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

9637898 小行星9628627 Solomon 地下水中放射性4He的浓度，从U/Th衰变的结果已被用作一个测年工具，在103至106年的时间尺度。 最近的测量表明，在许多浅流系统中的含水层固体可能会释放4He的速率比U/Th衰变大得多，使之有可能使用4He作为一个地下水测年工具，在短至101年的时间尺度。 这项建议是调查的机制，导致和地理范围的含水层，释放4He的速度大于U/Th衰变。 拟议的研究的主要目标是评估使用4He作为地下水示踪剂在10到几百年的时间尺度。 具体的研究问题包括： (1)控制4He在含水层固体和地下水之间交换的机制是什么？ 这种汇率可以预测吗？如果可以，不确定性是什么？ (2)He释放率的空间变化有多大？ 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）的变化相关？ (3)以显著大于U/Th衰变的速率释放4He的浅层含水层的地理范围是多少？ 是否有可能预测发生的高4He释放率的基础上一般地质概念？ (4)碳酸盐岩和其他裂隙岩石系统中的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含水层原岩的古水文？ 总4He含量能用来重建含水层的水文历史吗？ 4He从含水层固体释放的直接测量将进行比较与释放率的基础上放射性4He浓度在地下水和直接测定地下水年龄使用3H/3He和氯氟烃。 释放机制的评价，包括释放速率的潜在空间变化，将用于评估在50至500的时间尺度上用4He测定地下水年代的可行性;现有的地下水年代测定方法在这个对环境重要的时间尺度上不准确。 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化，传统的水力方法是有问题的。 地下水定年方法的实用性在于，在水流系统中的任何一点，地下水年龄本身是沿整个上游水流路径沿着的速度场的积分，因此，单一年龄测量包含记忆，比点速度（或渗透率）测量包含更多的信息。 因此，提供10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以目前不可用但解决水质和供水问题所需的详细程度来表征流动系统。 ??

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. ??