The movement of water, matter, organisms, and energy can be altered substantially at ecohydrological interfaces, the dynamic transition zones that often develop within ecotones or boundaries between adjacent ecosystems. Interdisciplinary research over the last two decades has indicated that ecohydrological interfaces are often “hot spots” of ecological, biogeochemical, and hydrological processes and may provide refuge for biota during extreme events. Ecohydrological interfaces can have significant impact on global hydrological and biogeochemical cycles, biodiversity, pollutant removal, and ecosystem resilience to disturbance. The organizational principles (i.e., the drivers and controls) of spatially and temporally variable processes at ecohydrological interfaces are poorly understood and require the integrated analysis of hydrological, biogeochemical, and ecological processes. Our rudimentary understanding of the interactions between different drivers and controls critically limits our ability to predict complex system responses to change. In this paper, we explore similarities and contrasts in the functioning of diverse freshwater ecohydrological interfaces across spatial and temporal scales. We use this comparison to develop an integrated, interdisciplinary framework, including a roadmap for analyzing ecohydrological processes and their interactions in ecosystems. We argue that, in order to fully account for their nonlinear process dynamics, ecohydrological interfaces need to be conceptualized as unique, spatially and temporally dynamic entities, which represents a step change from their current representation as boundary conditions at investigated ecosystems.

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Ecohydrological interfaces as hot spots of ecosystem processes

Recognition of sustainability issues around water resource consumption is gaining traction under global warming and land utilization complexities. These concerns increase the challenge of gaining an appropriate comprehension of the anthropogenic activities and natural processes, as well as how they influence the quality of surface water and groundwater systems. The characteristics of water resources cause difficulties in the comprehensive assessment regarding the source types, pathways, and pollutants behaviors. As the behavior and prediction of widely known contaminants in the water resources remain challenging, some new issues have developed regarding heavy metal pollutants. The main aim of this review is to focus on certain essential pollutants’ discharge from anthropogenic activities categorized based on land-use sectors such as industrial applications (solid/liquid wastes, chemical compounds, mining activities, spills, and leaks), urban development (municipal wastes, land use practices, and others), and agricultural practices (pesticides and fertilizers). Further, important pollutants released from natural processes classified based on climate change, natural disasters, geological factors, soil/matrix, and hyporheic exchange in the aquatic environment, are also discussed. Moreover, this study addresses the major inorganic substances (nitrogen, fluoride, and heavy metals concentrations). This study also emphasizes the necessity of transdisciplinary research and cross-border communication to achieve sustainable water quality using sound science, adaptable legislation, and management systems.

Various Natural and Anthropogenic Factors Responsible for Water Quality Degradation: A Review

https://escholarship.org/content/qt6b447227/qt6b447227.pdf?t=pexqpm

Geophysical characterisation of the groundwater–surface water interface

Simulation of dynamic expansion, contraction, and connectivity in a mountain stream network

The hyporheic zone is the interface beneath and adjacent to streams and rivers where surface water and groundwater interact. The hyporheic zone presents unique conditions for reaction of solutes from both surface water and groundwater, including reactions which depend upon mixing of source waters. Some models assume that hyporheic zones are well-mixed and conceptualize the hyporheic zone as a surface water-groundwater mixing zone. But what are the controls on and effects of hyporheic mixing? What specific mechanisms cause the relatively large (>∼1m) mixing zones suggested by subsurface solute measurements? In this commentary, we explore the various processes that might enhance mixing in the hyporheic zone relative to deeper groundwater, and pose the question whether the substantial mixing suggested by field studies may be due to the combination of fluctuating boundary conditions and multi-scale physical and chemical spatial heterogeneity. We encourage investigation of hyporheic mixing using numerical modeling and laboratory experiments to ultimately inform field investigations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016WR020005

The importance and challenge of hyporheic mixing

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016GL068286

Hyporheic exchange controlled by dynamic hydrologic boundary conditions

Expansion and contraction of the hyporheic zone due to temporal hydrologic changes between stream and riparian aquifer influence the biogeochemical cycling capacity of streams. Theoretical studies have quantified the control of groundwater discharge on the depth of the hyporheic zone; however, observations of temporal groundwater controls are limited. In this study, we develop the concept of groundwater-dominated differential hyporheic zone expansion to explain the temporal control of groundwater discharge on the hyporheic zone in a third-order stream reach flowing through glacially derived terrain typical of the Great Lakes region. We define groundwater-dominated differential expansion of the hyporheic zone as: differing rates and magnitudes of hyporheic zone expansion in response to seasonal vs. storm-related water table fluctuation. Specific conductance and vertical hydraulic gradient measurements were used to map changes in the hyporheic zone during seasonal water table decline and storm events. Planar and riffle beds were monitored in order to distinguish the cause of increasing hyporheic zone depth. Planar bed seasonal expansion of the hyporheic zone was of a greater magnitude and longer in duration (weeks to months) than storm event expansion (hours to days). In contrast, the hyporheic zone beneath the riffle bed exhibited minimal expansion in response to seasonal groundwater decline compared to storm related expansion. Results indicated that fluctuation in the riparian water table controlled seasonal expansion of the hyporheic zone along the planar bed. This groundwater induced hyporheic zone expansion could increase the potential for biogeochemical cycling and natural attenuation.

Temporal Hyporheic Zone Response to Water Table Fluctuations.

The volume of the water stored in and exchanged with the hyporheic zone is an important factor in stream metabolism and biogeochemical cycling. Previous studies have identified groundwater direction and magnitude as one key control on the volume of the hyporheic zone, suggesting that fluctuation in the riparian water table could induce large changes under certain seasonal conditions. In this study, we analyze the transient drivers that control the volume of the hyporheic zone by coupling the Brinkman-Darcy equation to the Navier-Stokes equations to simulate annual and storm induced groundwater fluctuations. The expansion and contraction of the hyporheic zone was quantified based on temporally dynamic scenarios simulating annual groundwater fluctuations in a humid temperate climate. The amplitude of the groundwater signal was varied between scenarios to represent a range of annual hydrologic forcing. Storm scenarios were then superimposed on the annual scenario to simulate the response to short-term storm signals. Simulations used two different groundwater storm responses; one in-phase with the surface water response and one 14 h out-of-phase with the surface water response to represent our observed site conditions. Results show that annual groundwater fluctuation is a dominant control on the volume of the hyporheic zone, where increasing groundwater fluctuation increases the amount of annual variation. Storm responses depended on the antecedent conditions determined by annual scenarios, where the time of year dictated the duration and magnitude of the storm induced response of the hyporheic zone.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015WR018056

Response of the hyporheic zone to transient groundwater fluctuations on the annual and storm event time scales

Groundwater surface water interaction in the hyporheic zone remains an important challenge for water resources management and ecosystem restoration. In heterogeneous stratified glacial sediments, reach-scale environments contain an uneven distribution of focused groundwater flow occurring simultaneously with diffusely discharging groundwater. This results in a variation of stream-aquifer interactions, where focused flow systems are able to temporally dominate exchange processes. The research presented here investigates the direct and indirect influences focused groundwater discharge exerts on the hyporheic zone during baseflow recession. Field results demonstrate that as diffuse sources of groundwater deplete during baseflow recession, focused groundwater discharge remains constant. During baseflow recession the hyporheic zone is unable to expand, while the high nitrate concentration from focused discharge changes the chemistry of the stream. The final result is a higher concentration of nitrate in the hyporheic zone as this altered surface water infiltrates into the subsurface. This indirect coupling of focused groundwater discharge and the hyporheic zone is unaccounted for in hyporheic studies at this time. Results indicate important implications for the potential reduction of agricultural degradation of water quality.

Focused groundwater controlled feedbacks into the hyporheic zone during baseflow recession.

Central Kentucky could be considered a critical source area of nutrients in water ways because of low permeability soils, fast groundwater flow through bedrock fractures, and pervasive agriculture and development. Of particular concern is rising development in rural areas, which creates mixed land cover (MLC) watersheds, i.e., watersheds with development, agriculture, and other land cover types. MLC watersheds add complexity to spatial and temporal releases of dissolved constituents, leading to less predictable water quality patterns. The goal of this research was to examine the export of dissolved substances from a small, upland MLC catchment in central Kentucky with a focus on how the interaction between discharges from developed agricultural land cover and groundwater influence base flow water quality. Our approach was to spatially sample a representative catchment monthly over 1 year, characterize the major dissolved constituents, and evaluate catchment processes with statistical analyses and Piper diagrams. Principal component analysis, factor analysis, and Piper diagrams indicate base flow was composed of groundwater influenced by two different host rocks and an outfall draining a developed region. Base flow nutrient export was dominated by mixing nitrate-sulfate rich groundwater with ammonium-phosphate-chloride rich outfall drainage. High nitrate groundwater dominated nitrogen export in the winter, whereas high ammonium outfall drainage dominated summer export. Spatial analysis revealed that ~ 10% of the basin may have similar land cover and hydrologic processes, suggesting that MLC catchments are small but collectively significant nitrogen sources to river networks due to development and agriculturally impacted groundwater.

Jonathan M. Malzone

Papers

Hyporheic exchange controlled by dynamic hydrologic boundary conditions

Temporal Hyporheic Zone Response to Water Table Fluctuations.

Response of the hyporheic zone to transient groundwater fluctuations on the annual and storm event time scales

Focused groundwater controlled feedbacks into the hyporheic zone during baseflow recession.

The impact of small-scale land cover and groundwater interactions on base flow solute and nutrient export in a small agricultural stream