Exchange-Corrected CFC Dating of Stream Flow

0309212.Solomon地下水的平均通过时间是地下水流系统的基本和健壮的特性。在非承压含水层中，地下水的平均通过时间与(1)含水层中储存的水量和(2)进出含水层的水流量有关。环境示踪剂，如氚，已被用来估计平均通过时间，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，这样的数据集非常罕见。使用氯氟烃(CFCs)等溶解气体对地下水进行精确测年已变得相对常见。如果将氯氟化碳地下水年龄保存在河流的基本流量中，即使在没有长期时间序列的情况下，也有可能获得流量加权的平均通过时间估计数。虽然人们可能认为溪流中的CFCs会与大气平衡，但在基流条件下从各种溪流中采集的样本与大气CFCs(在溪流的温度和海拔)之间并不平衡，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，那么获得的溪流将不会与大气中的CFCs平衡。进一步的假设是，通过量化特定溪流中的气体交换速率，可以估计地下水中CFCs的平均浓度。如果这些假设能够被证实，将会有一种新的工具来获得地下的综合水文特性。然后，这些积分值可以形成将以钻孔尺度测量的特性转换为地面流域尺度(即，放大尺度)的基础。这项研究将主要在犹他州瓦萨奇山脉的Red Butte分水岭进行，同时将结果应用于排放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。含水层特性(厚度、比产量等)的空间变异性。将通过比较短屏幕压力计中的地下水年龄和在河流基流中获得的积分值来考虑优先(大孔隙)流。开发一种评估地下分水岭平均通过时间的工具将为开发基于水和溶质在地下平均通过时间的可测量值的国家水资源调控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.