Known locally as the water mountain, for millennia Japan’s iconic Mt Fuji has provided safe drinking water to millions of people via a vast network of groundwater and freshwater springs. Groundwater, which is recharged at high elevations, flows down Fuji’s flanks within three basaltic aquifers, ultimately forming countless pristine freshwater springs among Fuji’s foothills. Here we challenge the current conceptual model of Fuji being a simple system of laminar groundwater flow with little to no vertical exchange between its three aquifers. This model contrasts strongly with Fuji’s extreme tectonic instability due to its unique location on top of the only known continental trench–trench–trench triple junction, its complex geology and its unusual microbial spring water communities. On the basis of a unique combination of microbial environmental DNA, vanadium and helium tracers, we provide evidence for prevailing deep circulation and a previously unknown deep groundwater contribution to Fuji’s freshwater springs. The most substantial deep groundwater upwelling has been found along Japan’s most tectonically active region, the Fujikawa-kako Fault Zone. Our findings broaden the hydrogeological understanding of Fuji and demonstrate the vast potential of combining environmental DNA, on-site noble gas and trace element analyses for groundwater science.

Environmental gas tracers allow inferring groundwater travel times and mixing ratios. Their concentrations are commonly interpreted with simplified and indirect approaches that are conceptually at odds with the high degree of complexity found in natural systems. However, the information content of the tracers can potentially be fully explored through the explicit simulation of an advection-dispersion transport equation, for example using integrated surface-subsurface hydrological models (ISSHMs). These integrated models can be used to explicitly simulate environmental tracers in complex environments. ISSHMs are usually variably saturated flow models. However, these models do not explicitly simulate gas partitioning with the aqueous phase, restricting explicit simulation of gas tracers to fully saturated conditions or to tracers with very low solubilities. We propose a mathematical formulation for the production of environmental gas tracers that are emanated in the subsurface. The production is scaled according to gas/water partitioning and water saturation, which is already computed by the model. Therefore, ISSHMs can now be used to their full potential to explicitly simulate tracer concentrations under variably saturated and dynamic conditions. The new formulation has been successfully verified against reference simulations provided with a multi-phase flow and transport model. In addition, explicit simulation of ²²²Rn and ³⁷Ar groundwater concentrations in a synthetic alluvial river-groundwater system was demonstrated, for the first time, with an ISSHM.

Für die Erstellung multipler Grundwassermodelle und deren Unsicherheitsanalyse existiert zwar eine Vielzahl an Methoden, aber in der Praxis bleibt dieses Potenzial oftmals ungenützt. Da Standard-Grundwassermessdaten nicht genügend Informationen für das Identifizieren hydraulischer Parameter beinhalten und auch das Hinzuziehen von unkonventionellen Messdaten Parameterunsicherheiten nicht komplett eliminiert, ist eine Unsicherheitsanalyse von Grundwassermodellen unabdingbar. Dieser Artikel zeigt Vorteile und Wege auf, wie dies in der Praxis umgesetzt werden kann.

Alluvial sand and gravel (ASG) aquifers are highly heterogeneous and exhibit strong, spatially variable anisotropy, often interspersed by buried paleo-channels of increased hydraulic conductivity. Groundwater flow and solute transport is often characterized by preferential flow caused by anisotropic properties in ASG aquifers. Connected ASG subsurface structures such as buried paleo-channels, however, are difficult to reproduce with commonly used techniques, and anisotropy is rarely considered in applied groundwater models. To ease the notoriously difficult problem of how to consider anisotropy, we propose a novel modeling framework based on calibration of an integrated surface-subsurface hydrological model via spatially varying, preferred anisotropy pilot point inversion. The inversion leverages hydraulic and tracer-based observations representing multiple spatial and temporal scales. We demonstrate the applicability of the framework on a real-world ASG site used for drinking water production, and we quantify the information content of observations to identify connected paleo-channels and provide guidance for optimal field-data acquisition.

Understanding the mixing between surface water and groundwater as well as groundwater travel times in vulnerable aquifers is crucial to sustaining a safe water supply. Age dating tracers used to infer apparent travel times typically refer to the entire groundwater sample. A groundwater sample, however, consists of a mixture of waters with a distribution of travel times. Age dating tracers only reflect the proportion of the water that is under the dating range of the used tracer, thus their interpretation is typically biased. Additionally, end-member mixing models are subject to various sources of uncertainties, which are typically neglected. In this study, we introduce a new framework that untangles groundwater mixing ratios and travel times using a novel combination of in-situ noble gas analyses. We applied this approach during a groundwater pumping test carried out in a pre-alpine Swiss valley. First, we calculated transient mixing ratios between recently infiltrated river water and regional groundwater present in a wellfield, using helium-4 concentrations combined with a Bayesian end-member mixing model. Having identified the groundwater fraction of recently infiltrated river water (F_rw) consequently allowed us to infer the travel times from the river to the wellfield, estimated based on radon-222 activities of F_rw. Furthermore, we compared tracer-based estimates of F_rw with results from a calibrated numerical model. We demonstrate (i) that partitioning of major water sources enables a meaningful interpretation of an age dating tracer of the water fraction of interest and (ii) that the streambed has a major control on the estimated travel times.

Snowmelt contributes a significant fraction of groundwater recharge in snow‐dominated regions, making its accurate quantification crucial for sustainable water resources management. While several components of the hydrological cycle can be measured directly, catchment‐scale recharge can only be quantified indirectly. Stable water isotopes are often used as tracers to estimate snowmelt recharge, even though estimates based on stable water isotopes are biased due to the large variations of δ²H and δ¹⁸O in snow and the difficulty to measure snowmelt directly. To overcome this gap, a new tracer method based on on‐site measurements of dissolved He, ⁴⁰Ar, ⁸⁴Kr, N₂, O₂ and CO₂ is presented. The new method was developed alongside classical tracer methods (stable water isotopes, ²²²Rn, ³H/³He) in a highly instrumented boreal catchment. By revealing (noble gas) recharge temperatures and excess air, dissolved gases allow (i) the contribution of snowmelt to recharge, (ii) the temporal recharge dynamics, and (iii) the primary recharge pathways to be identified. In contrast to stable water isotopes, which produced highly inconsistent snowmelt recharge estimates for the experimental catchment, dissolved gases produced consistent estimates even when the temperature of snowmelt during recharge was not precisely known. As dissolved gases are not controlled by the same processes as stable water isotopes, they are not prone to the same biases and represent a highly complementary tracer method for the quantification of snowmelt recharge dynamics in snow‐dominated regions. Furthermore, an observed systematic depletion of N₂ in groundwater provides new evidence for the pathways of biological N‐fixation in boreal forest soils.

To provide a sound understanding of the sources, pathways, and residence times of groundwater water in alluvial river-aquifer systems, a combined multitracer and modeling experiment was carried out in an important alluvial drinking water wellfield in Switzerland. ²²²Rn, ³H/³He, atmospheric noble gases, and the novel ³⁷Ar-method were used to quantify residence times and mixing ratios of water from different sources. With a half-life of 35.1 days, ³⁷Ar allowed to successfully close a critical observational time gap between ²²²Rn and ³H/³He for residence times of weeks to months. Covering the entire range of residence times of groundwater in alluvial systems revealed that, to quantify the fractions of water from different sources in such systems, atmospheric noble gases and helium isotopes are tracers suited for end-member mixing analysis. A comparison between the tracer-based mixing ratios and mixing ratios simulated with a fully-integrated, physically-based flow model showed that models, which are only calibrated against hydraulic heads, cannot reliably reproduce mixing ratios or residence times of alluvial river-aquifer systems. However, the tracer-based mixing ratios allowed the identification of an appropriate flow model parametrization. Consequently, for alluvial systems, we recommend the combination of multitracer studies that cover all relevant residence times with fully-coupled, physically-based flow modeling to better characterize the complex interactions of river-aquifer systems.