Exchange-Corrected CFC Dating of Stream Flow

0309212 . .地下水的平均运移时间是地下水流系统的一个基本的、可靠的特征。在无承压含水层中，地下水平均运移时间与(1)含水层中储水量和(2)进出含水层的水量有关。氚等环境示踪剂已被用于估计平均过境时间，但通常需要从20世纪60年代初到大约1980年的时间序列测量，而这样的数据集非常罕见。使用溶解气体如氯氟烃（CFCs）精确测定地下水年代已经变得相对普遍。如果在溪流的基流中保存氯氟化碳地下水年龄，即使没有长期时间序列，也有可能获得流量加权平均过境时间估计。虽然人们可能认为溪流中的氟氯化碳会与大气平衡，但在基流条件下从各种溪流中收集的样品与大气中的氟氯化碳并不平衡（在溪流的温度和高度下），这表明与大气的交换是缓慢的。拟议研究的中心假设是，如果非现代地下水的通量可以抵消与大气的交换速率，则获得的溪流将无法与大气中的氟氯化碳平衡。进一步假设，通过量化某一特定溪流的气体交换速率，可以估计地下水中氯氟烃的平均浓度。如果这些假设能够得到验证，那么将存在一种获取地下综合水文特性的新工具。然后，这些综合值可以构成将钻孔尺度测量的属性转换为地面流域尺度（即升级）的基础。这项研究将主要在犹他州瓦萨奇山脉的红丘流域进行，并将研究结果应用于科罗拉多高原的区域溪流。气体交换机制，包括气泡形成的影响，将通过在溪流中进行气体示踪剂测试来评估。通过将短屏压力表中的地下水年龄与溪流基流的综合值进行比较，考虑含水层性质（厚度、比产率等）和优先（大孔）流动的空间变异性。开发一种工具来评估地下流域的平均传输时间，将为开发基于地下水和溶质平均传输时间的可测量值的国家水资源管理和保护的新范例提供基础。

0309212..SolomonThe mean transit time of groundwater is a fundamental and robust characteristic of a subsurface flow system. In unconfined aquifers, the mean groundwater transit time is related to (1) the volume of water stored in the aquifer and (2) the flux of water into or out of the aquifer. Environmental tracers such as tritium have been used to estimate the mean transit time, but generally require a time series of measurements from the early 1960s to approximately 1980 and such data sets are very rare. Precise groundwater dating using dissolved gases such as chlorofluorocarbons (CFCs) has become relatively common. If CFC groundwater ages were preserved in the base flow of streams it might be possible to obtain flow-weighted mean transit time estimates even when long-term time series are not available. Although one might think that CFCs in streams would equilibrate with the atmosphere, samples collected from a variety of streams during base flow conditions are NOT in equilibrium with atmospheric CFCs (at the temperature and elevation of the stream) suggesting that exchange with the atmosphere is sluggish.The central hypothesis of the proposed research is that gaining streams will not equilibrate with CFCs in the atmosphere provided that the flux of non-modern groundwater can offset the rate of exchange with the atmosphere. It is further hypothesized that by quantifying the rate of gas exchange in a particular stream, the mean concentration of CFCs in groundwater can be estimated. If these hypotheses can be validated, a new tool will exist for obtaining integrated hydrologic properties in the subsurface. Such integrated values can then form the basis for transforming properties measured at the scale of boreholes to the scale of a groundwatershed (i.e. upscaling.)This research will be conducted primarily in the Red Butte watershed in the Wasatch Mountains, Utah, along with an application of the results to regional streams draining the Colorado Plateau. Gas exchange mechanisms, including the effects of bubble formation, will be evaluated by conducting gas tracer tests in streams. The spatial variability of aquifer properties (thickness, specific yield, etc.) and preferential (macropore) flow will be considered by comparing groundwater ages in short-screen piezometers with integrated values obtained in stream baseflow.The development of a tool to evaluate the mean transit time of a subsurface watershed will provide the basis for developing new paradigms for the regulation and protection the Nation's water resources that are based on measurable values for the average transit time of water and solutes in the subsurface.