20;808:151914. doi: 10.1016/j.scitotenv.2021.151914. Epub 2021 Nov 29.
This critical review presents the key factors that control the occurrence of natural elements from the uranium- and thorium-decay series, also known as naturally occurring radioactive materials (NORM), including uranium, radium, radon, lead, polonium, and their isotopes in groundwater resources. Given their toxicity and radiation, elevated levels of these nuclides in drinking water pose human health risks, and therefore understanding the occurrence, sources, and factors that control the mobilization of these nuclides from aquifer rocks is critical for better groundwater management and human health protection. The concentrations of these nuclides in groundwater are a function of the groundwater residence time relative to the decay rates of the nuclides, as well as the net balance between nuclides mobilization (dissolution, desorption, recoil) and retention (adsorption, precipitation). This paper explores the factors that control this balance, including the relationships between the elemental chemistry (e.g., solubility and speciation), lithological and hydrogeological factors, groundwater geochemistry (e.g., redox state, pH, ionic strength, ion-pairs availability), and their combined effects and interactions. The various chemical properties of each of the nuclides results in different likelihoods for co-occurrence. For example, the primordial U, Rn, and, in cases of high colloid concentrations also Po, are all more likely to be found in oxic groundwater. In contrast, in reducing aquifers, Ra nuclides, Pb, and in absence of high colloid concentrations, Po, are more mobile and frequently occur in groundwater. In highly permeable sandstone aquifers that lack sufficient adsorption sites, Ra is often enriched, even in low salinity and oxic groundwater. This paper also highlights the isotope distributions, including those of relatively long-lived nuclides (U/U) with abundances that depend on geochemical conditions (e.g., fractionation induced from redox processes), as well as shorter-lived nuclides (U/U, Ra/Ra, Ra/Ra, Pb/Rn, Po/Pb) that are strongly influenced by physical (recoil), lithological, and geochemical factors. Special attention is paid in evaluating the ability to use these isotope variations to elucidate the sources of these nuclides in groundwater, mechanisms of their mobilization from the rock matrix (e.g., recoil, ion-exchange), and retention into secondary mineral phases and ion-exchange sites.
本综述介绍了控制铀和钍衰变系列中天然元素（即天然放射性物质，NORM）发生的关键因素，包括铀、镭、氡、铅、钋及其同位素在地下水中的含量。由于这些核素有一定毒性且能放射辐射，因此饮用水中这些核素含量过高会对人体健康造成危害。因此，了解这些核素在含水层岩石中的赋存、来源和迁移控制因素对于更好地进行地下水管理和保护人类健康至关重要。地下水中这些核素的浓度是与核素衰变率相比地下水居留时间的函数，也是核素迁移（溶解、解吸、反冲）和保留（吸附、沉淀）之间净平衡的函数。本文探讨了控制这种平衡的因素，包括元素化学（例如，溶解度和形态）、岩性和水文地质因素、地下水地球化学（例如，氧化还原状态、pH 值、离子强度、离子对可用性）及其综合影响和相互作用。每个核素的各种化学性质导致其共同发生的可能性不同。例如，原始 U、Rn，在高胶体浓度的情况下还有 Po，都更有可能存在于氧化地下水环境中。相比之下，在还原含水层中，Ra 核素、Pb，以及不存在高胶体浓度的情况下，Po 则更具迁移性，经常出现在地下水中。在缺乏足够吸附点位的高渗透性砂岩含水层中，即使在低盐度和氧化地下水中，Ra 也会富集。本文还重点介绍了同位素分布，包括相对长寿命核素（U/U）的同位素分布，其丰度取决于地球化学条件（例如，由氧化还原过程引起的分馏），以及短寿命核素（U/U、Ra/Ra、Ra/Ra、Pb/Rn、Po/Pb）的同位素分布，这些同位素分布受物理（反冲）、岩性和地球化学因素的强烈影响。本文特别关注评估利用这些同位素变化来阐明地下水中这些核素的来源、它们从岩石基质中迁移的机制（例如，反冲、离子交换）以及保留在次生矿物相和离子交换点位上的能力。
