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

0229120麦卡锡地下水中的化学迁移经常在实验室环境中进行研究，在沉积物的异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不同。 为了评估原地迁移率，该项目利用了含水层系统中天然铀系和钍系同位素的分布情况。 岩石中的~（238）U和~（232）Th通过一系列中间子体衰变为Pb的稳定同位素，这些子体由具有不同化学性质和广泛衰变平均寿命的元素组成。 这些衰变系同位素由于水-岩相互作用而进入地下水。 它们在地下水中的丰度和比例随着水穿过岩石介质而演变，由此产生的亲子不平衡关系提供了对地下地球化学反应和各种时间尺度上的运输过程的定量约束。 我们将在田纳西州西部谢尔比县地区的一个被称为孟菲斯含水层的区域含水层系统中进行这种多示踪剂研究。 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、环境示踪剂、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，进行238 U和234 U）、钍（234 Th、230 Th、228 Th和232 Th）、镭（226 Ra、228 Ra和224 Ra）、铅（210 Pb）、钋（210 Po）和氡（222 Rn）的同位素测量。 还将对含水层固体和胶体颗粒进行测量。 将试图对流体和胶体中观察到的放射性不平衡进行建模，以量化水-岩相互作用的动力学（例如，吸附/解吸和沉淀/溶解速率）及其对化学输运和地下水流的影响。 模型的结果，测试所测得的不平衡的吸附阶段的岩石，允许校准的运输模型开发的小规模，自然，不受干扰的流动系统，并可能揭示新的过程是不明显的实验室研究或现场操作实验。 预计该研究将为地下水中放射性同位素的胶体缔合提供新的见解。 通过加强对地下污染物长期迁移的了解，该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好处。

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.