Imaging the Subsurface: The Effect of Logjams on Groundwater-Surface Water Exchange

Conventional methods of quantifying subsurface interactions in streams are often insufficient in characterizing hydrological processes and demand better techniques to measure complex dynamics. Point-measurements are incomplete and do not provide direct data on hyporheic exchange, a mixing of surface and subsurface water under and around a stream capable of influencing ecosystem processes. Fortunately, geophysical techniques can improve analysis of the hyporheic zone. For the first time in a field setting, we apply electrical resistivity imaging (ERI) to a mountainous stream below a channel-spanning logjam to estimate the extent of hyporheic exchange. Previous studies suggest logjams increase hydraulic resistance and drive water deeper into the hyporheic zone; our results imply water remains in the subsurface longer around a logjam, thus increasing a stream’s ability to process nutrients and solutes. A comparison between fluid conductivity and bulk conductivity proved ERI as a more spatially comprehensive technique in quantifying the extent of hyporheic exchange. These results highlight the critical role that logjams play in hyporheic zone dynamics, which could affect ecosystem health: future applications of this study can aid in conserving, managing, and restoring riverine systems

Data Associated with "Logjam Characteristics as Drivers of Transient Storage in Headwater Streams"

This repository contains data for the experimental flume and numerical model runs referenced in Marshall et al., 2022. Source code, data csv files for statistical analysis, and calculated temporal moments are included.Logjams in a stream create backwater conditions and locally force water to flow through the streambed, creating zones of transient storage within the surface and subsurface of a stream. We investigate the relative importance of logjam distribution density, logjam permeability, and discharge on transient storage in a simplified experimental channel. We use physical flume experiments in which we inject a salt tracer, monitor fluid conductivity breakthrough curves in surface water, and use breakthrough-curve skew to characterize transient storage. We then develop numerical models in HydroGeoSphere to reveal flow paths through the subsurface (or hyporheic zone) that contribute to some of the longest transient-storage timescales. In both the flume and numerical model, we observe an increase in backwater and hyporheic exchange at logjams. Observed complexities in transient storage behavior may depend largely on surface water flow in the backwater zone. As expected, multiple successive logjams provide more pervasive hyporheic exchange by distributing the head drop at each jam, leading to distributed but shallow flow paths. Decreasing the permeability of a logjam or increasing the discharge both facilitate more surface water storage and elevate the surface water level upstream of a logjam, thus increasing hyporheic exchange. Multiple logjams with low permeability result in the greatest magnitude of transient storage, suggesting that this configuration maximizes solute retention in backwater zones, while hyporheic exchange rates also increase. Understanding how logjam characteristics affect solute transport through both the channel and hyporheic zone has important management implications for rivers in forested, or historically forested, environments.This research was supported by National Science Foundation Hydrologic Sciences (award 1819068)

The emergence of hydrogeophysics for improved understanding of subsurface processes over multiple scales

Geophysics provides a multi-dimensional suite of investigative methods that are transforming our ability to see into the very fabric of the subsurface environment, and monitor the dynamics of its fluids and the biogeochemical reactions that occur within it. Here, we document how geophysical methods have emerged as valuable tools for investigating shallow subsurface processes over the past two decades and offer a vision for future developments relevant to hydrology and also ecosystem science. The field of “hydrogeophysics” arose in the late 1990s, prompted, in part, by the wealth of studies on stochastic subsurface hydrology that argued for better field-based investigative techniques. These new hydrogeophysical approaches benefited from the emergence of practical and robust data inversion techniques, in many cases with a view to quantify shallow subsurface heterogeneity and the associated dynamics of subsurface fluids. Furthermore, the need for quantitative characterization stimulated a wealth of new investigations into petrophysical relationships that link hydrologically relevant properties to measurable geophysical parameters. Development of time-lapse approaches provided a new suite of tools for hydrological investigation, enhanced further with the realization that some geophysical properties may be sensitive to biogeochemical transformations in the subsurface environment, thus opening up the new field of “biogeophysics”. Early hydrogeophysical studies often concentrated on relatively small ‘plot-scale’ experiments. More recently, however, the translation to larger-scale characterization has been the focus of a number of studies. Geophysical technologies continue to develop, driven, in part, by the increasing need to understand and quantify key processes controlling sustainable water resources and ecosystem services

Anomalous solute transport in saturated porous media:linking transport model parameters to electrical and nuclear magnetic resonance properties

The advection-dispersion equation (ADE) fails to describe commonly observed non-Fickian solute transport in saturated porous media, necessitating the use of other models such as the dual-domain mass-transfer (DDMT) model. DDMT model parameters are commonly calibrated via curve fitting, providing little insight into the relation between effective parameters and physical properties of the medium. There is a clear need for material characterization techniques that can provide insight into the geometry and connectedness of pore spaces related to transport model parameters. Here, we consider proton nuclear magnetic resonance (NMR), direct-current (DC) resistivity, and complex conductivity (CC) measurements for this purpose, and assess these methods using glass beads as a control and two different samples of the zeolite clinoptilolite, a material that demonstrates non-Fickian transport due to intragranular porosity. We estimate DDMT parameters via calibration of a transport model to column-scale solute tracer tests, and compare NMR, DC resistivity, CC results, which reveal that grain size alone does not control transport properties and measured geophysical parameters; rather, volume and arrangement of the pore space play important roles. NMR cannot provide estimates of more-mobile and less-mobile pore volumes in the absence of tracer tests because these estimates depend critically on the selection of a material-dependent and flow-dependent cutoff time. Increased electrical connectedness from DC resistivity measurements are associated with greater mobile pore space determined from transport model calibration. CC was hypothesized to be related to length scales of mass transfer, but the CC response is unrelated to DDMT

A field comparison of multiple techniques to quantify groundwater–surface-water interactions

Groundwater–surface-water (GW-SW) interactions in streams are difficult to quantify because of heterogeneity in hydraulic and reactive processes across a range of spatial and temporal scales. The challenge of quantifying these interactions has led to the development of several techniques, from centimeter-scale probes to whole-system tracers, including chemical, thermal, and electrical methods. We co-applied conservative and smart reactive solute-tracer tests, measurement of hydraulic heads, distributed temperature sensing, vertical profiles of solute tracer and temperature in the stream bed, and electrical resistivity imaging in a 450-m reach of a 3rd-order stream. GW-SW interactions were not spatially expansive, but were high in flux through a shallow hyporheic zone surrounding the reach. NaCl and resazurin tracers suggested different surface–subsurface exchange patterns in the upper ⅔ and lower ⅓ of the reach. Subsurface sampling of tracers and vertical thermal profiles quantified relatively high fluxes through a 10- to 20-cm deep hyporheic zone with chemical reactivity of the resazurin tracer indicated at 3-, 6-, and 9-cm sampling depths. Monitoring of hydraulic gradients along transects with MINIPOINT streambed samplers starting ∼40 m from the stream indicated that groundwater discharge prevented development of a larger hyporheic zone, which progressively decreased from the stream thalweg toward the banks. Distributed temperature sensing did not detect extensive inflow of ground water to the stream, and electrical resistivity imaging showed limited large-scale hyporheic exchange. We recommend choosing technique(s) based on: 1) clear definition of the questions to be addressed (physical, biological, or chemical processes), 2) explicit identification of the spatial and temporal scales to be covered and those required to provide an appropriate context for interpretation, and 3) maximizing generation of mechanistic understanding and reducing costs of implementing multiple techniques through collaborative research.This is the publisher’s final pdf. The article is copyrighted by the Society for Freshwater Science and published by the University of Chicago Press. It can be found at: http://www.jstor.org/action/showPublication?journalCode=fresscie

Final Report for "Toward Quantifying Kinetics of Biotic and Abiotic Metal Reduction with Electrical Geophysical Methods" DE-FG02-08ER64520

Although changes in the bulk electrical conductivity in aquifers have been attributed to microbial activity, electrical conductivity has never been used to infer biogeochemical reaction rates quantitatively. To explore the use of electrical conductivity to measure reaction rates, we conducted iron oxide reduction experiments of increasing biological complexity. To quantify reaction rates, we proposed composite reactions that incorporated the stiochiometry of five different types of reactions: redox, acid-based, sorption, dissolution/precipitation, and biosynthesis. In batch and column experiments, such reaction stiochiometries inferred from a few chemical measurements allowed quantification of the Fe-oxide reduction rate based on changes in electrical conductivity. The relationship between electrical conductivity and fluid chemistry did not hold during the latter stages of the column experiment when electrical conductivity increased while fluid chemistry remained constant. Growth of an electrically conductive biofilm could explain this late stage electrical conductivity increase. This work demonstrates that measurements of electrical conductivity and flow rate, combined with a few direct chemical measurements, can be used to quantify biogeochemical reaction rates in controlled laboratory situations and may be able to detect the presence of biofilms