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Brunner, Philip

Nom
Brunner, Philip
Affiliation principale
Fonction
Professeur ordinaire
Email
philip.brunner@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/165
_
0000-0001-6304-6274

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  • Publication
    Accès libre
    Noble gases as tracers of surface water – groundwater interactions: insights from novel field and modelling approaches
    (Neuchâtel : Université de Neuchâtel, 2023)
    Peel, Morgan 
    ;
    Brunner, Philip 
    Understanding the interactions between surface water (SW) and groundwater (GW) in alluvial aquifer systems is of critical importance for the sustainable management of water resources. Advances in realtime and continuous measurement of a range of hydrological tracers provide new opportunities for the characterization of SW-GW dynamics at unprecedented spatiotemporal resolutions. Amongst several promising tracer methods, noble gases are particularly well-adapted to the study of SW-GW interactions, and provide an integrated signal on the flow paths and travel times of water. Capitalizing on the insights offered by novel measurement technologies requires tracer interpretation methods that appropriately capture tracer transport processes in dynamic environmental conditions. However, recourse to highly-simplified tracer interpretation methods, conceptually detached from the complexity of natural systems, is still widespread. In such cases, the potentially rich information content of tracer measurements may be underutilized. This thesis aims at investigating how established and emerging noble gas tracer methods can be optimally used - and when they should be avoided - for the study of SW-GW interactions in alluvial aquifer systems. To this end, a range of novel laboratory, field, and modelling approaches are employed to systematically assess the applicability, limitations, and potential of some gas tracer methods toward informing SW-GW exchange processes. The first gas tracer method examined is the 222Rn apparent age model, which is widely used to estimate the ages of very young GW (days to weeks). High-resolution measurements of the spatial distribution of 222Rn emanation rates in an alluvial aquifer reveal significant spatial heterogeneity in 222Rn production. The explicit simulation of 222Rn in synthetic mass-transport models shows that this level of heterogeneity, combined with mixing of GW, can result in strongly biased estimates of GW age, effectively limiting the applicability of the 222Rn apparent age method to a limited range of field conditions. Although temporal changes in 222Rn concentrations may reveal insights into GW age dynamics, the information content of 222Rn measurements may be best extracted through the integration of 222Rn observations in the calibration process of physics-based flow and transport models. Indeed, the second part of this thesis is devoted to exploring how the explicit simulation of tracer concentrations and the assimilation of untransformed tracer data in highly parameterized, physicsbased models may inform model parameters and ultimately predictions of management interest. Within this framework, the joint assimilation of hydraulic and noble gas data (222Rn and helium), acquired over the course of a novel tracer injection experiment, is shown to strongly inform model parameters and reduce predictive uncertainty of several important water management quantities, such as GW age, SW-GW mixing ratios, and SW infiltration fluxes, far beyond what is achieved with “traditional” hydraulic data alone. These results build upon mounting evidence as to the benefits of explicitly simulating and assimilating diverse observation types with physically-based flow and transport models, avoiding the layer of conceptual simplification and potential bias accrued with simplified tracer interpretation models. Finally, the successful combination of novel gas injection methods, developed over the course of this project, and the assimilation of high-resolution gas tracer measurements in an explicit tracer simulation framework strongly support further developments of (noble) gas tracer methods and tracer-numerical model synergies.
  • Publication
    Accès libre
    The influence of riverbed heterogeneity patterns on river-aquifer exchange fluxes under different connection regimes
    (2017-9)
    Tang, Qi
    ;
    Kurtz, W.
    ;
    Schilling, Oliver 
    ;
    Brunner, Philip 
    ;
    Vereecken, H.
    ;
    Hendricks Franssen, Harrie-Jan
    Riverbed hydraulic conductivity (K) is a critical parameter for the prediction of exchange fluxes between a river and an aquifer. In this study, the role of heterogeneity patterns was explored using the fully integrated hydrological model HydroGeoSphere simulating complex, variably saturated subsurface flow. A synthetic 3-D river-aquifer reference model was constructed with a heterogeneous riverbed using nonmulti-Gaussian patterns in the form of meandering channels. Data assimilation was used to test the ability of different riverbed K patterns to reproduce hydraulic heads, riverbed K and river-aquifer exchange fluxes. Both fully saturated as well as variably saturated conditions underneath the riverbed were tested. The data assimilation experiments with the ensemble Kalman filter (EnKF) were carried out for four types of geostatistical models of riverbed K fields: (i) spatially homogeneous, (ii) heterogeneous with multiGaussian distribution, (iii) heterogeneous with non-multi-Gaussian distribution (channelized structures) and (iv) heterogeneous with non-multi-Gaussian distribution (elliptic structures). For all data assimilation experiments, state variables and riverbed K were updated by assimilating hydraulic heads. For saturated conditions, heterogeneous geostatistical models allowed a better characterization of net exchange fluxes than a homogeneous approximation. Among the three heterogeneous models, the performance of non-multi-Gaussian models was superior to the performance of the multi-Gaussian model, but the two tested non-multi-Gaussian models showed only small differences in performance from one another. For the variably saturated conditions both the multi-Gaussian model and the homogeneous model performed clearly worse than the two non-multi-Gaussian models. The two non-multi-Gaussian models did not show much difference in performance. This clearly shows that characterizing heterogeneity of riverbed K is important. Moreover, particularly under variably saturated flow conditions the mean and the variance of riverbed K do not provide enough information for exchange flux characterization and additional histogram information of riverbed K provides crucial information for the reproduction of exchange fluxes.
  • Publication
    Accès libre
    Characterisation of river–aquifer exchange fluxes: The role of spatial patterns of riverbed hydraulic conductivities
    (2015-12)
    Tang, Qi
    ;
    Kurtz, W.
    ;
    Brunner, Philip 
    ;
    Vereecken, H.
    ;
    Hendricks Franssen, Harrie-Jan
    Interactions between surface water and groundwater play an essential role in hydrology, hydrogeology, ecology, and water resources management. A proper characterisation of riverbed structures might be important for estimating river–aquifer exchange fluxes. The ensemble Kalman filter (EnKF) is commonly used in subsurface flow and transport modelling for estimating states and parameters. However, EnKF only performs optimally for MultiGaussian distributed parameter fields, but the spatial distribution of streambed hydraulic conductivities often shows non-MultiGaussian patterns, which are related to flow velocity dependent sedimentation and erosion processes. In this synthetic study, we assumed a riverbed with non-MultiGaussian channel-distributed hydraulic parameters as a virtual reference. The synthetic study was carried out for a 3-D river–aquifer model with a river in hydraulic connection to a homogeneous aquifer. Next, in a series of data assimilation experiments three different groups of scenarios were studied. In the first and second group of scenarios, stochastic realisations of non-MultiGaussian distributed riverbeds were inversely conditioned to state information, using EnKF and the normal score ensemble Kalman filter (NS-EnKF). The riverbed hydraulic conductivity was oriented in the form of channels (first group of scenarios) or, with the same bimodal histogram, without channelling (second group of scenarios). In the third group of scenarios, the stochastic realisations of riverbeds have MultiGaussian distributed hydraulic parameters and are conditioned to state information with EnKF. It was found that the best results were achieved for channel-distributed non-MultiGaussian stochastic realisations and with parameter updating. However, differences between the simulations were small and non-MultiGaussian riverbed properties seem to be of less importance for subsurface flow than non-MultiGaussian aquifer properties. In addition, it was concluded that both EnKF and NS-EnKF improve the characterisation of non-MultiGaussian riverbed properties, hydraulic heads and exchange fluxes by piezometric head assimilation, and only NS-EnKF could preserve the initial distribution of riverbed hydraulic conductivities.