Erich T. Hester

Moving beyond the banks: hyporheic restoration is fundamental to restoring ecological services and functions of streams.

Stream restoration needs to consider the hyporheic zone just as much as the surface and benthic regions.

Hydraulic and thermal effects of in-stream structure-induced hyporheic exchange across a range of hydraulic conductivities

In-stream structure-induced hyporheic exchange and associated thermal dynamics affect stream ecosystems. Their importance is controlled by spatial variability of sediment hydraulic conductivity (K). We calibrated a computational fluid dynamics (CFD) model of surface and groundwater hydraulics near a channel-spanning weir (represents log dams, boulder weirs) to field data and varied K from 10−7 to 10−2 m/s (silt to gravel). Surface water stopped cresting the weir for K > 10−3 m/s. Non-Darcy hyporheic flow was also prevalent for K > 10−3 m/s, and velocity errors using non-CFD models ranged up to 32.2%. We also modeled weir-induced heat transport during summer. As K increased from 10−7 to 10−3 m/s, weir-induced hyporheic heat advection steadily increased. Cooling and buffering along hyporheic flow paths decreased with increasing K, particularly above K = 10−5 and 10−4 m/s, respectively. Vertical heat conduction between surface water and groundwater near the weir decreased with increasing K, particularly for K > 10−5 m/s. Conduction between hyporheic flow paths and adjacent groundwater helped cool hyporheic flow. Downstream surface water cooling by hyporheic advection increased steadily with K as increases in hyporheic flow overwhelmed decreases in cooling along hyporheic flow paths. Yet such effects were small (0.016°C) even at K = 10−3 m/s. The largest thermal effect of weir-induced exchange was therefore spatial expansion of subsurface diel variability (particularly for K > 10−5 m/s) which affects benthic habitat and chemical reactions. The specific values of K where such trend shifts occur is likely variable among streams based on flow conditions, but we expect the presence of such trend shifts to be widespread.

Controls on mixing‐dependent denitrification in hyporheic zones induced by riverbed dunes: A steady state modeling study

The hyporheic zone is known to attenuate contaminants originating from surface water, yet the ability of the hyporheic zone to attenuate contaminants in upwelling groundwater plumes as they exit to surface water is less understood. We used MODFLOW and SEAM3D to simulate hyporheic flow cells induced by riverbed dunes and upwelling groundwater together with mixing-dependent denitrification of an upwelling nitrate ( NO3−) plume. Our base case modeled labile dissolved organic carbon (DOC) and dissolved oxygen (DO) advecting from surface water, and DO and NO3− advecting from groundwater, typical of certain agricultural areas. We conducted sensitivity analyses that showed mixing-dependent denitrification in the hyporheic zone increased with increasing hydraulic conductivity (K), decreasing lower boundary flux, and increasing DOC in surface water or NO3− in groundwater. Surface water DOC, groundwater NO3−, and K were the most sensitive parameters affecting mixing-dependent denitrification. Nonmixing-dependent denitrification also occurred when there was surface water NO3−, and its magnitude was often greater than mixing-dependent denitrification. Nevertheless, mixing-dependent reactions provide functions that nonmixing-dependent reactions cannot, with potential for hyporheic zones to attenuate upwelling NO3− plumes, depending on geomorphic, hydraulic, and biogeochemical conditions. Stream and river restoration efforts may be able to increase mixing-dependent reactions by promoting natural processes that promote bedform creation and augment labile carbon sources.

Measuring Environmental Sustainability of Water in Watersheds

Environmental sustainability assessment is a rapidly growing field where measures of sustainability are used within an assessment framework to evaluate and compare alternative actions. Here we argue for the importance of evaluating environmental sustainability of water at the watershed scale. We review existing frameworks in brief before reviewing watershed-relevant measures in more detail. While existing measures are diverse, overlapping, and interdependent, certain attributes that are important for watersheds are poorly represented, including spatial explicitness and the effect of natural watershed components, such as rivers. Most studies focus on one or a few measures, but a complete assessment will require use of many existing measures, as well as, perhaps, new ones. Increased awareness of the broad dimensions of environmental sustainability as applied to water management should encourage integration of existing approaches into a unified assessment framework appropriate for watersheds.

Comparison of effects of inset floodplains and hyporheic exchange induced by in‐stream structures on solute retention

The pollution of streams and rivers is a growing concern, and environmental guidance increasingly suggests stream restoration to improve water quality. Solute retention in off-channel storage zones, such as hyporheic zones and floodplains, is typically necessary for significant reaction to occur. Yet, the effects of two common restoration techniques, in-stream structures and inset floodplains, on solute retention have not been rigorously compared. We used MIKE SHE to model hydraulics and solute transport in the channel, on inset floodplains, and in structure-induced hyporheic zones of a third-order stream. We varied hydraulic conditions (winter base flow, summer base flow, and stormflow), geology (hydraulic conductivity), and stream restoration design parameters (inset floodplain length and presence of in-stream structures). The in-stream structures induced hyporheic exchange for approximately 20% of the year (during summer base flow) while inset floodplains were active for approximately 1% of the year (during stormflow). Flow onto inset floodplains and residence times in both the channel and on the floodplains increased nonlinearly with the fraction of bank with floodplains installed. The fraction of streamflow that flowed onto the inset floodplains was 1–3 orders of magnitude higher than that which flowed through the structure-induced hyporheic zone. Yet, residence times and mass storage in the hyporheic zone were 1–5 orders of magnitude larger than that on individual inset floodplains. In our modeling, neither in-stream structures nor inset floodplains had sufficient percent flow and residence times simultaneously to have a substantial impact on dissolved contaminants flowing downstream.

Assessing and Enhancing Environmental Sustainability: A Conceptual Review

While sustainability is an essential concept to ensure the future of humanity and the integrity of the resources and ecosystems on which we depend, identifying a comprehensive yet realistic way to assess and enhance sustainability may be one of the most difficult challenges of our time. We review the primary environmental sustainability assessment approaches, categorizing them as either being design-based or those that employ computational frameworks and/or indicators. We also briefly review approaches used for assessing economic and social sustainability because sustainability necessitates integrating environmental, economic, and social elements. We identify the collective limitations of the existing assessment approaches, showing that there is not a consistent definition of sustainability, that the approaches are generally not comprehensive and are subject to unintended consequences, that there is little to no connection between bottom-up and top-down approaches, and that the field of sustainability is largely fragmented, with a range of academic disciplines and professional organizations pursuing similar goals, but without much formal coordination. We conclude by emphasizing the need for a comprehensive definition of sustainability (that integrates environmental, economic, and social aspects) with a unified system-of-systems approach that is causal, modular, tiered, and scalable, as well as new educational and organizational structures to improve systems-level interdisciplinary integration.

The importance and challenge of hyporheic mixing

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.

Abundance and dimensions of naturally occurring macropores along stream channels and the effects of artificially constructed large macropores on transient storage

Macropores are connected void spaces in the subsurface and can act as preferential flow paths for groundwater transport, but their dimensions and distribution patterns have not been well characterized in stream banks and hyporheic sediments. We ran field surveys in 5 streams of varying size, bed slope, and watershed land use in the Appalachian Mountains of southwestern Virginia. Natural surface-connected macropores were nearly ubiquitous. Macropores in this region arise by a variety of mechanisms, including soil piping, tree root decay, erosion around hardened structures, macroinvertebrate burrows, and invertebrate burrows. Macropore openings were slightly wider than tall, with median cross-sectional widths and heights of 3.5 cm and 3.0 cm, respectively. The median and maximum macropore lengths to first bend were 15.0 cm and 120.5 cm into the bank, respectively. True (tortuous) macropore lengths probably are often greater, and methods to map macropores are needed. Median interspacing of macropores across all streams was 0.38 m, but mean interspacing was 1.12 m, indicating that macropores were clustered in space. Macropores were inundated at different times because of differences in their heights on the stream bank, channel geometry, and stream stage. HEC-RAS modeling of channel hydraulics indicated that only 1 to 32% of macropores were inundated by channel water at base flow, whereas up to 97% were inundated during the largest storms. We ran conservative-tracer injection experiments at base flow in a 30-m reach of a small tributary to 1 study stream. We calculated transient-storage parameters for 2 treatments: 1 without macropores and 1 with artificially created macropores. Our results suggest that constructed macropores weakly increased transient storage (channel:storage zone cross-sectional area [As/A] by 15.0% and fraction of median travel time spent in the transient-storage zone [Fmed] by 31.0%) and may affect surface-water–groundwater interactions. Many macropore openings were above typical baseflow water levels, so macropores may enhance bank storage, a possibility that bears further research.

Seasonal Variation in Floodplain Biogeochemical Processing in a Restored Headwater Stream.

Stream and river restoration activities have recently begun to emphasize the enhancement of biogeochemical processing within river networks through the restoration of river-floodplain connectivity. It is generally accepted that this practice removes pollutants such as nitrogen and phosphorus because the increased contact time of nutrient-rich floodwaters with reactive floodplain sediments. Our study examines this assumption in the floodplain of a recently restored, low-order stream through five seasonal experiments. During each experiment, a floodplain slough was artificially inundated for 3 h. Both the net flux of dissolved nutrients and nitrogen uptake rate were measured during each experiment. The slough was typically a source of dissolved phosphorus and dissolved organic matter, a sink of NO3(-), and variable source/sink of ammonium. NO3(-) uptake rates were relatively high when compared to riverine uptake, especially during the spring and summer experiments. However, when scaled up to the entire 1 km restoration reach with a simple inundation model, less than 0.5-1.5% of the annual NO3(-) load would be removed because of the short duration of river-floodplain connectivity. These results suggest that restoring river-floodplain connectivity is not necessarily an appropriate best management practice for nutrient removal in low-order streams with legacy soil nutrients from past agricultural landuse.

Hydrologic Effects of Surface Coal Mining in Appalachia (U.S.)

Surface coal mining operations alter landscapes of the Appalachian Mountains, United States, by replacing bedrock with mine spoil, altering topography, removing native vegetation, and constructing mine soils with hydrologic properties that differ from those of native soils. Research has demonstrated hydrologic effects of mining and reclamation on Appalachian landscapes include increased peakflows at newly mined and reclaimed watersheds in response to strong storm events, increased subsurface void space, and increased base flows. We review these investigations with a focus on identifying changes to hydrologic flow paths caused by surface mining for coal in the Appalachian Mountains. We introduce two conceptual control points that govern hydrologic flow paths on mined lands, including the soil surface that partitions infiltration vs. surface runoff and a potential subsurface zone that partitions subsurface storm flow vs. deeper percolation. Investigations to improve knowledge of hydrologic pathways on reclaimed Appalachian mine sites are needed to identify effects of mining on hydrologic processes, aid development of reclamation methods to reduce hydrologic impacts, and direct environmental mitigation and public policy.