Advancing measurements and representations of subsurface heterogeneity and dynamic processes: towards 4D hydrogeology

Abstract

Abstract. Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability needs to be considered when studying hydrogeological processes in order to employ adequate mechanistic models or perform upscaling. The scale at which a hydrogeological system should be characterized in terms of its spatial heterogeneity and temporal dynamics depends on the studied process and it is not always necessary to consider the full complexity. In this paper, we identify a series of hydrogeological processes for which an approach coupling the monitoring of spatial and temporal variability, including 4D imaging, is often necessary: (1) groundwater fluxes that control (2) solute transport, mixing and reaction processes, (3) vadose zone dynamics, and (4) surface-subsurface water interaction occurring at the interface between different subsurface compartments. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. Then, we highlight some recent innovations that have led to significant breakthroughs in this domain. We finally discuss how spatial and temporal fluctuations affect our ability to accurately model them and predict their behavior. We thus advocate a more systematic characterization of the dynamic nature of subsurface processes, and the harmonization of open databases to store hydrogeological data sets in their four-dimensional components, for answering emerging scientific question and addressing key societal issues.

Machine Learning for Bayesian Experimental Design in the Subsurface

“…Chapter 5: Groundwater-surface water interaction. The groundwater-surface water (GW-SW) exchange fluxes are driven by a complex interplay of subsurface processes and their interactions with surface hydrology (Hermans et al, 2022), which have a significant impact on the water and contaminant exchanges (e.g., Dujardin et al, 2014;Ghysels et al, 2021;Hermans et al, 2022;Irvine et al, 2016;Kikuchi and Ferré, 2017;Kurylyk et al, 2019;Moghaddam et al, 2022). Due to the complexity of these systems, the accurate estimation of GW-SW fluxes is important for quantitative hydrological studies and should be based on relevant data and careful experimental design.…”

“…Beven et al (2020) recognizes the need for improved observational technologies and network designs to support hypothesis testing in real catchments of interest that go beyond current monitoring capabilities, necessitating information from other studies as well as direct observations, remote sensing, intensive field campaigns, or other strategies. In a recent discussion about the future of hydrogeology, Hermans et al (2022) also advocate for the sharing of data in an open format. Quality.…”

“…Methods for quantifying groundwater-surface water (GW-SW) exchange flux have long been of interest in order to better understand water and solute exchange across the sediment-water interface as well as the magnitude, spatial distribution, and temporal dynamics of fluxes between subsurface and surface compartments of the Earth (Hermans et al., 2022). Therefore, understanding and quantifying the exchange processes between rivers and groundwater is crucial for the best possible protection of our water resources: the provision of drinking water, the characterization and management of environmental flow regimes, the preservation or restoration of riverine ecosystem health and functions, the attenuation of contaminants and water storage in aquifers are some of the challenges that are impacted by processes that occur at the GW-SW interface (Dujardin et al, 2014;Ghysels et al, 2021;Hermans et al, 2022;Irvine et al, 2016;Kikuchi and Ferré, 2017;Kurylyk et al, 2019;Moghaddam et al, 2022).…”

“…Water flux has traditionally been measured using temperature tracers, shallow piezometers, tracer experiments, and differential-discharge measurements. For a comprehensive review of the various types of water flux measurements and their outlooks, see Hermans et al (2022). Because GW and SW typically have different temperatures and advective…”

Estimation of Groundwater Flow Rate by an Actively Heated Fiber Optics Based Thermal Response Test in a Grouted Borehole

“…Proper knowledge of the groundwater flow rate is vital to water resource management, contaminant control, hydraulic structure design, and shallow geothermal energy exploitation (Bense et al., 2016; Herrera‐García et al., 2021; Stauffer et al., 2019; Xue et al., 1990). Due to the diversity of hydraulic conditions and the often‐significant heterogeneity of aquifers, reliable estimation is only feasible by field investigation (Brunner et al., 2017; Hermans et al., 2022; Ye et al., 2004). Most commonly, flow meters or tracer tests are applied for direct measurement, or piezometers are used to determine the direction and rate of groundwater flow.…”