COLLABORATIVE RESEARCH: Radiogenic Helium as a Chronologic Tracer for Young Groundwater

9637898 9628627波雷达所罗门U/Th衰变产生的地下水中放射性成因的~4He浓度，在103年至106年的时间尺度上已被用作测年工具。最近的测量表明，许多浅层流动系统中的含水层固体释放4He的速率可能远远大于U/Th衰变，这使得使用4He作为地下水测年工具的时间尺度短至101年是可能的。这项提议是为了调查导致4He的释放速率大于U/Th衰变的含水层的机制和地理范围。拟议研究的主要目标是评估4He作为地下水示踪剂在10至数百年的时间尺度上的使用情况。具体的研究问题包括：(1)含水层固体与地下水之间4He交换的控制机制是什么？这一汇率可以预测吗？如果是的话，不确定性是什么？(2)他的释放率在空间上有多大的变化？空间变化是否与基本水力性质(孔隙度、渗透率)和/或地球化学性质(矿物学、沉积相、成岩作用)的变化有关？(3)以显著大于U/Th衰变的速率释放4He的浅含水层的地理范围是什么？是否有可能根据一般地质概念预测高4He释放率的发生？(4)碳酸盐和其他裂隙岩石系统的4He释放率是多少？(5)含水层固体的4He总含量在多大程度上代表了含水层原岩的古水文？总的4He含量能用来重建含水层的水文历史吗？从含水层固体中直接测量4He的释放量将与基于地下水中放射性成因的4He浓度的释放率以及使用~3H/3He和氯氟化碳直接确定地下水年龄进行比较。对释放机制的评估，包括释放速率的潜在空间变化，将被用来评估4He测年地下水在50到500个时间尺度上的可行性；现有的地下水测年方法在这个对环境具有重要意义的时间尺度上是不准确的。评估人类活动对地下水资源质量的潜在影响取决于对流体速度和旅行时间的准确测量。然而，由于渗透率的极端变异性，传统的水力方法存在问题。地下水测年方法的实用性源于这样一个事实，即地下水年龄本身在流动系统中的任何一点都是沿整个上游流动路径的速度场的积分。因此，单一的年龄测量包含了记忆，包含了比点速度(或渗透率)测量更多的信息。因此，提供10年至数百年时间尺度上地下水传播时间的准确测量的方法，可以在目前无法获得的、解决水质和供水问题所需的详细程度上描述流动系统。?？

9637898 9628627 Poreda Solomon The concentration of radiogenic 4He in groundwater that results from U/Th decay has been used as a dating tool over time scales of 103 to 106 years. Recent measurements indicate that aquifer solids in many shallow flow systems may be releasing 4He at rates that are much greater than U/Th decay, making it possible to use 4He as a groundwater dating tool over time scales as short as 101 years. This proposal is to investigate the mechanisms leading to and the geographical extent of aquifers that are releasing 4He at rates greater than U/Th decay. The primary objective of the proposed research is to evaluate the use of 4He as a groundwater tracer over time scales of 10 to several hundred years. Specific research questions include the following: (1) What are the mechanisms controlling the exchange of 4He between aquifer solids and groundwater? Can this exchange rate be predicted and if so what is the uncertainty? (2) How spatially variable are He release rates? Are spatial variations correlated with variations in basic hydraulic properties (porosity, permeability) and/or geochemical properties (mineralogy, sedimentary facies, diagenesis)? (3) What is the geographical extent of shallow aquifers that are releasing 4He at rates that are significantly greater than U/Th decay? Is it possible to predict the occurrence of high 4He release rates based on general geologic concepts? (4) What are 4He release rates in carbonate and other fractured rock systems? (5) To what extent does the total 4He content of aquifer solids represent the paleohydrology of the aquifer protolith? Can total 4He contents be used to reconstruct the hydrologic history of an aquifer? Direct measurements of the 4He release from aquifer solids will be compared with release rates based on radiogenic 4He concentrations in groundwater and direct determinations of groundwater age using 3H/3He and chlorofluorocarbons. An evaluation of release mechanisms includi ng potential spatial variations in release rates will be used to assess the viability of dating groundwater with 4He over the time scale of 50 to 500; existing groundwater dating methods are not accurate over this environmentally important time scale. Evaluating the potential impacts of anthropogenic practices on the quality of groundwater resources depends on an accurate measure of fluid velocities and travel times. However, traditional hydraulic approaches are problematic due to extreme variability in permeability. The utility of groundwater dating methods results from the fact that the groundwater age itself, at any point in a flow system, is an integration of the velocity field along the entire upstream flow path. Thus, a single measurement of age contains memory and embodies much more information than a point measurement of velocity (or permeability). Methods that provide accurate measures of groundwater travel times over time scales of 10 to hundreds of years may thus allow a characterization of flow systems at a level of detail that is presently not available and yet is needed to solve water quality as well as water supply problems. ??