Many studies in ecohydrology focusing on hydrologic
transport argue that longer residence times across a stream ecosystem should
consistently result in higher biological uptake of carbon, nutrients, ...and
oxygen. This consideration does not incorporate the potential for
biologically mediated reactions to be limited by stoichiometric imbalances.
Based on the relevance and co-dependences between hydrologic exchange,
stoichiometry, and biological uptake and acknowledging the limited amount
of field studies available to determine their net effects on the retention
and export of resources, we quantified how microbial respiration is
controlled by the interactions between and the supply of essential nutrients (C, N, and P)
in a headwater stream in Colorado, USA. For this, we conducted two rounds of
nutrient experiments, each consisting of four sets of continuous injections
of Cl− as a conservative tracer, resazurin as a proxy for aerobic
respiration, and one of the following nutrient treatments: (a) N, (b) N+C,
(c) N+P, or (d) C+N+P. Nutrient treatments were considered to be known
system modifications that alter metabolism, and statistical tests helped
identify the relationships between reach-scale hydrologic transport and
respiration metrics. We found that as discharge changed significantly
between rounds and across stoichiometric treatments, (a) transient storage
mainly occurred in pools lateral to the main channel and was proportional to
discharge, and (b) microbial respiration remained similar between rounds and
across stoichiometric treatments. Our results contradict the notion that
hydrologic transport alone is a dominant control on biogeochemical
processing and suggest that complex interactions between hydrology, resource
supply, and biological community function are responsible for driving
in-stream respiration.
The hydrologic connectivity between streams and their valley bottoms (stream corridor) is a critical determinant of their ecological function. Ecological functions are known to be spatially and ...temporally variable, but spatial dimensions of the problem are not easily quantified and thus they are usually overlooked. To estimate the spatial patterns of connectivity, and how connectivity varies with changes in discharge, we developed the hyporheic potential model. We used the model to interpret a series of solute tracer injections in two headwater mountain streams with contrasting valley bottom morphologies to estimate connectivity in the stream corridor. The distributions of flow path origination locations and the lengths of hyporheic flow paths appear to vary with base flow recession, even in cases where transport timescales are apparently unchanged. The modeled distribution of origination locations further allowed us to define a spatial analog to the temporal window of detection associated with solute tracer studies, and enables assessment of connectivity dynamics between streams and their corridors. Altogether, the reduced complexity hyporheic potential model provides an easy way to anticipate the spatial distribution and origination locations of hyporheic flow paths from a basic understanding of the valley bottom characteristics and solute transport timescales.
Plain Language Summary
The manuscript details a simple method to assess the spatial connectivity of streams and their riparian zones. While the timescales of exchange in the river corridor have been broadly studied, the complimentary spatial dimension (i.e., the geometry of exchange flowpaths) remains largely unknown. The major challenge in assessing the spatial dimensions of exchange is the limited information available in the subsurface. Here, we develop a reduced complexity model of valley bottom transport to overcome these information limitations. With this model, relatively simple field site characterization and solute tracer data are combined to assess the spatial distribution of downwelling along a headwater mountain stream. We validate the model with a numerical experiment, and demonstrate its application in two watersheds of contrasting geology, repeated through baseflow recession.
Key Points
Hyporheic flow path geometry varies with discharge, even in cases where transport times remain unchanged
In‐stream discharge and along‐stream morphology cannot be used to identify flow path origination locations
Observations of solute tracers have a spatial window of detection in addition to the more broadly recognized temporal window of detection
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
•Multi-scale preferential flow is investigated in an urban streambed.•Less-mobile porosity dynamics are quantified under variable hydraulic conditions.•Systematic flow dependence of cm-scale ...less-mobile porosity dynamics is observed.•Reach-scale hyporheic and groundwater discharge flowpaths evolve over a storm pulse.
Spatially preferential flow processes occur at nested scales at the sediment-water interface (SWI), due in part to sediment heterogeneities, which may be enhanced in flashy urban streams with heavy road sand influence. However, several factors, including the flow-rate dependence of preferential hyporheic flow and discrete groundwater discharge zones are commonly overlooked in reach-scale models of groundwater/surface water exchange. Using a series of controlled-head tracer-injection experiments coupled with cm-scale geophysics within the highly reactive upper 30 cm of the hyporheic zone of an urban stream, we quantified the flow dependence of local less-mobile porosity volume, mass-transfer rate coefficient, and the resulting local residence time in the less-mobile pore space at three controlled downward fluid fluxes (0.8, 2, and 3 m/d). Experiments were performed in two adjacent streambed locations, representing different sediment bulk vertical permeability. Less-mobile porosity parameters were generally substantial and similar between the two streambed locations; though a more competent, thin, organic layer at ∼15 cm depth in one location strongly impacted tracer loading, flushing dynamics, and local residence times. Increased downward flux led to (1) a decrease in less-mobile porosity residence time in all experiments, and (2) an increase in less-mobile porosity fraction for most experiments. Additionally, at the larger stream reach-scale, surface electrodes for electrical resistivity measurement were installed along 22 m of the wetted stream channel. These surface electrode measurements were collected during a natural storm flow event, which revealed widespread, short-term, flushing (e.g. <3 h) of the hyporheic zone with stream water, followed by longer-term (e.g. >60 h) flushing of the SWI with riparian zone groundwater. Flow dependence of preferential hyporheic zone flowpaths, like in the controlled tracer experiments, was also observed in these reach-scale electrical resistivity tomography measurements. Our findings reveal that the spatial and temporal dependence of preferential flow processes create highly dynamic SWI conditions that will affect the physical and coupled biogeochemical functions of the SWI in urbanized, sand-impacted streams.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Solute transport along riparian and hyporheic flow paths is broadly expected to respond to dynamic hydrologic forcing by streams, aquifers, and hillslopes. However, direct observation of these ...dynamic responses is lacking, as is the relative control of geologic setting as a control on responses to dynamic hydrologic forcing. We conducted a series of four stream solute tracer injections through base flow recession in each of two watersheds with contrasting valley morphology in the H.J. Andrews Experimental Forest, monitoring tracer concentrations in the stream and in a network of shallow riparian wells in each watershed. We found hyporheic mean arrival time, temporal variance, and fraction of stream water in the bedrock‐constrained valley bottom and near large roughness elements in the wider valley bottom were not variable with discharge, suggesting minimal control by hydrologic forcing. Conversely, we observed increases in mean arrival time and temporal variance and decreasing fraction stream water with decreasing discharge near the hillslopes in the wider valley bottom. This may indicate changes in stream discharge and valley bottom hydrology control transport in less constrained locations. We detail five hydrogeomorphic responses to base flow recession to explain observed spatial and temporal patterns in the interactions between streams and their valley bottoms. Models able to account for the transition from geologically dominated processes in the near‐stream subsurface to hydrologically dominated processes near the hillslope will be required to predict solute transport and fate in valley bottoms of headwater mountain streams.
Key Points:
Hyporheic transport in constrained valleys is controlled by geologic setting
Hyporheic transport near large in‐stream features is not variable with discharge
Transport in broad riparian zones is controlled by dynamic hydrologic forcing
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In this manuscript, I provide ideas that may help early‐career colleagues on their paths in science, especially in research and academia. I discuss the inevitability of failure at times, the ...importance of finding great collaborators and mentors and making time for the things that bring you joy in your life, and suggest a few practices that I hope make us more pleasant human beings. I share a few difficulties I've navigated and advice I've shared with my students, postdocs, and early‐career colleagues through the years. I hope such thoughts are useful, and help others find the joy in being a scientist.
Key Points
The world of science can be frustrating and difficult at times. Be kind to yourself. Be kind to others
Life is short, so go do something more fun than reading this article
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The hydrologic connectivity between hillslopes and streams impacts the geomorphological evolution of catchments. Here, we propose a conceptual model for hydrogeomorphological evolution of the ...Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), a first-order catchment developed on shale in central Pennsylvania, U.S.A. At SSHCZO, the majority of available water (the difference between incoming meteoric water and outgoing evapotranspiration) flows laterally to the catchment outlet as interflow, while the rest is transported by regional groundwater flow. Interflow, shallow hillslope flow, is limited to the upper 5 to 8 m of highly fractured bedrock, thought to have formed during periglacial conditions in the late Pleistocene. In contrast, groundwater flowpaths are influenced by the primary permeability of the varying stratigraphic units. Both flowpaths respond to weathering-related secondary permeability. O^sub 2^-rich interflow mixes with deep O^sub 2^-poor groundwater under the catchment outlet at depths of 5-8 m. Penetration of this oxygenated interflow under the valley results in pyrite oxidation, release of sulfuric acid, dissolution of minerals, and weakening of bedrock. This is hypothesized to enhance channel incision and, in turn, to promote drainage of deep groundwater from the ridges. Drainage subsequently lowers the catchment water table, advancing the cascade of reactions that produce regolith. Weathering in the catchment is characterized by both sharp and diffuse reaction fronts. Relatively sharp fronts (pyrite, carbonate) mark where vertical, unsaturated flow changes to horizontal, saturated flow, while diffuse fronts (illite, chlorite, feldspar) mark where flow is largely vertical and unsaturated. According to this model, catchment morphology reflects subsurface pyrite reactions due to mixing of interflow and groundwater flow under the valley floor that ultimately results in clay weathering and regolith production nearer the land surface.
Measurements of transient storage in coupled surface‐water and groundwater systems are widely made during base flow periods and rarely made during storm flow periods. We completed 24 sets of slug ...injections in three contiguous study reaches during a 1.25 year return interval storm event (discharge ranging from 21.5 to 434 L s−1) in a net gaining headwater stream within a steep, constrained valley. Repeated studies over a 9 day period characterize transient storage and channel water from prestorm conditions through storm discharge recession. Although the valley floor was always gaining from the hillslopes based on hydraulic gradients, we observed exchange of water from the stream to the valley floor throughout the study and flow conditions. Interpretations of transient storage and channel water balance are complicated by dynamic in‐stream and near‐stream processes. Metrics of transient storage and channel water balance were significantly different (95% confidence level) between the three study reaches and could be identified independently of stream discharge via analysis of normalized breakthrough curves. These differences suggest that the morphology of each study reach was the primary control on solute tracer transport. Unlike discharge, metrics of transient storage and channel water balance did not return to the prestorm values. We conclude that discharge alone is a poor predictor of tracer transport in stream networks during storm events. Finally, we propose a perceptual model for our study site that links hydrologic dynamics in 3‐D along the hillslope‐riparian‐hyporheic‐stream continuum, including down‐valley subsurface transport.
Key Points
Gains and losses of stream water persist through storm events.
Strong down‐valley hyporheic transport persists through a storm event.
The window of detection helps partition short‐ and long‐term storage in streams.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Groundwater–surface-water (GW-SW) interactions in streams are difficult to quantify because of heterogeneity in hydraulic and reactive processes across a range of spatial and temporal scales. The ...challenge of quantifying these interactions has led to the development of several techniques, from centimeter-scale probes to whole-system tracers, including chemical, thermal, and electrical methods. We co-applied conservative and smart reactive solute-tracer tests, measurement of hydraulic heads, distributed temperature sensing, vertical profiles of solute tracer and temperature in the stream bed, and electrical resistivity imaging in a 450-m reach of a 3rd-order stream. GW-SW interactions were not spatially expansive, but were high in flux through a shallow hyporheic zone surrounding the reach. NaCl and resazurin tracers suggested different surface–subsurface exchange patterns in the upper ⅔ and lower ⅓ of the reach. Subsurface sampling of tracers and vertical thermal profiles quantified relatively high fluxes through a 10- to 20-cm deep hyporheic zone with chemical reactivity of the resazurin tracer indicated at 3-, 6-, and 9-cm sampling depths. Monitoring of hydraulic gradients along transects with MINIPOINT streambed samplers starting ∼40 m from the stream indicated that groundwater discharge prevented development of a larger hyporheic zone, which progressively decreased from the stream thalweg toward the banks. Distributed temperature sensing did not detect extensive inflow of ground water to the stream, and electrical resistivity imaging showed limited large-scale hyporheic exchange. We recommend choosing technique(s) based on: 1) clear definition of the questions to be addressed (physical, biological, or chemical processes), 2) explicit identification of the spatial and temporal scales to be covered and those required to provide an appropriate context for interpretation, and 3) maximizing generation of mechanistic understanding and reducing costs of implementing multiple techniques through collaborative research.
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BFBNIB, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Hyporheic hydrodynamics are a control on stream ecosystems, yet we lack a thorough understanding of catchment controls on these flow paths, including valley constraint and hydraulic gradients in the ...valley bottom. We performed four whole‐stream solute tracer injections under steady state flow conditions at the H. J. Andrews Experimental Forest (Oregon, United States) and collected electrical resistivity (ER) imaging to directly quantify the 2‐D spatial extent of hyporheic exchange through seasonal base flow recession. ER images provide spatially distributed information that is unavailable for stream solute transport modeling studies from monitoring wells alone. The lateral and vertical extent of the hyporheic zone was quantified using both ER images and spatial moment analysis. Results oppose the common conceptual model of hyporheic “compression” by increased lateral hydraulic gradients toward the stream. We found that the extent of the hyporheic zone increased with decreasing vertical gradients away from the stream, in contrast to expectations from conceptual models. Increasing hyporheic extent was observed with both increasing and decreasing down‐valley (i.e., parallel to the valley gradient) and cross‐valley (i.e., from the hillslope to the stream, perpendicular to the valley gradient) hydraulic gradients. We conclude that neither cross‐valley nor down‐valley hydraulic gradients are sufficient predictors of hyporheic exchange flux nor flow path network extent. Increased knowledge of the controls on hyporheic exchange, the temporal dynamics of exchange flow paths, and their the spatial distribution is the first step toward predicting hyporheic exchange at the scale of individual flow paths. Future studies need to more carefully consider interactions between spatiotemporally dynamic hydraulic gradients and subsurface architecture as controls on hyporheic exchange.
Key Points
No evidence of hydraulic gradients constricting hyporheic zones in steep valleys
Hyporheic response to changing hydraulic gradients varies with valley constraint
Hydrogeophysical methods characterize hyporheic spatial and temporal dynamics
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Methane leakage from hydrocarbon wells plays an important role in the groundwater‐quality impacts of hydrocarbon development and presents a more likely hazard than hydraulic fracturing or formation ...fluids. Methane released from contaminated water wells has been linked with combustion risks and degraded water quality. Potentially, methane can serve as a precursor to other fluids associated with hydrocarbon extraction, such as volatile organics. In this review, we surveyed studies relating to contamination of drinking‐water aquifers by methane gas from leaking hydrocarbon wells. Challenges associated with linking methane in groundwater to hydrocarbon extraction are identified, highlighting the need for groundwater‐quality and well‐integrity databases. Science‐based policy recommendations are made, including deeper surface casings and greater cement coverage for wells with deviated wellbores, remediation of faulty abandoned wells, and increased gas‐migration monitoring. We suggest four hypotheses to quantify risks to groundwater quality from methane leakage. First, differentiation between thermogenic methane occurring in groundwater due to natural migration and thermogenic methane present due to hydrocarbon development can be used to alleviate the need for baseline measurements of methane in groundwater. Second, methane newly discovered in freshwater aquifers is unlikely to have originated from leaks beginning decades ago. Third, pertaining to the zone separating methane leakage from groundwater, relative permeability will have a larger impact on plume diameter than heterogeneity in intrinsic permeability. Fourth, thermogenic methane in groundwater will serve as a precursor to benzene, toluene, ethybenzene, and xylene (BTEX) under conditions where methane and BTEX coexist in a hydrocarbon reservoir and leakage is transported primarily in the aqueous phase.
This article is categorized under:
Engineering Water > Sustainable Engineering of Water
Science of Water > Water Quality
We synthesize studies on groundwater‐quality impacts of hydrocarbon‐well leakage and call for:
Publicly available groundwater‐quality and well‐integrity databases;
Increased efforts to locate abandoned wells;
Study of geochemical effects of hydrocarbon‐well leakage to groundwater, including potential benzene, toluene, ethybenzene, and xylenes contamination; and
Interdisciplinary collaborations to evaluate groundwater‐quality impacts of anthropogenic methane.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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