A Framework for Untangling Transient Groundwater Mixing and Travel Times

Abstract

Although these changes will profoundly influence groundwater recharge and discharge in mountainous environments (Hayashi, 2019), they have largely been ignored so far (Somers et al., 2019). Since surface water and groundwater resources are closely coupled, an improved understanding of surface water-groundwater interactions is highly relevant for a sustainable water governance as well as for water-dependent ecosystems in mountainous regions (e.g., Holman, 2006;Krause et al., 2014;Schilling et al., 2020).Within the last two decades, studies on river-aquifer exchange dynamics have substantially improved the understanding of the drivers (e.g., river discharge) and controls (e.g., riverbed hydraulic conductivity) of water exchange patterns and their impact on biogeochemical cycling of solutes (e.g., reviews by Boano et al., 2014;Brunner et al., 2017; Lewandowski et al., 2019 and references therein). Particularly, the continued recognition and investigation of riverbed dynamics as key controls on river-aquifer exchange have brought substantial scientific progress in the field of surface water-groundwater interactions (e.g., Mutiti & Levy, 2010;Tang et al., 2018). However, the spatiotemporal dynamics of surface water-groundwater interactions still remain elusive, mainly due to a lack of high-resolution field data (Barthel & Banzhaf, 2016;

Buried Paleo‐Channel Detection With a Groundwater Model, Tracer‐Based Observations, and Spatially Varying, Preferred Anisotropy Pilot Point Calibration

“…In agreement with the observations and the independent measurements of Popp et al. (2021), the upstream wells 1–3 and downstream wells 6–8 receive water primarily via the paleo‐channel, whereas the mid‐section wells 4 and 5 receive more regional GW (illustrated by flowlines in Figures 1e and 1f). Rather than K, the dominant control on exchange fluxes are the weirs, with the most complex patterns arising where the aquifer is thinnest (Figures 1c, 1d, and 1f).…”

“…Model calibration was sequential against GW level, residence time and f stream observations, revealing K aq = 550 m/d, K sb = 2.4 m/d and porosity = 0.1 as optimal homogeneous parameters. The homogeneous model robustly predicted f stream of an independent pumping and tracer experiment conducted 4 years later (Popp et al., 2021). While Tang et al.…”

“…Available borelogs confirm the existence of a buried paleo‐channel (classification: G2 and OF) at the reported depth (AWA, 2021), but they are too sparse for reliable paleo‐channel delineation. Recent tracer analyses revealed that upstream and downstream wells receive significantly larger f stream compared to mid‐section wells, hinting at a meandering, high‐K paleo‐channel (Popp et al., 2021; Schilling, Gerber, et al., 2017).…”

Rapid Advances in Mobile Mass Spectrometry Enhance Tracer Hydrology and Water Management

“…Dissolved noble gases analyzed by mobile MS systems have successfully been used to assess groundwater flow (Moeck et al., 2017), mixing between surface water and groundwater (Popp et al., 2021; 2019), and apparent groundwater ages (Moeck et al., 2021). Further examples of potential advances using mobile MS applications relate to groundwater recharge, particularly in mountainous regions.…”

“…Further examples include, but are not limited to, combining measurements of conservative water tracers like stable water isotopes with the analysis of reactive gases like CO 2 and CH 4 (concentration and/or δ 13 C), or dissolved organic carbon, to identify biogeochemical processes or leakage along flowpaths. Coupling radon measurement devices with mobile MS systems is also a promising way to locate groundwater inputs to streams or untangle mixing and travel times of very young water (Popp et al., 2021). Combining different measurement techniques on different spatial and temporal scales further provides an opportunity to monitor and detect long‐term changes and improve the integration of hydrology and biogeochemistry (e.g., Li et al., 2021; Vonk et al., 2019).…”