Collaborative Research: Groundwater transit time distributions: bridging the gap between advanced tracer techniques and numerical modeling

地下水从补给到排入溪流的过渡时间是污染物从含水层冲入溪流的速度的重要控制因素。实地研究和地下水模型都表明，在任何给定的时间内，具有一定过渡时间范围的地下水对径流有贡献。然而，实地研究表明，年轻地下水(即运输时间较短的地下水)对河流的相对贡献比模型研究中计算的要小得多。地下水通过时间分布(TTD)的这种差异导致了对径流来源的完全不同的理解，对地下水排放对河流水质的未来影响的根本不同的预测，以及对管理含水层非点源污染努力的设计和评估的潜在不同观点。该项目将进行实地测量和地下水建模，以调查地下水TTD的差异，并确定和了解在一系列空间尺度上驱动TTD的过程。该项目的成果将有助于水资源管理者、监管者和顾问更好地了解和管理水量和水质。该项目的总体目标是：(1)评估从地下水测量(测年示踪剂的现场采样)得出的TTD与地下水模型之间存在差异的原因，(2)调查如何通过在一定范围(本项目中为14-3900平方公里)的嵌套流域中使用现场测量和模型来更好地理解这些过程，以及(3)确定控制地下水TTD形状的过程。该项目将在内布拉斯加州沙丘的High Plains含水层上的一个分水岭进行，那里的地下水约占年河流流量的90%。将使用年轻和古老地下水(~3H/3He、SF6、14C、4He)的测年示踪剂来确定TTD。地下水年龄信息将与溪流流量测量和河床管式渗流计地下水流量测量相结合。基于示踪剂的TTD将与在河段和分水岭尺度上的地下水数值模型(MODFLOW)中的颗粒跟踪模拟得到的地下水TTD进行比较。该模型将被用作调查示踪剂TTD和基于模型的TTD之间的差异的工具。例如，该项目将调查地下水补给相对于河道的变化，以及模型中更高的分辨率和更准确的补给表示是否会导致基于现场的TTD和基于模型的TTD之间更好的一致性。该模型还将使用地下水年龄数据进行重新校准，以评估改进和重新校准后TTD与现场估计的比较情况。信息将通过合作推广计划传播，包括www.nebraskawatershedScience ence.org上的互动在线培训材料。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The transit time of groundwater from recharge to discharge into streams is an important control on how quickly contaminants are flushed out of aquifers and into streams. Field studies and groundwater models both indicate that groundwater with a range of transit times contributes to streamflow at any given time. However, field studies suggest the relative contribution of young groundwater (i.e., groundwater with short transit times) to streams is substantially less than calculated in modeling studies. This discrepancy in the groundwater transit time distribution (TTD) leads to a very different understanding of streamflow sources, fundamentally different predictions of the future impact of groundwater discharge on stream water quality, and potentially different perspectives on the design and assessment of efforts to manage non-point-source contamination in aquifers. This project will conduct field measurements and groundwater modeling to investigate differences in groundwater TTDs, and to determine and understand the processes that drive TTD at a range of spatial scales. Results of the project will assist water resources managers, regulators, and consultants to better understand and manage both water quantity and quality. Information will be disseminated through Cooperative Extension programs.The overall goals of this project are to (1) evaluate why there is a discrepancy between TTDs derived from groundwater measurements (field sampling of age-dating tracers) and groundwater models, (2) investigate how these processes may be better understood by using both field measurements and models in nested watersheds at a range of scales (14-3900 km2 in this project) and (3) determine the processes that control the shape of the groundwater TTDs,. This project will be conducted in a watershed overlying the High Plains aquifer in the Nebraska Sandhills, where groundwater accounts for approximately 90% of the annual stream discharge. The TTDs will be determined using age-dating tracers for both young and old groundwater (3H/3He, SF6, 14C, 4He). Groundwater age information will be coupled with discharge measurements in streams and groundwater discharge measurement from tube seepage meters in streambeds. The tracer-based TTDs will be compared to groundwater TTDs derived from particle tracking simulations in a numerical groundwater model (MODFLOW), at both the reach and watershed scale. The model will be used as a tool to investigate differences that arise between tracer- and model-based TTDs. For example, the project will investigate the variation in groundwater recharge relative to the stream channel, and if a higher resolution and more accurate representation of recharge in the model leads to better agreement between field- and model-based TTDs. The model will also be re-calibrated using groundwater age data to evaluate how TTDs compare to field-based estimates after refinement and recalibration. Information will be disseminated through Cooperative Extension programs, including interactive online training materials at www.nebraskawatershedscience.org.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.