Hydrological and biogeochemical dynamics of nitrate production and removal at the stream – ground water interface

The feedbacks between hydrology and biogeochemical cycling of nitrogen (N) are of critical importance to global bioavailable N budgets. Human activities are dramatically increasing the amount of bioavailable N in the biosphere, which is causing increasingly frequent and severe impacts on ecosystems and human welfare. Streams are important features in the landscape for N cycling, because they integrate many sources of terrestrially derived N and control export to downgradient systems via internal source and sink processes. N transformations in stream ecosystems are typically very complex due to spatiotemporal variability in the factors controlling N biogeochemistry. Thus, it is difficult to predict if a particular stream system will function as a net source or sink of bioavailable N. A key location for N transformations in stream ecosystems is the hyporheic zone, where stream and ground waters mix. The hyporheic zone can be a source of bioavailable N via nitrification or a sink via denitrification. These N transformations are regulated by the physical and biogeochemical conditions of hyporheic zones. Natural heterogeneity in streams leads to unique combinations of both the physical and biogeochemical conditions which in turn result in unique N source and sink conditions. This dissertation investigates the relationships between physical and biogeochemical controls and the resulting fate of bioavailable N in hyporheic zones. The key physical factor investigated is the supply rate of solutes which is a function of transport processes - advection and dispersion, and transport conditions - hydraulic conductivity and flowpath length. Different physical conditions result in different characteristic residence times of water and solutes in hyporheic zones. The key biogeochemical factors investigated are the dynamics of oxygen (O₂), labile dissolved organic carbon (DOC), and inorganic bioavailable N (NH₄⁺ and NO₃⁻). This dissertation uses ¹⁵N isotope experiments, numerical modeling of coupled transport of the bioavailable N species, O₂ and DOC, and a suite of geophysical measurements to identify the key linkages between hydrological and biogeochemical controls on N transformations in hyporheic zones. Specifically, it was determined that the conditions governing the fate of hyporheic N are both the physical transport and reaction kinetics – the residence time of water and the O2 uptake rate. An important scaling relationship is developed by relating the characteristic timescales of residence time and O₂ uptake. The resulting dimensionless relationship, the Damköhler number for O₂, is useful for scaling different streams hyporheic zones and their role on stream N source – sink dynamics. More generally, these investigations demonstrate that careful consideration and quantification of hydrological processes can greatly inform the investigation of aquatic biogeochemical dynamics and lead to the development of process-based knowledge. In turn, this process-based knowledge will facilitate more robust approaches to quantifying and predicting biogeochemical cycles and budgets

Coupling multiscale observations to evaluate hyporheic nitrate removal at the reach scale

Excess NO₃⁻ in streams is a growing and persistent problem for both inland and coastal ecosystems, and denitrification is the primary removal process for NO₃⁻. Hyporheic zones can have high denitrification potentials, but their role in reach- and network-scale NO₃⁻ removal is unknown because it is difficult to estimate. We used independent and complementary multiscale measurements of denitrification and total NO₃⁻ uptake to quantify the role of hyporheic NO₃⁻ removal in a 303-m reach of a 3rd-order agricultural stream in western Oregon, USA. We characterized the reach-scale NO₃⁻ dynamics with steady-state ¹⁵N-NO₃⁻ tracer-addition experiments and solute-transport modeling, and measured the hyporheic conditions via in-situ biogeochemical and groundwater modeling. We also developed a method to link these independent multiscale measurements. Hyporheic NO₃⁻ removal (rate coefficient λ[subscript HZ] = 0.007/h) accounted for 17% of the observed total reach NO₃⁻ uptake and 32% of the reach denitrification estimated from the ¹⁵N experiments. The primary limitations on hyporheic denitrification at the reach scale were availability of labile dissolved organic C and the restricted size of the hyporheic zone caused by anthropogenic channelization (sediment thickness ≤ 1.5 m). Linking multiscale methods made estimates possible for hyporheic influence on stream NO₃⁻ dynamics. However, it also demonstrated that the traditional reach-scale tracer experimental designs and subsequent transport modeling cannot be used alone to directly investigate the role of the hyporheic zone on reach-scale water and solute dynamics.Keywords: residence time, denitrification, nitrogen, nutrient cycling, surface-water–groundwater interactionKeywords: residence time, denitrification, nitrogen, nutrient cycling, surface-water–groundwater interactio

Hyporheic Exchange and Water Chemistry of Two Arctic Tundra Streams of Contrasting Geomorphology

The North Slope of Alaska’s Brooks Range is underlain by continuous permafrost, but an active layer of thawed sediments develops at the tundra surface and beneath streambeds during the summer, facilitating hyporheic exchange. Our goal was to understand how active layer extent and stream geomorphology influence hyporheic exchange and nutrient chemistry. We studied two arctic tundra streams of contrasting geomorphology: a high-gradient, alluvial stream with riffle-pool sequences and a low-gradient, peat-bottomed stream with large deep pools connected by deep runs. Hyporheic exchange occurred to ~50 cm beneath the alluvial streambed and to only ~15 cm beneath the peat streambed. The thaw bulb was deeper than the hyporheic exchange zone in both stream types. The hyporheic zone was a net source of ammonium and soluble reactive phosphorus in both stream types. The hyporheic zone was a net source of nitrate in the alluvial stream, but a net nitrate sink in the peat stream. The mass flux of nutrients regenerated from the hyporheic zones in these two streams was a small portion of the surface water mass flux. Although small, hyporheic sources of regenerated nutrients help maintain the in-stream nutrient balance. If future warming in the arctic increases the depth of the thaw bulb, it may not increase the vertical extent of hyporheic exchange. The greater impacts on annual contributions of hyporheic regeneration are likely to be due to longer thawed seasons, increased sediment temperatures or changes in geomorphology

Influence of Morphology and Permafrost Dynamics on Hyporheic Exchange in Arctic Headwater Streams under Warming Climate Conditions

We investigated surface-subsurface (hyporheic) exchange in two morphologically distinct arctic headwater streams experiencing warming (thawing) sub-channel conditions. Empirically parameterized and calibrated groundwater flow models were used to assess the influence of sub-channel thaw on hyporheic exchange. Average thaw depths were at least two-fold greater under the higher-energy, alluvial stream than under the lowenergy, peat-lined stream. Alluvial hyporheic exchange had shorter residence times and longer flowpaths that occurred across greater portions of the thawed sediments. For both reaches, the morphologic (longitudinal bed topography) and hydraulic conditions (surface and groundwater flow properties) set the potential for hyporheic flow. Simulations of deeper thaw, as predicted under a warming arctic climate, only influence hyporheic exchange until a threshold depth. This depth is primarily determined by the hydraulic head gradients imposed by the stream morphology. Therefore, arctic hyporheic exchange extent is likely to be independent of greater sub-stream thaw depths

Labile dissolved organic carbon supply limits hyporheic denitrification

We used an in situ steady state ¹⁵N-labeled nitrate (¹⁵NO₃⁻) and acetate (AcO⁻) well-to-wells injection experiment to determine how the availability of labile dissolved organic carbon (DOC) as AcO⁻ influences microbial denitrification in the hyporheic zone of an upland (third-order) agricultural stream. The experimental wells receiving conservative (Cl⁻ and Br) and reactive (¹⁵NO₃⁻) solute tracers had hyporheic median residence times of 7.0 to 13.1 h, nominal flowpath lengths of 0.7 to 3.7 m, and hypoxic conditions (<1.5 mg O₂ L⁻¹). All receiving wells demonstrated ¹⁵N₂ production during ambient conditions, indicating that the hyporheic zone was an environment with active denitrification. The subsequent addition of AcO⁻ stimulated more denitrification as evidenced by significant δ¹⁵N₂ increases by factors of 2.7 to 26.1 in receiving wells and significant decreases of NO₃⁻ and DO in the two wells most hydrologically connected to the injection. The rate of nitrate removal in the hyporheic zone increased from 218 kg ha⁻¹ yr⁻¹ to 521 kg ha⁻¹ yr⁻¹ under elevated AcO⁻ conditions. In all receiving wells, increases of bromide and ¹⁵N₂ occurred without concurrent increases in AcO⁻, indicating that 100% of AcO⁻ was retained or lost in the hyporheic zone. These results support the hypothesis that denitrification in anaerobic portions of the hyporheic zone is limited by labile DOC supply

Dynamics of nitrate production and removal as a function of residence time in the hyporheic zone

Biogeochemical reactions associated with stream nitrogen cycling, such as nitrification and denitrification, can be strongly controlled by water and solute residence times in the hyporheic zone (HZ). We used a whole‐stream steady state ¹⁵N‐labeled nitrate (¹⁵NO₃⁻) and conservative tracer (Cl⁻) addition to investigate the spatial and temporal physiochemical conditions controlling the denitrification dynamics in the HZ of an upland agricultural stream. We measured solute concentrations (¹⁵NO₃⁻, ¹⁵N₂ (g), as well as NO₃⁻, NH₃, DOC, DO, Cl⁻), and hydraulic transport parameters (head, flow rates, flow paths, and residence time distributions) of the reach and along HZ flow paths of an instrumented gravel bar. HZ exchange was observed across the entire gravel bar (i.e., in all wells) with flow path lengths up to 4.2 m and corresponding median residence times greater than 28.5 h. The HZ transitioned from a net nitrification environment at its head (short residence times) to a net denitrification environment at its tail (long residence times). NO₃⁻ increased at short residence times from 0.32 to 0.54 mg‐N L⁻¹ until a threshold of 6.9 h and then consistently decreased from 0.54 to 0.03 mg‐N L⁻¹. Along these same flow paths, declines were seen in DO (from 8.31 to 0.59 mg‐O₂ L⁻¹) and DOC (from 3.0 to 1.7 mg‐C L⁻¹). The rates of the DO and DOC removal and net nitrification were greatest during short residence times, while the rate of denitrification was greatest at long residence times. ¹⁵NO₃⁻ tracing confirmed that a fraction of the NO₃⁻ removal was via denitrification as ¹⁵N₂ was produced across the entire gravel bar HZ. Production of ¹⁵N₂ across all observed flow paths and residence times indicated that denitrification microsites are present even where nitrification was the net outcome. These findings demonstrate that the HZ is an active nitrogen sink in this system and that the distinction between net nitrification and denitrification in the HZ is a function of residence time and exhibits threshold behavior. Consequently, incorporation of HZ exchange and water residence time characterizations will improve mechanistic predictions of nitrogen cycling in streams

Comparison of Instantaneous and Constant-Rate Stream Tracer Experiments Through Parametric Analysis of Residence Time Distributions

Artificial tracers are frequently employed to characterize solute residence times in stream systems and infer the nature of water retention. When the duration of tracer application is different between experiments, tracer breakthrough curves at downstream locations are difficult to compare directly. We explore methods for deriving stream solute residence time distributions (RTD) from tracer test data, allowing direct, non-parametric comparison of results from experiments of different durations. Paired short- and long-duration field experiments were performed using instantaneous and constant-rate tracer releases, respectively. The experiments were conducted in two study reaches that were morphologically distinct in channel structure and substrate size. Frequency- and time domain deconvolution techniques were used to derive RTDs from the resulting tracer concentrations. Comparisons of results between experiments of different duration demonstrated few differences in hydrologic retention characteristics inferred from short- and long-term tracer tests. Because non-parametric RTD analysis does not presume any shape of the distribution, it is useful for comparisons across tracer experiments with variable inputs and for validations of fundamental transport model assumptions

Coupled transport and reaction kinetics control the nitrate source-sink function of hyporheic zones

The fate of biologically available nitrogen (N) and carbon (C) in stream ecosystems is controlled by the coupling of physical transport and biogeochemical reaction kinetics. However, determining the relative role of physical and biogeochemical controls at different temporal and spatial scales is difficult. The hyporheic zone (HZ), where groundwater–stream water mix, can be an important location controlling N and C transformations because it creates strong gradients in both the physical and biogeochemical conditions that control redox biogeochemistry. We evaluated the coupling of physical transport and biogeochemical redox reactions by linking an advection, dispersion, and residence time model with a multiple Monod kinetics model simulating the concentrations of oxygen (O₂), ammonium (NH₄), nitrate (NO₃), and dissolved organic carbon (DOC). We used global Monte Carlo sensitivity analyses with a nondimensional form of the model to examine coupled nitrification-denitrification dynamics across many scales of transport and reaction conditions. Results demonstrated that the residence time of water in the HZ and the uptake rate of O₂ from either respiration and/or nitrification determined whether the HZ was a source or a sink of NO₃ to the stream. We further show that whether the HZ is a net NO₃ source or net NO₃ sink is determined by the ratio of the characteristic transport time to the characteristic reaction time of O₂ (i.e., the Damköhler number, Da[subscript O2]), where HZs with Da[subscript O2] < 1 will be net nitrification environments and HZs with Da[subscript O2] ≪ 1 will be net denitrification environments. Our coupling of the hydrologic and biogeochemical limitations of N transformations across different temporal and spatial scales within the HZ allows us to explain the widely contrasting results of previous investigations of HZ N dynamics which variously identify the HZ as either a net source or sink of NO₃. Our model results suggest that only estimates of residence times and O₂ uptake rates are necessary to predict this nitrification-denitrification threshold and, ultimately, whether a HZ will be either a net source or sink of NO₃

Transient Storage as a Function of Geomorphology, Discharge, and Permafrost Active Layer Conditions in Arctic Tundra Streams

Transient storage of solutes in hyporheic zones or other slow-moving stream waters plays an important role in the biogeochemical processes of streams. While numerous studies have reported a wide range of parameter values from simulations of transient storage, little field work has been done to investigate the correlations between these parameters and shifts in surface and subsurface flow conditions. In this investigation we use the stream properties of the Arctic (namely, highly varied discharges, channel morphologies, and subchannel permafrost conditions) to isolate the effects of discharge, channel morphology, and potential size of the hyporheic zone on transient storage. We repeated stream tracer experiments in five morphologically diverse tundra streams in Arctic Alaska during the thaw season (May–August) of 2004 to assess transient storage and hydrologic characteristics. We compared transient storage model parameters to discharge (Q), the Darcy-Weisbach friction factor (f), and unit stream power (ω). Across all studied streams, permafrost active layer depths (i.e., the potential extent of the hyporheic zone) increased throughout the thaw season, and discharges and velocities varied dramatically with minimum ranges of eight-fold and four-fold, respectively. In all reaches the mean storage residence time (tstor) decreased exponentially with increasing Q, but did not clearly relate to permafrost active layer depths. Furthermore, we found that modeled transient storage metrics (i.e., tstor, storage zone exchange rate (αOTIS), and hydraulic retention (Rh)) correlated better with channel hydraulic descriptors such as f and ω than they did with Q or channel slope. Our results indicate that Q is the first-order control on transient storage dynamics of these streams, and that f and ω are two relatively simple measures of channel hydraulics that may be important metrics for predicting the response of transient storage to perturbations in discharge and morphology in a given stream

Human domination of the global water cycle absent from depictions and perceptions

International audienceHuman water use, climate change and land conversion have created a water crisis for billions of individuals and many ecosystems worldwide. Global water stocks and fluxes are estimated empirically and with computer models, but this information is conveyed to policymakers and researchers through water cycle diagrams. Here we compiled a synthesis of the global water cycle, which we compared with 464 water cycle diagrams from around the world. Although human freshwater appropriation now equals half of global river discharge, only 15% of the water cycle diagrams depicted human interaction with water. Only 2% of the diagrams showed climate change or water pollution—two of the central causes of the global water crisis—which effectively conveys a false sense of water security. A single catchment was depicted in 95% of the diagrams, which precludes the representation of teleconnections such as ocean–land interactions and continental moisture recycling. These inaccuracies correspond with specific dimensions of water mismanagement, which suggest that flaws in water diagrams reflect and reinforce the misunderstanding of global hydrology by policymakers, researchers and the public. Correct depictions of the water cycle will not solve the global water crisis, but reconceiving this symbol is an important step towards equitable water governance, sustainable development and planetary thinking in the Anthropocene