Dissolved oxygen (DO) is a key environmental variable that drives and feeds back with numerous processes. In the aquatic sediment that makes up the hyporheic zone, DO may exhibit pronounced spatial gradients and complex patterns which control the distribution of a series of redox processes. Yet, little is known regarding the dynamics of hyporheic zone DO, especially under transitional flow regimes. Considering the natural tendency of rivers to be highly responsive to external forcing, these temporal dynamics are potentially just as important and pronounced as the spatial gradients. Here we use laboratory flume experiments and multiphysics flow and reactive transport modeling to investigate surface flow controls on the depth of oxygen penetration in the bed as well as the area of oxygenated sediment. We show that the hyporheic zone DO conditions respond over time scales of hours‐to‐days when subjected to practically instantaneous surface flow perturbations. Additionally, the flume experiments demonstrate that hyporheic zone DO conditions respond faster to surface flow acceleration than to deceleration. Finally, we found that the morphology of the dissolved oxygen plume front depends on surface flow acceleration or deceleration. This study thus shows that the highly dynamic nature of typical streams and rivers drives equally dynamic redox conditions in the hyporheic zone. Because the redox conditions and their distribution within the hyporheic zone are important from biological, ecological, and contaminant perspectives, this hyporheic redox dynamism has the potential to impact system scale aquatic chemical cycles.

Many fish lay their eggs in nests, or redds, which they construct in sediment. The viability of eggs depends on many factors, particularly their oxygenation. Because dissolved oxygen is typically saturated within the stream channel, the dissolved oxygen distribution within the redd depends on whether or not hyporheic flow and transport occur within the sediment. We conducted a series of flume and numerical flow and age transport modeling experiments with the aim of understanding the effects of salmonid redds on the hyporheic transport of young oxygenated water. Hyporheic flow was visualized directly through dye injections. Dissolved oxygen throughout the fish nest was measured using a planar optode. Experiments were conducted at various open channel flow velocities in order to understand their effect on dissolved oxygen, and computational simulations considered various sediment textures and ambient groundwater upwelling rates to add process‐level insight. We found that, as also shown by previous studies, the redd topography induces multiscale hyporheic flow that effectively flushes the egg pocket location with younger presumably oxygenated water; older water upwells and forms anoxic zones. This pattern persists even at the lowest channel flow rates and at small upwelling velocities of older ambient groundwater which splits the multiscale hyporheic flow cells into isolated pockets. Large groundwater upwelling rates can shut down all the hyporheic flushing. The relatively coarse texture of the redd further promotes hyporheic flushing of the redd sediment with oxygenated water. Thus, redd morphology and sediment texture optimally combine to induce hyporheic exchange flow that delivers young oxygenated water to the egg pocket.

Health risks of fine particle air pollution (PM 2.5 ) are an important public health concern that has the potential to be mitigated in part by interventions such as air cleaning devices that reduce personal exposure to ambient PM 2.5 . To characterize exposure to ambient PM 2.5 indoors as a function of residential air cleaners, a multi-zone indoor air quality model was used to integrate spatially resolved data on housing, meteorology, and ambient PM 2.5 , with performance testing of residential air cleaners to estimate short-term and annual average PM 2.5 of outdoor origin inside residences of three metropolitan areas. The associated public health impacts of reduced ambient PM 2.5 exposure were estimated using a standard health impact assessment methodology. Estimated indoor levels of ambient PM 2.5 varied substantially among ventilation and air cleaning configurations. The median 24-h average indooroutdoor ratio of ambient PM 2.5 was 0.57 for homes with natural ventilation, 0.35 for homes with central air conditioning (AC) with conventional filtration, and 0.1 for homes with central AC with high efficiency in-duct air cleaner. Median modeled 24-h average indoor concentrations of PM 2.5 of outdoor origin for those three configurations were 8.4, 5.3, and 1.5 mg/m 3 , respectively. The potential public health benefits of reduced exposure to ambient PM 2.5 afforded by air cleaning systems were substantial. If the entire population of single-family homes with central AC in the modeling domain converted from conventional filtration to high-efficiency in-duct air cleaning, the change in ambient PM 2.5 exposure is estimated to result in an annual reduction of 700 premature deaths, 940 hospital and emergency room visits, and 130,000 asthma attacks in these metropolitan areas. In addition to controlling emissions from sources, high-efficiency whole-house air cleaner are expected to reduce exposure to particles of outdoor origin and are projected to be an effective means of managing public health impacts of ambient particle pollution.

BackgroundLaboratory research studies indicate that aerosolized influenza viruses survive for longer periods at low relative humidity (RH) conditions. Further analysis has shown that absolute humidity (AH) may be an improved predictor of virus survival in the environment. Maintaining airborne moisture levels that reduce survival of the virus in the air and on surfaces could be another tool for managing public health risks of influenza.MethodsA multi-zone indoor air quality model was used to evaluate the ability of portable humidifiers to control moisture content of the air and the potential related benefit of decreasing survival of influenza viruses in single-family residences. We modeled indoor AH and influenza virus concentrations during winter months (Northeast US) using the CONTAM multi-zone indoor air quality model. A two-story residential template was used under two different ventilation conditions - forced hot air and radiant heating. Humidity was evaluated on a room-specific and whole house basis. Estimates of emission rates for influenza virus were particle-size specific and derived from published studies and included emissions during both tidal breathing and coughing events. The survival of the influenza virus was determined based on the established relationship between AH and virus survival.ResultsThe presence of a portable humidifier with an output of 0.16 kg water per hour in the bedroom resulted in an increase in median sleeping hours AH/RH levels of 11 to 19% compared to periods without a humidifier present. The associated percent decrease in influenza virus survival was 17.5 - 31.6%. Distribution of water vapor through a residence was estimated to yield 3 to 12% increases in AH/RH and 7.8-13.9% reductions in influenza virus survival.ConclusionThis modeling analysis demonstrates the potential benefit of portable residential humidifiers in reducing the survival of aerosolized influenza virus by controlling humidity indoors.

CH4 emissions from inland waters are highly uncertain in the current global CH4 budget, especially for streams, rivers, and other lotic systems. Previous studies have attributed the strong spatiotemporal heterogeneity of riverine CH4 to environmental factors such as sediment type, water level, temperature, or particulate organic carbon abundance through correlation analysis. However, a mechanistic understanding of the basis for such heterogeneity is lacking. Here, we combine sediment CH4 data from the Hanford reach of the Columbia River with a biogeochemical-transport model to show that vertical hydrologic exchange flows (VHEFs), driven by the difference between river stage and groundwater level, determine CH4 flux at the sediment–water interface. CH4 fluxes show a nonlinear relationship with the magnitude of VHEFs, where high VHEFs introduce O2 into riverbed sediments, which inhibit CH4 production and induce CH4 oxidation, and low VHEFs cause transient reduction in CH4 flux (relative to production) due to reduced advective CH4 transport. In addition, VHEFs lead to the hysteresis of temperature rise and CH4 emissions because high river discharge caused by snowmelt in spring leads to strong downwelling flow that offsets increasing CH4 production with temperature rise. Our findings reveal how the interplay between in-stream hydrologic flux besides fluvial-wetland connectivity and microbial metabolic pathways that compete with methanogenic pathways can produce complex patterns in CH4 production and emission in riverbed alluvial sediments.