H14B-01 INVITED
Scaling Hyporheic Flow and Biogeochemical Reactions across a Wide Range of Flow and Sediment Conditions in Aquatic Systems
Aquatic ecosystems are strongly influenced by advective transport from surface water into shallow sediments
of the hyporheic zone. The delivery of energy and nutrient-rich materials to microbially and geochemically
reactive sediment stimulates high rates of biogeochemical reactions that influence the overall metabolism of
the ecosystem as well as influencing the chemistry of downstream receiving waters. Predicting hyporheic flow
is difficult because of the potential involvement of many physical processes, including diffusion, shear,
bedform-scale advective pumping, bed mobility and bioturbation, turbulence penetration, and head potential-
driven groundwater exchange. We used published data from carefully controlled laboratory flume
experiments to develop a scaling relationship that predicts hyporheic exchange based on physical
descriptors (e.g. shear stress velocity, roughness height, and sediment permeability) that summarize fluid-
flow and sediment characteristics. We tested the scaling relationship's predictions by comparing them with
more time and labor intensive measurements of solute and reactive tracer transport made in situ in hyporheic
zones. In situ measurements were acquired using the USGS MINIPOINT sampler, which allows detailed
subsurface measurements without significant disturbance of sediment or the ambient surface or subsurface
water fluxes. Fieldwork was undertaken in several streams that varied widely in surface water flow velocities,
grain type, median grain size, sediment porosity, sediment organic content, sediment hydraulic conductivity,
and groundwater specific discharge. The comparison generally supported the predictive capability of the
scaling relationship in complex field settings. The value of the scaling relationship is also indicated for
improving rate measurements of biogeochemical reactions in hyporheic zones (e.g. oxygen uptake,
denitrification, and manganese oxidation), as well as for estimating the cumulative influence of hyporheic
reactions on chemistry and water quality of downstream receiving waters.
http://water.usgs.gov/nrp/jharvey/site/index.html
H14B-02
Quantifying the Role of Hyporheic Zone of Gravel Bed Rivers in the Nitrogen Cycle
The hyporheic exchange resulting from the interaction between the stream and its surrounding saturated streambed sediments, has a profound impact on fluvial ecosystems. Bedforms cause near bed pressure gradients that induce a complex flow pattern within the hyporheic zone. This surface-subsurface water exchange influences important processes occurring at the interface between surface and subsurface waters, including nutrients and solute export. In the present work, we focus on the export of ammonium (NH4+), nitrate (NO3-) and their fate within the streambed of a gravel bed river. We modelled the hyporheic exchange by combining analytical solutions of the intra-gravel flows induced by alternate bars with a set of transport equations for NH4+ and NO3- coupled with first order kinetics. Transport is solved by particle tracking, assuming that local dispersion is negligible and considering that temperature affects the reaction rate coefficients. With this simple, yet powerful, model, we studied the interplay between streambed morphology and nitrogen fate within the hyporheic zone. We observed that while the hyporheic zone acts as a sink of ammonium to an extent that depends on the nitrification rate, it may act as a source or a sink of nitrate. Additionally, it can influence the emission of nitrogen gases (N2 and N2O), depending on the ratio between ammonium and nitrate concentrations in the stream and on the role of biomass uptake. We also observed that because of the shorter residence time nitrification dominates in small steep streams, while denitrification plays a major role in low-gradient large streams. Furthermore, the emission of nitrogen gases increases with the pore water temperature in small steep streams, but not with the thickness of alluvium depth, due to fact that the hyporheic flow mostly develops near the surface. On the other hand, the emission of nitrogen gases increases with both temperature and alluvium depth in low-gradient streams. The overall conclusion of our analysis is that river morphology has a major impact on processes as nitrification and denitrification with a direct impact on the export of nutrients.
H14B-03
Assessing the influence of various aquatic ecosystem types on biogeochemical fluxes at river network scales
River networks are comprised of a wide array of linked aquatic ecosystem types (e.g. channels of different sizes with associated transient storage zones, ponds, lakes, reservoirs, and floodplains). Recent field and modeling results demonstrate that river channels become less effective at retaining nutrients under high flow conditions when most nutrient fluxes occur. During these periods, lakes, reservoirs, and floodplains may play a more important role than river channels in basin scale nutrient removal. We applied a river network N removal model, modified to integrate lakes, to a hypothetical 7th order river network to assess how lake number, position in the network, and size influence biogeochemical fluxes at river network scales. We assumed a frequency distribution of lake size and number consistent with Downing et al. (2006), that lake position and size is correlated with river size, and that biological reactivity (uptake velocity) was constant with respect to concentration and 5x higher in rivers than lakes. Under mean annual conditions, whole network removal was 55% of total inputs, with roughly 2/3 occurring in rivers and 1/3 in lakes. Small headwater lakes (< 0.01 km2) accounted for half the lake removal because they were numerous and therefore more likely to intercept nonpoint inputs near their source. However, a single moderate sized lake located in a higher order stream can dominate river network N removal because of small hydraulic loads and interception of a large proportion of basin runoff. These results suggest that spatially integrating multiple aquatic ecosystem types into river network models is necessary to understand the mechanisms controlling biogeochemical fluxes and variability across basins and over time.
H14B-04
Topographical scaling of surface water-groundwater interaction in the hyporheic zone
Landscape topography controls the surface water exchange with groundwaters from mountain ranges to the smallest hills. Separating the topography in a Fourier spectrum both represents the fractal ground surface topography in fluvial and glacial landscapes and provides an exact solution for the three-dimensional groundwater flows including the surface water interaction. The fractal topography implies that all landscape features have a significant impact on the surface–subsurface water interaction. However, because of the decaying permeability with depth there is a clear tendency that the short-term interfacial flux is dominated by small-scale features, while the large-scale features seem to cause generally larger surface fluxes over much longer times. With depth the larger topographical scales become increasingly important for the groundwater fluxes. In the order of 10 percents of the precipitation in Scandinavia infiltrates the ground, but only about 1 permille of the groundwater recharge reaches deeper than 400 m. The effect of large-scale topography to water fluxes with the hyporheic zone is generally seen as gaining and losing reaches, whereas on the landscape scale this flux is slightly larger than hyporheic exchange caused by in-stream topographical effects. The fractal nature of surface-subsurface waterfluxes yields a scale-independent distribution of surface fluxes and subsurface water residence times that can be formally implemented in surface hydrological models on the catchment scale. This new approach is expected to provide a better understanding of the overall water balance in watersheds and the interaction with biogeochemistry.
H14B-05
The Influence of Landscape Drainage on Biogeochemical Cycling of Carbon in Agricultural Ecosystems
The movement of water through agricultural ecosystems is often modified by the presence of open ditches and subsurface tile drainage systems. Despite the common occurrence of these practices, particularly in the corn- and soybean-producing regions of the midwestern United States, much remains unknown about how altered drainage patterns may influence carbon export from agricultural landscapes. In this study, we examined the role of subsurface drainage systems on the quantity and quality of dissolved carbon export from experimental agricultural fields located in south-central Minnesota. Results from two years of observations show that fields with more intense drainage designs (e.g., greater density of subsurface drain lines) have dissolved organic carbon (DOC) concentrations that are similar to conventionally drained fields. However, fields with more intense drainage exhibit greater annual DOC loads due to higher water yields resulting from more intense drainage. In contrast, dissolved inorganic carbon (DIC) concentrations were consistently greater in fields with more intense drainage practices across all flow conditions. Our ongoing work is focused on determining if these differences in DIC concentrations are the result of either increased weathering or increased soil/plant root respiration resulting in increased soil CO2 concentrations. Molecular weight characterization of samples from our experimental fields shows that DOC from subsurface tile drainage is generally comprised of low molecular weight compounds. This low molecular weight signal is less apparent in samples from downstream ditch and river sites which are dominated by higher molecular weight compounds; suggesting that differences in organic matter source and/or processing are apparent over spatial scales transitioning from the field to small watershed. Overall, these results show that subsurface drainage practices fundamentally alter annual DOC and DIC carbon export from agricultural ecosystems as well as influence qualitative characteristics of DOC that could influence downstream biological and chemical transformations and the ultimate fate of terrestrially-derived carbon in aquatic environments.
H14B-06
Multi-scale Dynamics of Surface and Subsurface Processes and Effects on the Hydrochemistry and Hydroecology of a Montane Salmon Spawning Stream
Multi-scale investigations in a 31km2 catchment in the Scottish Highlands have used high resolution monitoring and long-term sampling of multiple-tracers to conceptualise the dynamic interactions between surface and subsurface processes and their influence on stream water chemistry and aquatic ecology. At the catchment scale, geochemical and isotopic tracers demonstrate that circum-neutral baseflows are derived from a wide range of older groundwater sources that discharge into the stream via spatially discrete hotspots in the riparian and hyporheic zones. However, extensive hyporheic exchange throughout the channel network also regulates stream water quality at low flows via nutrient retention, diurnal cycling of carbon and moderation of stream temperatures. At higher flows, tracers show that surface processes in riparian wetlands generate large volumes of acidic, "young" overland flow that dominates surface waters and "resets" the hyporheic hydrochemistry as local hydraulic head reversals cause ingress of more acidic stream waters that are rich in DOC. The marked temporal dynamics between periods where surface or subsurface hydrological sources dominate the stream network and hyporheic zone are climatically driven and are evident at event, seasonal and inter-annual scales. The ecology of salmon populations in the stream reflects the imprint of spatial and temporal variations in catchment scale interactions between surface and subsurface processes. For example, timing of the upstream migration of salmon for spawning is triggered by the onset of higher storm flows in the early winter. Spatially, spawning itself is concentrated at hotspots of groundwater discharge through the hyporheic zone where changes in reach scale geomorphology also result in suitable hydraulic and sedimentary conditions. Although the resident salmon population is adapted to long-term hydrological conditions, climatic variability can result in stochastic inter-annual perturbations to different life stages. For example, dry autumns can greatly reduce the numbers of salmon entering to spawn and wet winters can result in prolonged periods of discharge of de-oxygenated groundwater through spawning gravels which can cause egg mortality. Both conditions can have adverse impacts on recruitment of juveniles. Understanding such interactions, and predicting how they will be affected by climatic change, is only possible through a long- term commitment to interdisciplinary, multi-scale studies.
H14B-07 INVITED
The Changing Role of Hyporheic Exchange Flows Across a Stream Network
The influence of hyporheic exchange on biogeochemical processing of solutes transported through stream networks depends on the time scale of the biogeochemical process, the residence-time distribution of hyporheic exchange flows, and the amount of stream water cycled through the hyporheic zone (QHEF). I examined the longitudinal and seasonal patterns in QHEF in a 5th-order, 62 km2 mountainous stream network. The size of exchange flows, relative to stream discharge (QHEF:Q), was large only in very small streams at low discharge (area = 100 ha; Q < 10 l/s). At higher flows (flow exceedance probability > 0.7) and in all larger streams, QHEF:Q was small. Clearly then, biogeochemical processes in the hyporheic zone of small streams can substantially influence the streams solute load, but these processes become hydrologically constrained at high discharge or in larger streams and rivers. In large streams, even at baseflow discharge, biogeochemical transformations are unlikely to reset the chemical signatures that affect downstream solute fluxes because QHEF is always small relative to stream discharge. How then, can hyporheic processing of solutes be important in larger streams and rivers? A groundwater flow model was used to examine spatial patterns of hyporheic exchange on the bed of a small stream. Results showed distinct patches of downwelling and upwelling. Downwelling water is brought into close contact with biofilms on sediment surfaces so that reactive solutes are exposed to biogeochemical processing. Return flows of hyporheic water may be rich in plant-available forms of limiting nutrients. Thus, hyporheic exchange can create hotspots of biological activity at both downwelling and upwelling patches. These patches are likely to be important to biological and ecosystem processes, even if their impact on bulk stream water solute loads is small.