Numerical modeling of an abiotic hyporheic mixing-dependent reaction: Chemical evolution of mixing and reactant production zones.

Mixing-dependent reactions occur where groundwater and surface water mix in shallow sediments (hyporheic zone) and can attenuate contaminants along upwelling flowpaths, thus reducing transport to surface water. Here we used MODFLOW/SEAM3D to numerically simulate prior laboratory observations of a mixing-dependent reaction between sodium sulfite (Na2SO3) and dissolved oxygen (DO) to produce sodium sulfate (Na2SO4). This reaction is not common in nature but is used as a surrogate for mixing-dependent DO consuming reactions of environmental significance. We evaluated how location and thickness of mixing zones and reaction product production zones dynamically respond to variations in hydraulic and chemical boundary conditions and reaction kinetic rate. Sensitivity analysis showed that location and thickness of mixing zones and reactant production zones were most sensitive to changes in the balance of hydrologic inflow from groundwater and surface water (inflow ratio). Mixing zone thickness for reactive DO calibrated to experimental data was thinner than that for the "DO tracer" (identical source location and concentration as DO but conservative tracer), indicating that as DO is consumed its mixing zone narrows. The SO4 production zone was consistently thicker than the DO mixing zone. Small changes in mixing/production zone thicknesses were linked to large changes in mass consumed and produced, indicating the potential for simpler field metrics like thickness to act as surrogates for more challenging measurements such as contaminant flux or consumption in monitoring natural attenuation. This study improves understanding of the evolution of hyporheic mixing-dependent reaction zones that occur even under steady state hydraulics, emphasizing their complex controls.

Hyporheic transverse mixing zones and dispersivity: Laboratory and numerical experiments of hydraulic controls.

Mixing of surface water and groundwater in shallow sediments is important to biogeochemical cycling and contaminant migration, and is often used to define the hyporheic zone. Yet knowledge of mixing processes in hyporheic zones is supported by surprisingly few rigorous lab or field observations, and differ from those in deeper groundwater by presence of enhanced head gradients, sediment heterogeneity, and temporal fluctuations. In a laboratory sediment (sand) tank we photographed a conservative dye to analyze transverse mixing zones between upwelling groundwater and bidirectional hyporheic exchange flows. We then conducted numerical modeling to investigate processes behind observed phenomena and estimate dispersivities. We found that transverse mixing zones were thin (i.e. mixing thickness measured in direction of steepest concentration gradient, δ, less than 5 cm), consistent with a small calibrated transverse dispersivity (~0.1 mm) and prior lab studies conducted at similar scales. In steady-state experiments and simulations, δ and estimated dispersion coefficients increased with the surface water head drop driving exchange flows. Given relatively constant deeper groundwater heads, increased Δh led to increased mixing zone length for both steady-state and transient conditions, indicating larger bedforms or weaker gaining conditions enhance subsurface mixing. However, Peclet number and flux-related dilution index simultaneously increased and decreased, respectively, indicating that enhancement of subsurface advection outpaced that of dispersion. In transient experiments and simulations, δ was greater than for steady-state, probably from temporary addition of longitudinal dispersion. During transient experiments, δ exhibited temporal noise, perhaps due to the mixing zone moving past varying patterns of sediment packing. Our results provide basic knowledge of mixing zone behavior in hyporheic zones with implications for hyporheic zone definitions, solute transport, mixing-dependent reaction, and water quality.

Effect of Floodplain Restoration on Photolytic Removal of Pharmaceuticals.

Floodplain restoration is popular to address excess nutrients, but its ability to enhance photolysis of emerging contaminants has not been evaluated. We used the numerical model MIKE-21 to simulate photolysis reactions within the inundated surface water of restored floodplains along a mid-size river. We examined both "high" and "low" floodplain scenarios where inundation occurs 5% (storms) and 50% (baseflow) of the year, respectively. We simulated photolysis of the pharmaceuticals morphine, codeine, and methamphetamine and, for context, compared it with nitrate removal (denitrification and plant uptake). Pollutant removal due to floodplain restoration was greater for the low floodplain (e.g., 18.8% for morphine) than for the high floodplain (5.6% for morphine) due to greater water exchange relative to channel flow. The fastest- and slowest-reacting pollutants (morphine and methamphetamine, respectively) were always transport- and reaction/kinetics-limited within floodplain surface water, respectively. Yet, those with intermediate decay-rate constants switched from reaction limitation to transport limitation as the floodplain length increased, and removal leveled off at an optimum length of ∼1000 m. However, as the floodplain width increased, the required floodplain length for 30% removal decreased. Optimal restored floodplain conditions for photolysis would maximize light exposure, which may differ from those for nutrients.

Variability of subsurface structure and infiltration hydrology among surface coal mine valley fills.

Surface coal mining alters landscapes including creating waste-rock fills or dumps. In Appalachia USA, mines fill valleys with waste rock, constructing valley fills that affect water quality and aquatic ecology downstream. Total dissolved solids (TDS) in mine effluent are elevated from exposure of mineral surfaces to weathering. Understanding TDS variability requires understanding valley fill internal structure and its effect on hydrology, yet prior studies focused on point measurements or did not address patterns among fills. Here we investigated subsurface structure and hydrologic flowpaths in two dimensions within four valley fills using electrical resistivity imaging (ERI). We used artificial rainfall to investigate the location and transit time of preferential flowpaths through the fills. We corroborated our ERI interpretations using borehole logs, downhole video, and shallow soil excavation. ERI results indicated variability in substrate type and widespread presence of preferential flowpaths. We estimated an average preferential flowpath vertical length of 6.6 m, average transit time of water along the flowpath of 1.4 h, and average minimum water velocity of 5.1 m/h (0.14 cm/s). These rates are higher than typical for undisturbed lands, and resemble highly preferential flow in karst terrain. ERI successfully distinguished fills using conventional loose-dump construction from experimental controlled-material compacted-lift construction. Conventional fills exhibited finer particles that retain water at the surface, with larger rocks and larger voids at depth. Conventional fills had greater ranges of subsurface resistivity (i.e. substrate types) and greater interior accumulation of water during artificial rainfall, indicating more quick/deep preferential infiltration flowpaths. We show experimental construction significantly alters hydrologic response, which in combination with use of low-TDS waste rock, may affect downstream water quality relative to conventional loose-dump methods. Our soil boring and pits corroborated ERI interpretation, thus demonstrating ERI to be a robust non-invasive technique that provides reliable information on valley fill structure and hydrology.

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.

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.

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.

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.