Advancing Physically‐Based Flow Simulations of Alluvial Systems Through Atmospheric Noble Gases and the Novel ³⁷Ar Tracer Method

Abstract

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.

Simulating Flood‐Induced Riverbed Transience Using Unmanned Aerial Vehicles, Physically Based Hydrological Modeling, and the Ensemble Kalman Filter

“…HGS has been used for the simulation of many different hydrological systems, for example, for the interactions between GW, SW and vegetation (e.g., Ala‐aho et al, ; Banks et al, ; Schilling et al, ; Schomburg et al, ), for the detection of the sensitivity of catchment scale dynamics to the different model parameters (Cornelissen et al, ), to track catchment scale SW levels and overland flow routing (Ameli & Creed, ), to explore the hydrological dynamics of wetlands (Liu et al, ) and microtopographic wetland runoff (Frei et al, ), and for large‐scale solute transport (Blessent et al, ). In a few instances HGS was calibrated against different types of data (e.g., Brunner et al, ; Karan et al, ; Schilling et al, ; Schilling, Gerber, et al, ) using the automatic inverse code PEST (Doherty, ). Recently, HGS has been coupled to a sequential DA routine (EnKF‐HGS) by Kurtz et al ().…”

Integrated Surface and Subsurface Hydrological Modeling with Snowmelt and Pore Water Freeze–Thaw

“…Investigations on the interactions between groundwater, surface water, and vegetation (e.g., Ala‐Aho et al [], Schilling et al [], Schomburg et al []), on the development of unsaturated zones between rivers and aquifers in heterogeneous systems (e.g., Irvine et al [], Schilling et al [], Tang et al []), or on contaminant transport and tile drainage in agricultural contexts (e.g., Bonton et al [], De Schepper et al []) are just some recent examples for which HGS was used. HGS has recently been coupled to the Weather Research and Forecast (WRF) model for the integrated simulation of atmosphere, surface, and subsurface interactions (Davison et al ), to particle tracking and flow tracking tools (Partington et al ; Partington et al ; Chow et al ; Schilling et al ), and to the ensemble Kalman filter (EnKF) data assimilation tool (Kurtz et al ; Tang et al ; Tang et al ).…”

“…The modified tilted‐V catchment with alluvial drinking water production is a typical setup in mountainous and peri‐alpine catchments, where winter processes play a key role (Kurtz et al ; Schilling et al ). If winter processes are not taken into account when managing groundwater pumping stations in such systems, during the winter season groundwater abstraction may exceed sustainable amounts because recharge is strongly reduced by the retention of precipitation as snow.…”

“…A total of 10 layers of elements are used to discretize the catchment in the vertical z -direction, with the following decreasing element thicknesses, from bottom to top: 10, 10, 6, 3, 2, 1.1, 0.5, 0.25, 0.1, and 0.5 m. The resulting mesh has a total of 80 × 50 × 11 nodes along the x -, yand z -directions, respectively. The modified tilted-V catchment with alluvial drinking water production is a typical setup in mountainous and peri-alpine catchments, where winter processes play a key role Schilling et al 2017a). If winter processes are not taken into account when managing groundwater pumping stations in such systems, during the winter season groundwater abstraction may exceed sustainable amounts because recharge is strongly reduced by the retention of precipitation as snow.…”

Using environmental tracers to determine the relative importance of travel times in the unsaturated and saturated zones for the delay of nitrate reduction measures

“…However, hydrogeological models require not only more detailed data about aquifer geometry and properties, but also enough suitable data for calibration of unknown parameters. Especially for the USZ and the mixing of different water masses in the SZ, a calibration based on groundwater heads might not be sufficient, and additional tracer data or measurements from the USZ might be necessary for robust results (Schilling et al, 2017;Visser et al, 2009). Furthermore, Eberts et al (2012) showed that if analogous conceptual models are used, LPMs and hydrogeological models often result in very similar contaminant predictions.…”