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

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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.