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Editorial

Advances in the Study and Understanding of Groundwater Discharge to Surface Water

1
WATEC, Department of Geoscience, Aarhus University, 8000 Aarhus, Denmark
2
Department of Geodynamics, University of Granada, 18071 Granada, Spain
3
United States Geological Survey, Water Mission Area, Denver, CO 80225, USA
*
Author to whom correspondence should be addressed.
Water 2022, 14(11), 1698; https://doi.org/10.3390/w14111698
Submission received: 11 May 2022 / Accepted: 18 May 2022 / Published: 25 May 2022
Groundwater discharge is vitally important for maintaining or restoring valuable ecosystems in surface water and at the underlying groundwater-surface-water ecotone [1,2,3]. Detecting and quantifying groundwater discharge is challenging because rates of flow can be very small and difficult to measure, exchange is commonly highly heterogeneous both in space and time, and surface-water hydrodynamics can influence the exchange and hinder measurements [4]. Fortunately, a growing number of methods developed during the last several decades has led to advancements in our capabilities to identify and quantify groundwater discharge to surface water, including better use of seepage meters [5,6], application of tracers such as heat [7,8] or isotopes [9,10], and improved groundwater-modeling capabilities [11,12,13]. This progress has led to coalescence in characterizing the complex mix of hydrological, biological, and chemical processes that occur at the groundwater-surface water interface [14,15,16], along with relevant societal effects [17]. Still, many uncertainties and assumptions show an incomplete knowledge of these processes, including the lack of studies in many regions of the world, insufficient sharing of practical methodologies between scientific disciplines (i.e., [18,19]), incomplete understanding of processes and parameters specific to the sediment-water interface [20], and challenges associated with measuring exchange at multiple scales of time and space [21,22,23].
The collection of tools and methods that can be applied to quantify groundwater-surface-water interaction has increased substantially in recent decades. Nevertheless, given the broad range of natural settings at which groundwater exchanges with surface water, adaptations of standard methods are often required to quantify exchanges at specific settings. Although site-specific modifications hinder the desired standardization of methods that can be applied worldwide, new tools and new ways to envision and quantify these exchanges further populates and enriches the range of tools required to broaden our understanding of this interface and important ecotone. This special issue focuses on summarizing new tools and methods for quantifying and understanding groundwater discharge to surface water and the resulting improved and broadened understanding of this interface.
Several ideas emerge from the collection of papers published in this special issue: (i) Improvements in methods and applications, whether measurement- or model-based, continue to increase our understanding of processes related to the discharge of groundwater to surface water and how to measure them. (ii) Case studies that require determination of groundwater discharge to surface water need to employ multiple methods to improve confidence in the results. (iii) A sufficient number of groundwater-discharge studies in a broad range of settings now exist such that comprehensive reviews of existing literature can yield new insights.
This special issue includes four original research papers focusing on innovative applications of existing tools, improvements in measurement and interpretation, and use of emerging techniques in the study of groundwater-surface-water interaction. Three case studies demonstrate the extension and use of multiple approaches to quantify groundwater discharge in challenging environments. Two review papers provide the latest understanding regarding groundwater discharge in nearshore margins, one for high-energy beach-aquifer settings and another for lower-permeability salt-marsh ecosystems. Collectively, these contributions demonstrate the increase in breadth and depth of understanding of processes at these interfaces, thanks largely to combinations of innovative new techniques with improvements of methods that have long been staples of the hydrogeological research community.
Engesgaard et al. [24] employed a simple, cost-efficient field method to characterize groundwater-surface-water interaction in a low-gradient setting where gradient-based methods were unreliable. Relying on the combination of multiple tracers (electrical conductivity and stable isotopes of water) and innovative end-member mixing analyses, they quantified both groundwater discharge to a small lake and recharge of lake water to adjacent groundwater. To add confidence to the tracer-based approach, the authors installed a dense network of piezometers that, when sampled to determine the percent lake water, allowed better determination of the direction of groundwater flow in the vicinity of the lake. These results will have transfer value for lakes and wetlands in low gradient settings that are common in lowland areas, particularly those in coastal margins.
Karan et al. [25] made use of a grid-refinement tool to improve discretization of a numerical model designed to represent discharge of groundwater to several streams in western Denmark. Results when using an unstructured-grid-refinement option of a popular groundwater-flow model (MODFLOW-USG) were compared with MODFLOW output when a standard grid structure was used. Simulated drainage to streams increased substantially with the refined-grid approach that more appropriately simulated the areas of exchange with streams than did the equal-grid model. Results from the refined-grid model also compared much better with measured results from an extensive network of instrumentation.
Briggs et al. [26] used thermal detection methods to identify locations of preferential groundwater discharge. Both handheld cameras and cameras attached to unmanned aerial vehicles were deployed along two river reaches of 12 and 26 km. Aerial imagery worked well except where hindered by overbank vegetation, at which point handheld cameras deployed from floating watercraft found more points of focused discharge. Locations and distributions of focused discharge were related to maps of soil types. Numerous points of focused groundwater discharge were co-located with tree-root masses, indicating the importance of local controls on spatial distribution of focused groundwater discharge.
Rosenberry et al. [27] demonstrated that seepage meters, long considered to have a constant efficiency associated with the design of the meter, have variable and substantially reduced efficiency when deployed in high-permeability settings. Seepage meters with an efficiency close to 100% in hydraulic conductivity settings typical of most lakebeds become far less efficient when hydraulic conductivity is greater than about 10 m/d. Efficiency decreases to as little as 10% in these settings commonly found in high-energy fluvial or wave-washed shoreline settings. Meter designs need to be particularly efficient, and extra care is required when operating meters in these high-permeability settings, to reduce the occurrence of flow that bypasses rather than flowing through the meter.
Three case studies demonstrate the importance of quantifying discharge to surface water in spite of the difficulty associated with particularly challenging settings. Wu et al. [28] studied Hongjiannao Lake, the largest freshwater desert lake in China, and the important implications of sources of water for associated ecosystems, including an endangered relict gull. The lake is fed by several surface-water sources, but the groundwater component was largely unknown. A full suite of hydrochemical and isotopic data characterized the groundwater adjacent to the lake. Lake-water chemistry is greatly affected by evaporation, which substantially increases concentrations of solutes in the lake.
Gil-Márquez et al. [29] conducted a similar study of two wetlands in the south of Spain with high ecological value. Despite being separated by just a few kilometers, they reacted differently to wet and dry periods. The authors unraveled differences in the wetland water budgets that required detailed analysis of the methods for assessing evaporation. Differences in wetland water budgets and associated water quality are largely attributed to relative positions in the regional hydrogeologic setting. This study highlights the challenge of spatial changes in groundwater-surface-water interaction for two wetlands in the same geological region but in different hydrogeological settings.
Sánchez-Martos et al. [30] compared changes at two other wetlands, also in southern Spain, but this time in response to changes in exploitation of groundwater that discharges to the wetlands. One wetland expanded in size over several decades while another, only a few kilometers distant, decreased in size. These opposite trends were attributed to differences in groundwater discharge. The expanding wetland was receiving an increased amount of groundwater discharge as upgradient withdrawals were decreasing due to increased aquifer salinity. Meanwhile, the use of deeper groundwater was increasing, which decreased discharge of deeper groundwater to the other nearby wetland.
Two reviews demonstrate the importance of groundwater discharge in nearshore margins, one focused on high-permeability beach aquifers and the other on low-permeability salt marshes. Kim and Heiss [31] reviewed methods for studying biogeochemical reactivity in sandy beaches. Constituents delivered via groundwater discharge to coastal margins often are attenuated or transformed due to mixing processes. Quantifying fluxes and the effects of physical and biogeochemical processes requires careful measurements at appropriate spatial and temporal resolution. The authors summarize the literature and make recommendations for employing specific methods to quantify specific processes and associated constituents.
Guimond and Tamborski [32] provided a similar review focused slightly inland, on lower-energy salt marshes. Complex processes associated with tidally driven oscillations in hydraulic gradient and salinity require adaptations of otherwise standard methods, including direct measurements, numerical modeling, and geochemical approaches. The authors indicate the need for the use of multiple approaches to address issues related to temporal and spatial heterogeneity, just as was suggested by the Kim and Heiss [31] (this issue) review. The authors also indicate that, in spite of scientifically and societally important processes related to carbon budgets, biological processes, and climate change, this is still an emerging field of research.
The development of new, modified, or alternative methods and analysis techniques for identifying and measuring groundwater discharge to surface water continues to be an active research field across the world. Whether triggered by new insights, technological advancement, reduction of cost, or simply driven by the necessity to study an especially challenging site, continuous improvement in the approaches for understanding groundwater-surface-water interaction is evident in the scientific literature.
This special issue adds to these improvements in methodology, instrumentation and interpretation. It presents applications of improved methods in particularly challenging settings. It synthesizes the current state of the science where groundwater discharges in marine margins. One common theme is that no one method rules them all. Multiple approaches are required in virtually all studies to achieve a more comprehensive understanding of processes and fluxes and implications for societal relevance. Challenges remain, particularly with regard to spatial scale and heterogeneity, but progress has indeed been impressive and the incorporation of the new or revised methods presented here will contribute to continued advancements in understanding during the ensuing years.

Funding

This research was partially funded by the Next-Generation EU funding: Programa María Zambrano Sénior (REF: MZSA03).

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Duque, C.; Rosenberry, D.O. Advances in the Study and Understanding of Groundwater Discharge to Surface Water. Water 2022, 14, 1698. https://doi.org/10.3390/w14111698

AMA Style

Duque C, Rosenberry DO. Advances in the Study and Understanding of Groundwater Discharge to Surface Water. Water. 2022; 14(11):1698. https://doi.org/10.3390/w14111698

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Duque, Carlos, and Donald O. Rosenberry. 2022. "Advances in the Study and Understanding of Groundwater Discharge to Surface Water" Water 14, no. 11: 1698. https://doi.org/10.3390/w14111698

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