Collaborative Research (USC and UT): A Study of Naturally Occurring Decay-Series Isotopes as Quantitative Hydro-Geochemical Tracers

地下水中的化学运移通常是在实验室环境中研究的，在沉积物的非均质性和所涉及的时间和空间尺度方面，实验室环境可能与自然环境有很大差异。为了评估就地运移速率，该项目利用了含水层系统中天然存在的铀和钍系列同位素的分布。岩石中的238U和232Th通过一系列具有不同化学性质和广泛衰变平均寿命的元素组成的中间子体衰变为稳定的铅同位素。这些衰变系列的同位素作为水-岩石相互作用的结果进入地下水。它们在地下水中的丰度和比例随着水通过岩石介质而演变，由此产生的母子不平衡关系对不同时间尺度上的地下地球化学反应和运移过程提供了定量的约束。我们将在田纳西州西部谢尔比县地区被称为孟菲斯含水层的区域含水层系统中进行这项多示踪研究。在确定地下水补给方面，它一直是一个跨学科研究的研究场所，涉及地球化学、环境示踪、数值模拟和沉积物分析。我们将从不同地点和含水层深度收集地下水，用于同位素测量，包括238U和234U)、钍(234、230、228和232Th)、镭(226Ra、228Ra和224Ra)、铅(210Pb)、钚(210Po)和氡(222Rn)。还将对含水层固体和胶体颗粒进行测量。将尝试对观察到的流体和胶体中的放射性不平衡进行建模，以量化水-岩石相互作用的动力学(例如，吸附/解吸和沉淀/溶解的速率)及其对化学传输和地下流动的影响。模型结果将通过岩石吸附相中测量的不平衡来检验，允许对从小尺度、自然、未受干扰的流动系统开发的传输模型进行校准，并可能揭示在实验室研究或现场操纵实验中不明显的新过程。预计这项研究将为地下水中放射性同位素的胶体结合提供新的见解。通过加强对地下污染物长期迁移的了解，这项研究具有加强地质系统中储存的核废物风险评估和管理的科学基础的实际益处。

0229120McCarthyChemical transport in groundwater is often studied in the laboratory setting which may differ considerably from the natural settings in terms of heterogeneity of the sediment and the scales involved, both temporal and spatial. To assess the in-situ transport rates, this project makes use of the distributions of naturally occurring uranium- and thorium-series isotopes in an aquifer system. 238U and 232Th in rocks decay to stable isotopes of Pb through a series of intermediate daughters consisting of elements with diverse chemical properties and a wide range of decay mean lives. These decay-series isotopes enter groundwater as a result of water-rock interaction. Their abundance and ratios in groundwater evolve as the water passes through rock media, with resultant parent-daughter disequilibrium relationships providing quantitative constraints on subsurface geochemical reactions and transport processes on various time scales. We will conduct this multi-tracer research in a regional aquifer system known as the Memphis aquifer at the Shelby County area in western Tennessee. In connection with the determination of groundwater recharge, it has been a research study site for interdisciplinary studies involving geochemistry, environmental tracers, numerical modeling and sediment analysis. We will collect groundwater from various locations and aquifer depths for isotopic measurements of 238U and 234U), thorium (234Th, 230Th, 228Th, and 232Th), radium (226Ra, 228Ra, and 224Ra),, lead (210Pb), polonium (210Po) and radon (222Rn). Measurements will also be made on aquifer solids and colloidal particles. Modeling the observed radioactive disequilibria in fluids and colloids will be attempted to quantify the kinetics of water-rock interactions (e.g., rates of adsorption/desorption and precipitation/dissolution) and their effects on chemical transport and subsurface flows. The model results, to be tested by the measured disequilibria in the sorbed phases of rocks, allow calibration of transport models developed from small-scale, natural, undisturbed flow system, and may reveal new processes that are not apparent in laboratory studies or field manipulation experiments. It is anticipated that the research will provide new insights into the colloidal associations of radioisotopes in groundwater. Through an enhanced understanding of long-term subsurface contaminant transport, the research has the practical benefit of strengthening the scientific basis for risk assessment and management of nuclear wastes stored in geologic systems.