H11B-0733
What Have We Done? The State of Hyporheic Characterization
Numerous studies of hyporheic zone processes have been completed in the past 50 years, yet we lack a universal, interdisciplinary set of criterion to facilitate comparison across spatial and temporal scales. Whereas these many studies addressed a variety of questions or hypotheses, comparison among them is difficult because there are few common, meaningful characterizations made that are appropriate for cross- site comparison. These studies encompass a breadth of hyporheic processes yet few provide meaningful data for use by researchers investigating related hyporheic phenomena. For example, a characterization of the physical flowpaths or hydraulic retention of a stream likely provides incomplete information to a researcher interested in microbial production. By expanding our observation sets to include a set of basic, inexpensive parameters we can facilitate additional analysis of hyporheic processes and their interactions across spatial and temporal scales. Review and analysis of selected literature identifies typical state variables, spatial and temporal scales of investigation, and the primary hyporheic processes(es) investigated. Channel dimension, stream gradient, and sediment grain size are among the most universally reported characterizations in the hyporheic literature. In contrast, few papers studying physical processes report even the simplest characterization of benthic community or water chemistry, preventing scientists studying chemical or biological processes from fully utilizing the data set. We identify additional trends and gaps in the characterization and assessment of hyporheic zones and processes as we move toward establishing a universal characterization.
H11B-0734
Prediction of lateral hyporheic flux in meandering rivers
The meandering of rivers results in head gradients across pointbar deposits leading to hyporheic exchange- the flow of groundwater from and back to the stream. This study presents two models for predicting horizontal fluid flux: one based solely on valley slope and the other uses both valley slope and channel sinuosity. Both models require knowledge of aquifer hydraulic conductivity. The models are developed via a suite of groundwater flow simulations with boundary conditions taking a variety of channel geometries, idealized as sinusoidal, and valley slopes which we assume is similar to that of the water table. Functional predictive models are developed by fitting a line or surface to discrete model output which cover the spectrum of most natural meandering rivers. Since the fitted models only require channel slope and/or sinuosity (plus hydraulic conductivity), they are amenable for regional estimation of hyporheic flux using digital terrain data.
H11B-0735
Surface-Subsurface Exchange and Transient Storage in Relation to Riparian Corridor Land Cover in an Urban Watershed
A suite of methods is being utilized by the Baltimore Waters Test Bed to develop an understanding of the interaction between groundwater and surface water at multiple space and time scales. As part of this effort, bromide tracer experiments were conducted over two 10-day periods in August 2007 and May 2008 along two 900-m reaches of Dead Run, a small urban stream located in Baltimore County, Maryland, to investigate the influence of distinct zones of riparian land cover on surface-subsurface exchange and transient storage under low- and high-baseflow conditions. Riparian land cover varied by reach along a gradient of land use spanning parkland, suburban/residential, and commercial, and included wooded, meadow, turf grass, and impervious cover. Reach-average surface-subsurface exchange was estimated using tracer-dilution and velocity-gaging stream flow measurements. Transient storage parameters (active storage area and storage exchange rate coefficient) were estimated using the USGS-code OTIS. Bed sediment grain size distribution was found to be unrelated to watershed position and riparian cover; however, a trend of finer bed sediment under low baseflow conditions was observed at most stations. In the lower section of Dead Run, gross inflow, gross outflow and surface-subsurface exchange (defined as the absolute value of net inflow) tended to be higher under high baseflow conditions (May 2008) compared to low baseflow conditions (August 2007). Surface-subsurface exchange was highest in reaches with greater riparian pervious cover while transient storage area and exchange rate were highest in reaches with greater impervious cover. Results from the tracer test are being incorporated into a coupled watershed model of the region. When working in urban watersheds, many complexities not typically encountered in more pristine areas must be addressed. Among these are high ambient salt concentrations (> 200 mg/L chloride), unexpected (and transient) stream discharges and withdrawals, and potential interactions with the public and police.
H11B-0736
Lateral Hyporheic Exchange Along a Beaver-Dammed Stream Draining a Montane Peatland
Hyporheic zones are dynamic areas important in increasing stream water transit time through basins and enhancing redox-sensitive biogeochemical reactions that influence downstream water quality and ecosystem health. Hyporheic flowpath length and complexity may be increased by beaver dams, which are common throughout lower-order streams in North America. We investigated lateral hyporheic exchange along a beaver dammed, second-order stream draining a ~1.3 km2 Canadian Rocky Mountain peatland. Observations of hydraulic heads and chloride concentrations in a network of 224 piezometers and 80 water table wells were used to document spatial and temporal patterns of hyporheic flow and zone extent. Heads were measured throughout the summer of 2006-2008 and water samples were taken weekly in summer 2008. Hyporheic fluxes were computed using Darcy's Law. Samples were analyzed for chloride concentrations, which were used as a conservative tracer in a two-component mixing model to separate subsurface water into its stream and groundwater components. The hyporheic zone was delineated where subsurface water contained >10% stream water. In contrast to findings in other geographic areas, lateral hyporheic fluxes and zone size did not vary with stream discharge. Instead, we show that beaver dams are a key driver of stream water into the banks, which resulted in greater extension of the hyporheic zone into the bank than along an undammed reach. Summer mean hyporheic flux was <0.005 L/s along the undammed reach and ranged from 0.002 to 0.015 L/s along the beaver dammed reach, and stream water flowed under the banks in a looping fashion around the dam. Chloride concentrations indicate that the size of the hyporheic zone lateral to the stream at the beaver dam was at least 30% greater than at the undammed reach. Chloride concentrations were >10% in nearly all piezometers and water table wells, suggesting that our reach may be nested within a larger hyporheic flow system generated by two relatively large beaver dams upstream of the study reach. In the future, lateral fluxes of nitrogen across the stream-hyporheic interface will be calculated and patterns in nitrogen and dissolved organic carbon chemistry will be examined in relation to the degree of stream water and groundwater mixing to look for hot spots and hot moments in nitrogen dynamics.
H11B-0737
Determining Surface Transient Storage Zone Residence Time Distributions from Whole Stream Solute Injections
Little is known about the impact of surface transient storage (STS) zones on reach-scale transport and fate
of dissolved nutrients in streams. Exchange with these locations may influence the rates of nutrient cycling
often observed in whole-stream tracer experiments, particularly because they are sites of organic matter
collection and lower flow velocities than those observed in the thalweg. We performed a conservative stream
tracer experiment (slug of dissolved NaCl) in the Ipswich River in northeastern Massachusetts, USA, and
collected solute breakthrough curve data both in the thalweg and 3 adjacent STS zones. Tracer
breakthrough curves observed in STS zones are a function of residence time distributions (RTDs) that are an
aggregate of the upstream transport to that point (RTDupstream) and that which is due to temporary
storage within the zones of interest (RTDSTS). Here we demonstrate the separation of these two RTDs
to determine the RTDSTS specifically. All three RTDupstream functions and two RTDSTS
functions demonstrate exponential behavior. The other RTDSTS demonstrates power-law behavior.
Thus, whereas there is some variability in the STS RTDs the overall signal observed in the channel is
consistently exponential. Transit times for these STS zones are on the order of hr, compared to minutes for
thalweg transport, suggesting that these zones have the potential to host important biogeochemical
transformations in stream systems.
http://water.engr.psu.edu/gooseff/ipswich.html
H11B-0738
Characterization of Surface Transient Storage Zone Exchange and Flow Dynamics in a 3rd-Order Stream, Massachusetts
There is little known about the influence of surface transient storage (STS) zones (backwaters, eddies, etc.)
on stream solute transport, though these are potential biogeochemical hotspots because they collect
particulate organic matter and retain stream water on timescales longer than the thalweg. Cycling of solutes
within STS zones greatly depends on the residence times, water depth, flow patterns, and substrate within
them. We characterized these properties of STS zones to assess the potential for biogeochemical processes
to occur. Solute injections and detailed velocity, depth, and topographic surveys were conducted in three
STS zones during various flow conditions in Fish Brook, a tributary to the Ipswich River in northeastern
Massachusetts. NaCl was used as a conservative tracer to obtain residence time estimates and velocity
grids with location and depth coordinates were made using a handheld ADV (Acoustic Doppler Velocimeter).
Wolman pebble counts were conducted in each of the surface transient storage zones to characterize
substrate. We have found that velocity patterns within the STS zones vary as expected at different stages,
and residence times are generally on the order of hours. Substrate characterization indicates that these are
locations of enhanced organic matter collection and retention. These assessments suggest that STS zones
have the potential to be important locations of biogeochemical reactions in streams.
http://water.engr.psu.edu/gooseff/ipswich.html
H11B-0739
Spatial and Temporal Trends in the Carbon, Nitrogen, and Sulfur Isotopes of Stream DOM From 10 Watersheds at the HJ Andrews Experimental Forest.
In order to better understand sources of dissolved organic matter (DOM) in streams from the small to large
watershed scales, we initiated a one-year investigation of the chemical and isotopic characteristics of DOM at
the HJ Andrews Experimental Forest (HJA) in Blue River, OR. DOM is a biologically significant loss from
these watersheds, but its sources (forest floor, mineral soil, riparian zones, stream biota) are debated.
Traditional chemical characterizations of DOM like SUVA and FI have been useful in conceptualizing and
modeling streamflow sources, however, an improved method for assessing DOM quality is needed to
adequately differentiate DOM from sources within the watershed. The isotopic characterization of inorganic
molecules like nitrate has provided insight to the role of subsurface and surface processes governing the
production and transport of critical nutrients, and yet to date, little work has been done to examine the
usefulness of isotopic characterization of organically bound nutrients.
We apply the isotopic characterization approach to DOM in order to better understand DOM production,
transformation, and transport to streams in a range of watershed sizes. Major questions addressed in this
research are: 1) Where in the watershed does stream DOM come from? 2) How do DOM sources vary
temporally? 3) How do physical attributes of the watershed mediate DOM quality?
A relatively new solid-phase extraction technique using C-18 resin was used to isolate DOM in water samples
from 10 watersheds, ranging in size from 10 to 6200 hectares, on 3-week intervals from May 2007 to June
2008. The modified technique allowed for small (1 Liter) sample sizes and short processing times to reduce
the costs of analysis. The capacity of carbon, nitrogen and sulfur isotopic characterizations of DOM, as well
as traditional methods like SUVA and C:N, to predict physical watershed attributes (i.e. mean residence time,
soil depth, elevation, gradient) and land use history (timber harvest treatments) was tested using multiple
linear regression. Analyses showed that isotopic characterization helped to partition the potential sources of
stream DOM.
http://oregonstate.edu/~frentrej/research/
H11B-0740
Progressive Salinization and Chemical Evolution of the Rio Grande (New Mexico) Driven by Interaction of Deep Brine Leakage with Agricultural Processes
The total dissolved solids content of the Rio Grande increases from 40 mg/L at its headwaters in the San Juan Mountains of Colorado to over 1,000 mg/L at El Paso, Texas, located 1,000 km downstream. Along this path the composition evolves from a Ca-HCO3 dominated water to a Na-(Ca-Mg) SO4-(Cl- HCO3) water. These changes are highly detrimental to use of the water for urban and agricultural purposes, but the causes have not previously been adequately understood. We show that this evolution is driven by the interaction of deep sedimentary brine leakage with geochemical reactions associated with irrigated agriculture processes. All these are modulated by the progression of lithology encountered by the river along its path. The initial water composition in the San Juan Mountains is fixed by classical aluminosilicate incongruent weathering reactions. As the river flows southward it encounters sedimentary basins where Na-Cl-(SO4) brines discharge along faults. Diversion of the water for irrigation and subsequent evapotranspiration concentrate these solutes. Upon entering the vadose zone beneath agricultural fields the waters encounter gypsum, dolomite, and very high pCO2. In this environment, increases in the Ca and HCO3 concentrations are suppressed by dedolomitization, while SO4 increases. After subsequent discharge to agricultural drains, remaining HCO3 is lost by CO2 degassing and additional carbonate minerals are precipitated. The Rio Grande effectively “spirals” through a succession of surface and subsurface pathways that extend over hundreds of kilometers and it is this "geochemical ratchet effect" associated with surface/subsurface exchange that drives the dramatic increase in the salinity of the river.
H11B-0741
Partitioning Surface and Hyporheic Transient Storage in Streams of Increasing Size
In comparison to the main channel of a stream, transient storage zones provide increased opportunity for flowing water and dissolved solutes to interact with microbial communities, both by extending residence times and by facilitating contact with biogeochemically active sediments. Our current understanding of transient storage has been limited by an inability to discriminate between stream main channel exchange with surface transient storage (STS) and with hyporheic transient storage (HTS) two locations with potentially very different conditions (light, redox state, temperature, etc.). In this study, we apply a two storage zone model to conservative solute breakthrough curves (BTCs) collected from main channel and STS zones to estimate both STS and HTS parameters. The estimation of two distinct sets of transient storage parameters is supported by the information from these BTCs and from detailed stream velocity cross-sections. This approach was implemented in streams throughout the low gradient Ipswich and Parker River Basins, Massachusetts, USA with contributing areas ranging from 1.0 km2 to 188.0 km2. Our results indicate that the flux of water between the main channel and STS is an order of magnitude higher than the main channel-HTS flux, and that both show a positive relationship with stream size. Conversely, mean water residence time within STS is at least an order of magnitude lower than the corresponding HTS, while mean residence time in both storage zones also increases with stream size. Finally, exchange with STS has a consistently large effect on median reach transport time in all streams studied, while exchange with HTS has a consistently negligible effect. We conclude that STS exchange may be of low biogeochemical importance in small headwater streams, but is increasingly influential in larger systems. Although HTS conditions may strongly promote internal biogeochemical processes, exchange with this storage zone may be of little importance to overall stream chemistry in low gradient basins.
H11B-0742
Water and Nutrient Sources for Floodplain Trees: Model-Based Inference
Hydrologic connectivity between floodplain forests and their rivers affects forest productivity, habitat quality, and river chemistry and function. Here we focus on one aspect of connectivity, plant water uptake from the shallow alluvial aquifer, as a key aquatic-terrestrial link and as a clue to nutrient uptake from the aquifer. We infer a depth profile of water accessed by trees by applying a one-dimensional hydrologic model to observations of sapflow, soil moisture, and meteorological variables at contrasting sites on the Nyack Floodplain of the Middle Fork Flathead River, Montana. Combined model simulations and field observations suggest that shallow groundwater access regulates the active growing season and spatio-temporal patterns of NPP. In the early summer, trees access water in the aquifer at each of the three instrumented sites despite differences in soil type and tree growth form. During the late summer simulations are especially sensitive to modeled rooting depth, with trees becoming either (1) increasingly dependent on the alluvial aquifer for water as surface soils dry or (2) fully disconnected from the aquifer. Ultimately, we plan to incorporate these results in the coupling of a hydrologic/biogeochemical model of the aquifer (WREN, Water-Borne Resource Exchange Network) with a forest growth model (Biome-BGC). With the coupled model we will investigate spatially explicit effects of forest nutrient uptake on the biogeochemistry of alluvial aquifers and floodplain- flanked river channels.
H11B-0743
Use of a Lumped Seasonal Agricultural Flow in an Inverse End-Member Mixing Analysis to Understand Transport of Water and Solutes in Irrigated Catchments
Irrigation water can be a major component of the hydrologic budgets of agricultural catchments in arid regions and can play a dominant role in the transport of solutes in those catchments. Hydrologic, chemical, isotopic, age-dating, and mineralogical data were collected over a full year from the irrigation water supply canal, ground water, overland flow, streambed pore water, and the catchment outflow of a small (6 km2) agricultural catchment in central Washington. During the irrigation season, outflow from the catchment was 2 to 3 times greater than during winter. The water chemistry at the outflow also varied substantially between the irrigation season and winter. Based on these observations, a simple, end-member mixing analysis (EMMA) was formulated for the catchment that expressed the outflow as the sum of ground-water base flow plus seasonal agricultural flow (SAF). Conceptually, SAF is a hypothetical composite of excess irrigation water and includes system spill, tailwater, overland flow, and applied irrigation water that is not incorporated into the ground-water system, but infiltrates and travels via shallow, short subsurface flow paths (e.g., tile drains) to the surface water system. Because winter catchment outflow was virtually all ground-water base flow, summer base flow was estimated by adjusting winter catchment outflow measurements to reflect seasonal changes in the hydraulic head gradient between ground-water and the stream. Winter chemistry data from the catchment outflow were used to approximate summer base-flow chemistry. Using measured summer catchment discharge and chemical characteristics (nutrients and major ions) and the estimates for summer base-flow characteristics, the discharge and chemical characteristics of SAF were calculated using the inverse formulation of the EMMA. Comparing and contrasting the chemical characteristics of SAF with canal water provided insight into the composite effects of agriculture on the catchment's water chemistry. In addition, because ground-water base flow in the catchment can virtually all be attributed to irrigation over the past century, differences between SAF and base flow provided information on water chemistry changes resulting from transport through the subsurface and(or) differences in agricultural practices over time. Finally, comparing characteristics of SAF to measured characteristics of water from a single field outflow provided insight into how well the net effect of processes at the field scale approximated the net effect of processes at the catchment scale. This simple inverse EMMA was a useful tool for testing and refining the conceptual model for this irrigated catchment.
H11B-0744
Use and modeling of the "smart" tracer resazurin to quantify metabolically-active transient storage: Field results
We have developed a tracer, resazurin, that is sensitive to metabolically-active transient storage (MATS water with velocity much slower than the mean in which there is significant primary production or respiration). Resazurin quickly reduces to resorufin in the presence of aerobic respiration, which is concentrated within MATS. Field and lab tests indicate that it is feasible to use resazurin in streams to measure MATS, and that a standardized methodology to measure MATS can be adopted similar to what is commonly done with a conservative tracer (e.g., NaCl) to measure transient storage. We have also significantly upgraded STAMMT- L (v 3.0) to enable the code to handle multiple decaying or reacting compounds simultaneously (e.g., a conservative tracer, resazurin, and resorufin) in both the channel and transient storage zones, and have given the code several other new capabilities. We present data and modeling results from three stream tests (Riera de Sta. Fe, Catalunya, Spain; WS3, HJ Andrews, Oregon, USA; and Drift Creek, Oregon, USA). Results show pseudo-first-order conversion of resazurin to resorufin and a qualitatively strong relationship to transient storage. We use STAMMT-L 3.0 to quantitatively compare results and parameters. The resazurin uptake length in the cobble-bed Catalan stream was 740 m and the uptake velocity was 0.7 mm/min. The uptake length in the upper, bedrock reach (little transient storage) of the WS3 was 1640 m and the uptake velocity was 0.022 mm/min. The uptake length in the lower, alluvial reach (large transient storage) of WS3 was 197 m and the uptake velocity was 0.25 mm/min. Pending results from Drift Creek will also compare resazurin and NO3 uptake.
H11B-0745
Temporal Geochemical Variations in a Mountain Stream: Expectations to Anomalies
This study describes unusual temporal variations in streamwater isotopic and geochemical composition in a 3rd order stream, Red Canyon Creek, draining carbonate-rich catchments in the southeastern flank of Wind River Range (WY). Although the isotopic ratios of oxygen in precipitation increased from about -30 ‰ to -15 ‰ during winters of 2005-2007, the oxygen isotopic composition of streams remained the same at about -18.5 ‰ despite expected dilution of base-metal and major anion concentrations in the major tributary of the system providing meltwater, Cherry Creek. We hypothesize that O-18 preferentially leached from snowpacks during early snowmelt and resulted in snowmelt that circumstantially was similar to the isotopic composition of groundwater that supports baseflow and which is controlled by mixing and dispersion of multiple rain events and snowmelt infiltrating into alluvial fans and terraces during the year. Nitrate concentration variations, distinct from all other solutes, increased between November and March when productivity diminished. The volume-weighted mean nitrate concentration and export rate are estimated to be 0.06 mg/L as N and 9.4 kg-N/m2/yr, respectively. This lower than the median export rate from watersheds elsewhere in the nation (0.087 mg/L as N and 26 kg-N/m2/yr), probably relates to low wet and dry atmospheric deposition of nitrate in the American West and/or rapid local plant uptake rate. In the transitional zone, nutrient cycling in Red Canyon Creek watershed acts more closely to the arid desert than the high peak watersheds.
H11B-0746
Hyporheic Denitrification in an Upland Agricultural Stream: a 15N Tracer Study
We used whole-stream steady-state 15N-labeled nitrate (15NO3-) and conservative tracer (Cl-) additions to investigate the hydraulic and physiochemical factors controlling denitrification in the hyporheic zone (HZ) of an upland agricultural stream. We measured solute concentrations (15NO3-, 15N2(g), as well as NO3-, DOC, DO, Cl-), and hydraulic transport parameters (head, flow rates, flowpaths, and residence time distributions) of the reach and along HZ flowpaths of an instrumented gravel bar. HZ exchange was observed across the entire gravel bar with nominal flowpath lengths up to 6.1 m and corresponding mean residence times greater than 21.5 hr. Along sampled HZ flowpaths from gravel bar head to distal well, NO3-N decreased from 0.34 to 0.02 mg/L, DO declined from 8.31 to 0.59 mg/L, and DOC dropped from 3.0 to 1.7 mg/L. The rate of the NO3-, DO, and DOC removal was greatest during the first 2 m of the flowpaths but removal continued across the entire gravel bar. Conversely, the surface water chemical and nutrient conditions were more uniform across the length of the reach (NO3-N: 0.31 - 0.34 mg/L, DO: 8.1 - 8.5 mg/L, DOC: 2.8 - 3.4 mg/L). 15NO3- tracing confirmed that the removal of the NO3- along the HZ flowpaths was primarily due to denitrification as 15N2 was produced across the entire gravel bar HZ (in all 12 wells). Relative to the HZ, surface water N2 concentrations were similar, but minimal 15N2-enrichment was observed. These findings demonstrate that the HZ is an active sink of nitrogen in this system. Overall, results support our hypothesis that HZ denitrification is an important nitrogen sink in mid-network streams with significant NO3-, because denitrification in these systems will be less limited by HZ biogeochemical and physical transport conditions compared to streams above and below them in the network.
H11B-0747
Effect of Vertical Flow Exchange on Biogeochemical Processes in Hyporheic Zones
Biogeochemical processes in hyporheic zones are of great interest because they make the hyporheic zones highly productive and complex environments. When contaminants or polluted water pass through hyporheic zones, in particular, biogeochemical processes play an important role in removing contaminants or attenuating contamination under certain conditions. The study site, a reach of Munsan stream (Paju-si, South Korea), exhibits severe contamination of surface water by nitrate released from Water Treatment Plant (WTP) nearby. The objectives of this study are to investigate the hydrologic and biogeochemical processes at the riparian area of the site which may contribute to natural attenuation of surface water driven nitrate, and analyze the effect of vertical (hyporheic) flow exchange on the biogeochemical processes in the area. To examine hydraulic mixing or dilution processes, vertical hydraulic gradients were measured at several depth levels using minipiezometers, and then soil temperatures were measured by using i-buttons installed inside the minipiezometers. The microbial analyses by means of polymerase chain reaction (PCR)-cloning methods were also done in order to identify the denitrification process in soil samples. In addition, correlation between vertical flow exchange, temperature data, and denitrifying bacteria activity was also investigated so as to examine the effects on one another. The results showed that there were significant effects of vertical flow exchange and hyporheic soil temperature on the biogeochemical processes of the site. This study found strong support for the idea that the biogeochemical function of hyporheic zone is a predictable outcome of the interaction between microbial activity and flow exchange.
H11B-0748
Comparison of Model Structure in Multiple-Transient Storage Modeling of Solute Transport in Streams: Nested Versus Competing Storage Zones
Recent progress has been made to adapt single transient storage zone models into two-storage zone (2-SZ) models that discriminate between surface and subsurface (hyporheic) exchange. Surface transient storage (STS) is the non-advective portion of channels that surround and sometimes divide the main channel, while hyporheic transient storage (HTS) is the wetted subsurface area adjacent to the stream. These storage zones are subject to different physical and biogeochemical conditions. Thus, to advance biogeochemical models, we want to separate the effects of these storage zones. Current stream tracer techniques to parse the two storage zones apply a competing storage zone model, where both are connected to the stream at the same location and the stream interacts with the STS and HTS separately. Since the STS acts as a intermediary between the main channel and HTS, we propose a nested storage zone model, where the STS interacts directly with the stream and the HTS interacts only with the STS. We investigated the transient storage characteristics of a 460-m reach of Laurel Run, a first-order stream in central Pennsylvania, by building and calibrating two-storage zone solute transport models for both nested and competing storage zone configurations. We modeled the solute breakthrough curves from three salt tracer experiments ranging from high to low flows and compared the results. Preliminary results suggest that both the competing and nested storage zone models may accurately simulate the BTCs obtained in the STS and main channel.
H11B-0749
Unusual seasonal patterns and inferred processes of nitrogen retention in forested headwater catchments of the Upper Susquehanna basin
The Susquehanna River provides two-thirds of the annual nitrogen (N) load to the Chesapeake Bay, and atmospheric deposition is a major contributor to the basin's N inputs. Yet, there are few measurements of the retention of atmospheric N in the Upper Susquehanna's forested headwaters. We characterized the amount, form (nitrate, ammonium, and dissolved organic nitrogen), isotopic composition (del18O- and del15N-nitrate), and seasonality of stream N over two years from 8-15 small forested headwater catchments of the Susquehanna Basin. We expected high rates of N retention and seasonal nitrate patterns typical of other seasonally snow-covered catchments: dormant season peaks and growing season minima. Annual nitrate exports were approximately 0.1-0.7 kg N ha-1 y-1, and correlated positively with the percent of catchment free from historical agriculture. DON export averaged 0.6 +/- 0.1 kg N ha-1 y-1. All catchments had high rates of N retention but with atypical seasonal nitrate patterns, consisting of summer peaks, fall crashes, and modest rebounds during the dormant season. The fall nitrate crash coincided with carbon inputs at leaffall, indicating in-stream heterotrophic uptake. Stream del18O-nitrate values indicated microbial nitrification as the dominant source of stream nitrate, with modest contributions directly from precipitation in early stages of snowmelt. Three hypothesized sources of summer nitrate peaks include: delayed release of nitrate flushed to groundwater at snowmelt, weathering of geologic N, and increased net nitrate production. Measurements of shale del15N as well as soil, well-, and springwater nitrate within one catchment point toward a summer increase in net nitrification in surface soils. Rather than plant demand, processes governing the production, retention, and hydrologic transport of nitrate in surface mineral soils may drive the unusual nitrate seasonality in this and other systems, and provide insights on N retention in general.
H11B-0750
Separation of River Network Scale Nitrogen Removal Between Surface and Hyporheic Transient Storage Compartments
Reach scale experiments have shown that the characteristics and distribution of surface transient storage (STS) and hyporheic transient storage (HTS) may be important controls on nitrogen (N) export to coastal waters. We investigated the relative impact that STS and HTS have on N removal at the river network scale using a daily time step river network N removal model applied to the Ipswich River (a 5th order basin) in northeastern Massachusetts, USA. Spatially distributed runoff and discharge were predicted using a daily time step Water Balance and Routing Model. Nitrogen inputs to streams were calculated using simulated runoff and N concentrations based on land use type. Field investigations in 1st through 5th order reaches of the Ipswich River provided the scaling rules for hydraulic characteristics of STS and HTS throughout the network. The size of the STS and HTS relative to the size of the channel cross section (As:A) had positive relationships with stream size whereas the coefficients of exchange between the transient storage compartments and the main channel remained generally constant. On average, the cross-sectional area of the HTS was 4x that of STS while the exchange coefficient of the STS was 18x greater than that of the HTS. Nitrogen removal was simulated in three channel compartments (STS, HTS and the main channel) for every river grid cell using hydraulic characteristics, simulated river discharge and a time specific removal rate (k). For our initial model runs we assumed that k was identical in all compartments to assess how gradients in STS and HTS hydraulic parameters as a function of stream size influence network scale N removal and its distribution across stream order. Model results indicate that N removed in the HTS potentially dominates N removal, both at the reach and river network scales, and that the relative importance of HTS increases in larger streams. This suggests that the longer residence time of water in the HTS compared to STS outweighs the effect of smaller exchange rates between the main channel and the HTS compared to STS in determining the fate of N. However, a better understanding of the rates of various N cycle processes in both the STS and HTS is needed to identify the fate of N in entire river systems.
H11B-0751
Scaling From Stream Reach Observations of Groundwater-Surfacewater Exchange to Network Scale Behavior and Effects
Streamwater gains from and losses to groundwater impact stream hydrology, solute transport, and biogeochemistry. We used dual instantaneous salt injections (slugs) to investigate stream gains and losses across the Bull Trout watershed stream network (1,180 ha), Sawtooth Mountains, Idaho. The dual slug injection method allows estimation of gross gains and losses in addition to net changes in discharge across each study reach. Our results indicate that gross gains and losses occurred across each study reach despite net discharge that ranged from negative 58 % to positive 130 %. The hydrologic turnover or exchange of water can impact solute transport, in-stream solute concentrations and inertia, and watershed mass export. We found persistent relationships between stream discharge, stream flow velocity, and stream losses. From these relationships we present a simple approach using reach scale observations/experiments to simulate network scale hydrologic turnover. This conceptual model can help address: 1) how far a parcel of water travels in the stream before it is likely replaced and how this varies with stream network location and structure, and 2) where the water or solutes measured at the outlet (or anywhere else along the network) may have originated in the watershed. Addressing these questions is critical for understanding the role of the stream network and its geometry in modifying watershed runoff and solute dynamics.
H11B-0752
Dissolved Organic Matter Discharge from the Six Largest Arctic Rivers: Chemical Composition and Seasonal Variability
The vulnerability of the Arctic to climate change has been realized due to disproportionately large increases in surface air temperatures. Effects of this temperature shift are widespread in the Arctic but likely include changes to the hydrological cycle and permafrost thaw, which have implications for the transport of organic carbon from the watersheds into rivers. The focus of this research was to describe the seasonal variability of the chemical composition of dissolved organic matter (DOM) in the six largest Arctic rivers (Yukon, Mackenzie, Ob, Yenisei, Lena and Kolyma) using optical properties (UV-Vis Absorbance and Fluorescence) and lignin phenol analysis. We also investigated differences between rivers and how watershed characteristics influence DOM composition. Dissolved organic carbon (DOC) concentrations followed the hydrograph with highest concentrations measured during snowmelt. The six rivers studied here shared a similar seasonal pattern and chemical composition. There were, however, large differences between rivers in terms of total carbon discharge reflecting the differences in watershed characteristics such as climate, catchment size, river discharge, slope, elevation, soil types, and permafrost distribution. Carbon and lignin flux estimates were highest from the large Siberian rivers (Lena, Yenisei), with a large proportion of permafrost. Although the Kolyma and Mackenzie rivers had the smallest carbon and lignin flux estimates; the Kolyma exported about twice the amount of organic material annually. Multiple linear regressions of the carbon (kg DOC km2 yr-1) export indicate the most important predictors are maximum elevation, temperature and percentage of continuous permafrost (Adj R2=0.45, p=0.05). Similarly, the most important predictors of lignin (kg lignin km2 yr-1) export were slope, maximum elevation, temperature and percentage of continuous permafrost (Adj R2=0.89, p<<0.001). The chemical composition of DOM during peak flow indicates a dominance of freshly leached material with elevated aromaticity, larger molecular weight, and elevated lignin yields relative to baseflow DOM. The seasonal difference in DOM quality likely reflects increased microbial processing of soil and river organics during summer. Other factors influencing the concentration and composition of river DOM include flow path (surface run off versus percolation) and sorption to different mineral soils.
H11B-0753
Seasonal dynamics of carbon and nitrate uptake in streams draining watersheds underlain by discontinuous permafrost
Permafrost plays an important role in shaping the chemistry of streams by restricting subsurface flows through catchments to soils. During the summer thaw of soil, subsurface flows migrate through deeper soil horizons presumably resulting in seasonal shifts in the inputs of carbon and nitrogen to the streams. Within streams, the extent of the hyporheic zone also shifts with seasonal thaw. Hyporheic zones have high mineralization and nitrification rates; thus expansion of the hyporheic zone throughout the season has important implications for the chemistry of the stream. This study examined nitrogen cycling in two streams draining watersheds with varying extents of underlying permafrost to understand how nitrate uptake is affected as carbon sources shift during soil thaw. Additionally, we examined the extent of transient storage (hyporheic zone, eddies, pools) in order to determine if storage increases as the active layer thaws and if storage differs between the two streams. The research was conducted in two streams draining subcatchments with low and high permafrost extents (5% and 50% permafrost) within Caribou-Poker Creeks Research Watershed located in interior Alaska. Steady-state solute injections, amended with acetate and 15NO3-, were performed in both streams throughout the summer of 2008 to capture the seasonal thaw of soils. In both streams, acetate and nitrate uptake was rapid at the start of summer. In the stream draining the low permafrost catchment, acetate and nitrate uptake rate declined later in the summer. In contrast, the stream draining the higher permafrost watershed maintained high acetate and nitrate uptake rates. In the low permafrost stream, carbon sources may shift seasonally from recalcitrant material leached from surface soils, to more labile sources as soils thaw. The high permafrost stream, in contrast, appears to receive relatively recalcitrant carbon inputs throughout the summer presumably because shallow subsurface flows dominate. These contrasting patterns in carbon and nitrogen cycling have important implications for future functioning of streams as permafrost thaws, subsurface flows through watersheds change, and the resulting inputs of materials into streams is altered.
H11B-0754
Combining End-Member and Reaction Path Modelling: A case study on Hydrologic Structure in an Alpine Catchment
End-member mixture models have been widely used to better discern the sources of stream water in catchment systems over the last 20 years. Similarly, the reaction path model (RPM) chemical weathering approach pioneered by Garrels and MacKenzie has been used to understand the geochemical sources of constituents in catchments. Unifying these two approaches would appear to offer the opportunity to connect the source of water in end-member mixing analysis to the structure of hydrologic flows between source waters, and thus shed light on the paths that water takes through a catchment. Here we apply this approach to the Green Lake 4 (GL4) catchment in Colorado, USA. In this study, a RPM utilizes end members during the 1996 snowmelt season. The inverse geochemical portion of PHREEQC was run using a mass balance approach between each combination of end member solute concentrations available in each of three time periods, as well as the average value for the year. Unique combinations of flowpaths occur during separate time periods (early, middle, late), and this change in flowpaths shows a dynamic hydrologic system. RPM results also show notable non-flowpaths; soil water is not geochemically connected to any other end member, showing isolation from the overall hydrogeochemical system. The weathering reactions that occur along the flowpaths also change dependent on the time period. Results show changes between time periods in alkalinity as well as CO2 in-gassing and out-gassing. These changes affect the catchment buffering capacity and are due both to dilution effects as well as changes in weathering reactions in the catchment, which are dependent on the duration and timing of snowmelt.
H11B-0755
Variability in stream flow and specific discharge along three headwater streams in central Montana, USA
Specific discharge is commonly used to quantify the runoff at a watershed outlet with respect to the
watershed area. However, little is known about how specific discharge is distributed along stream valleys
within watersheds. Analyses of stream flow and specific discharge distributions may provide insight into the
interactions of runoff generating processes and stream-subsurface exchange. We compare longitudinal
distributions of stream channel flow and specific discharge in 3 mountain headwater streams of the
Tenderfoot Creek Experimental Forest in central Montana, comprising 2.6-, 1.4-, and 2.3-km valley lengths
with 5.5, 4.0, and 4.5 km2 of total contributing area, respectively. We performed an instantaneous tracer
release every 100 m along each valley, and used dilution gauging to estimate stream channel flow from each
release. Multiple series of tracer tests were performed during the summer baseflow recession following
snowmelt. We used topographic analysis of digital elevation models to quantify sub-basin contributing areas
to each location where flow was measured. We then calculated specific discharges by normalizing each
estimate of stream channel flow by its corresponding sub-basin contributing area. The study streams
demonstrated substantial variability in specific discharge in both space and time. For example, a 1300-m
upstream segment showed consistently lower specific discharges than an 800-m downstream segment in the
same stream, where the ratio of specific discharges in the upstream segment to specific discharges in the
downstream segment generally ranged from 0.7 at higher baseflows to 0.3 at lower baseflows. The
differences in specific discharges over the segments were likely driven by both the variability in source water
input from contributing areas and the variability in the importance of segment-scale stream-subsurface
exchange relative to stream channel flow. We compare the stream flow and specific discharge distributions
across space and time in the context of multi-scaled structural characteristics of the stream valleys. These
comparisons reveal the influence of interactive, hierarchical controls on the distribution of stream channel
flow characteristics, which are important to applying watershed-scale hydrologic principles to stream system
understanding across networks.
http://water.engr.psu.edu/gooseff/hydroscapes.html
H11B-0756
Spatiotemporal Variability in Terminal Electron Accepting Processes in Groundwater Ecosystems at the Floodplain Scale
Montane alluvial floodplains are characterized by complex groundwater-surface water interactions resulting in gradients of electron donor and acceptor availability and redox states within alluvial aquifers. To improve floodplain scale models of solute dynamics in extensive (>10 km2) hyporheic systems, we characterized hyporheic water chemistry and microbial community structure and function in wells of contrasting floodplain position within the alluvial aquifer of a gravel-bed river system, the Middle Fork Flathead River in northwest Montana, USA. Data indicate that microbial community structure and function, and the relative occurrence of various terminal electron acceptors (O2, NO3-, SO4- 2, and Fe+2) are a function of flowpath characteristics and context, such as subsurface water residence time and organic matter availability, resulting in a shifting spatiotemporal mosaic of biogeochemical activity within the alluvial aquifer. In localized zones of high microbial abundance and respiration, low-oxygen conditions yielded methane production and iron reduction as the dominant biogeochemical processes. The incorporation of this dataset into a coupled hydrologic/biogeochemical simulation model will help identify drivers of key sites for biogeochemical transformation at the whole-floodplain scale.
H11B-0757
Mixing of Surface and Subsurface Organic Matter During Catchment Formation at Springs
When spring waters form or enter existing catchments, spring geochemistry and discharge can affect local surface aquatic biogeochemistry by introducing different chemical and biological species. In this study, we applied excitation-emission matrix fluorescence spectroscopy to water samples and extracted organic matter (OM) solutions from surface solids (soil, pine needles, microbial mats) to examine OM mixing in a small pool at the outlet of a sulfidic spring in a Louisiana forest. At this site, the spring water contributes hydrogen sulfide and subsurface microbes, which differ geochemically and biologically from soil pore water or meteoric sources. The original signature of the subsurface OM was obtained by isolation from spring water using a C- 18 adsorbent following filtration to remove microbes. Surface OM signatures from soil and microbial mats were obtained using traditional humic substance extraction techniques. Comparisons of fluorescence characteristics show that OM composition and concentration change significantly over < 2 m stream reach from the spring orifice. In addition to mixing of surface and subsurface OM, surface mechanisms influence the OM signatures, including photodegradation and photooxidation, as well as microbial OM turnover and processing. It is apparent from our results that the contribution of subsurface material to surface environments has great influence on the biogeochemistry near the point of emission, but that influence is rapidly mitigated as the waters equilibrate with the atmosphere and surrounding soil.
H11B-0758
MODELING FINE SEDIMENT INFILTRATION WITHIN THE HYPORHEIC ZONE
In rivers stream water is connected to groundwater through the hyporheic zone, a rich ecotone at the interface between surface and subsurface waters. The resulting exchange effects both surface and subsurface water quality and primarily depends on sediment characteristics, flow regime, and bed topography. Fine sediments, which are transported at virtually all flow conditions, deposit within the gravel bed primarily during low flow conditions by gravity or by a combination of hyporheic flow and gravity, depending on particle size. Fine sediment infiltration is important because their fraction (concentration) within the streambed sediment affects hyporeos and benthic species habitat. Additionally, nutrients and pollutants attached on sediment surface and are deposited in the streambed material where they could be temporally stored. We adopted three-dimensional analytical models for predicting surface and subsurface water flows in gravel-bed channels with pool-riffle morphology. Surface and subsurface models are coupled via the near- bed pressure distribution, which is calculated by the surface water model and used as boundary condition for the groundwater model. We calculate the trajectory of a fine sediment particle within the hyporheic zone in a Langrangian framework by superimposing the falling velocity of the sediment particle to the hyporheic flow field. The trapping capacity of the bed sediments, which depends on electrical forces, absorption, and mechanical clogging, is modeled with the following first-order irreversible exchange equation dC=-λf C ds, where C is the concentration of transported fine sediments in the pore water for a distance s traveled along a pathline in the bed, and λf is the filtration coefficient. Our investigation showed the effect of bed topography between small steep headwater streams and large-low gradient streams. Additionally we looked at the effect of particle size and hyporheic flow to determine when the gravitational deposition is the primarily infiltration-driving mechanism and the pore-water velocity can be neglected.
H11B-0759
Hyporheic Flow and Residence Times in Heterogeneous Cross-Bedded Sediment
The permeability (k) structure of stream sediment influences hyporheic zone depth, flux, and residence time distribution (RTD). We numerically simulate bedform-induced hyporheic exchange using turbulent open channel flow simulations coupled to groundwater flow models that consider high-resolution k fields. The k fields are from the cross-bedded Massillon Sandstone and modern climbing ripple deposits of the Brazos River (Texas). In both cases, k heterogeneity leads to nested scales of hyporheic exchange and increases the depth of exchange relative to equivalent homogeneous sediment. Solute flux and RTDs differ markedly for the Massillon and Brazos cases, although both exhibit power-law RTDs. In the Massillon case, heterogeneity extends the tail of the RTD, relative to equivalent homogeneous sediment. In the Brazos case, heterogeneity leads to earlier breakthrough of solutes and shorter solute residence times along shallow exchange paths, relative to equivalent homogeneous sediment. These complex transport dynamics would presumably influence the fate of biogeochemical constituents and the thermal regime of sediment. Our case studies suggest, however, that the impact of k heterogeneity on hyporheic zone hydrodynamics is small compared with bed topography and channel hydraulics.
H11B-0760
Phosphorus and Nitrogen Generation Processes and Stream Loadings Following Wildfire
Little is known about the biogeochemical responses of catchments to wildfire. In particular, the processes of nutrient generation, the temporal signature of constituent form, and consequent magnitude and time series of nutrient stream loadings are poorly characterised. Following a wildfire in south eastern Australia that burnt over 1 million hectares of forested land in early 2003, two former research catchments (136 and 244 ha) in the East Kiewa valley, Victoria, were re-instrumented. Discharge, suspended sediment, bedload and nitrogen (N) and phosphorus (P) were measured for three years post-fire. The nutrient data consisted of 15-minute estimation of particulate P and Total Kjeldahl N concentrations via a regression with turbidity, and concentrations of dissolved forms of P and N estimated from over 1100 water samples. The fire appears to have increased total P and N exports by around 5-6 fold, peaking at 1.6 kg ha-1 of P and 15.3 kg ha-1 of total combined N. Nutrients transported as particulate matter dominated the first post-fire year, with 94% of total combined P and 69% of total combined N. Although dissolved forms increased in importance over time, the particulate load comprised 86% of the total combined P load and 68% of the total combined N load over the three post-fire years. This suggests the dynamics of overland flow generation and erosion processes are the critical drivers of constituent production in these landscapes following fire. Concentrations and loads of P and N exhibited a rapid recovery to unburnt levels during the second post-fire year. Particulate forms declined sharply through a reduction in sediment delivery. Nitrate displayed the slowest relaxation time, suggesting a persistent subsurface pathway and the effect of nitrification. Notably, dissolved N fluxes were predominantly transported in baseflow even in the first post-fire year. A simple model with time as the single parameter proved to be a good predictor of mean three monthly concentrations. Phosphorus enrichment ratios from point to plot were 2 and from plot to catchment were 1.5, indicating the effect of particle size selection on particulate P transport was relatively low.
H11B-0761
In situ measurements of organic matter dynamics during a storm event in an agricultural watershed
Dissolved organic matter (DOM) from the breakdown of plant and animal material is a significant concern for drinking water quality in California due to the potential formation of carcinogenic disinfection byproducts during treatment. Winter storms are important forcing events on the California landscape, but the extent to which they impart rapid changes in DOM and other biogeochemical variables is poorly understood. In situ optical measurements are useful as they can be made autonomously at high temporal resolution, aiding in the quantification of rapid changes in the DOM pool. We collected in situ and discrete samples during a storm event period (Feb 22-March 3, 2008) at the mouth of the 415 km2 agricultural Willow Slough watershed. The watershed is characterized by steep grasslands in the headwaters and agriculture (largely in alfalfa, rice, tomato, grasses and orchard) in the valley. The in situ optical measurements included turbidity, chromophoric DOM fluorescence (cDOM), and nitrate (NO3-) concentrations, along with a suite of ancillary parameters. Discharge and turbidity were strongly correlated at peak flow and increased by over two orders of magnitude, while the peak cDOM lagged the peak in turbidity by ten hours. The cDOM values increased by nearly 4 fold and were highly correlated with dissolved organic carbon (DOC) concentrations (r2=0.97), providing a highly resolved proxy for DOC throughout the flow event. Specific UV absorbance (an indicator of DOM aromaticity) doubled at the DOC peak, while decreases in both the spectral slope (a proxy for DOM molecular weight) and δ13C-DOM during the same period support terrestrially- derived DOM contributions at peak flows. The lag to peak cDOM behind peak discharge presumably reflects the draining of watershed soils and delayed surface runoff of natural and agricultural landscapes. Together, laboratory and in situ data provide insights into the timing and magnitude of changes in DOM quantity and quality during storm events.
H11B-0762
Use of Continuous Specific Conductance to Differentiate the Sources of Water to an Agricultural Stream With Subsurface Drainage Networks
The sources of water to natural streams include direct precipitation, overland flow, and ground-water inflow. In glaciated areas, the presence of artificial surface and subsurface drainage networks, a common practice for removing excess water from agricultural fields, provides additional pathways of water movement to the stream. The artificial drainage of agricultural fields allows rainfall to move quickly through the catchment to the stream transporting nutrients, pesticides and other agricultural-related constituents. A largely agricultural (about 90%), 31 km2 subcatchment of the South Fork of the Iowa River in north-central Iowa was studied for two years. Discharge and specific conductance (SC) were measured continuously and discreet water samples were obtained for analyses of nutrients and other constituents. SC is an electrical measurement of the total ion content in the water. The SC of the rain and ground-water is about 10 microS/cm and 800-1,200 microS/cm, respectively. The typical, base-flow SC of the stream is 700-800 microS/cm. Within minutes after a substantial rain event, the stream discharge increases and the SC decreases (often times below 200nmicroS/cm). The rain water is processed through the catchment before it reaches the stream via direct overland flow, preferential flow to subsurface drains, vertical drains attached to subsurface drains in ponded areas, and/or soil infiltration to ground-water. Water moving through each of these pathways has different characteristic time scales and different degrees of interactions with the soil yielding different ionic content, thus different SC. Both the discharge and SC concurrently return to the typical base-flow values over the following days and weeks. This strong relation between rainfall, discharge and SC is used to calculate the relative importance and time scale of the various hydrologic pathways. In addition to the two-year stream record, complementary discharge and SC data were collected in two subsurface drains and continuous ground-water levels collected from nearby observation wells. These data compared to the stream record demonstrate the quick response of subsurface drains and shallow ground-water to rainfall.
H11B-0763
Deep Groundwater Contributions as a Primary Control on Stream Chemistry and Apparent Age in a Large Alpine Watershed in the Southern Rocky Mountains of Colorado
Considerable advances have been made in understanding runoff generation at small-scales yet our understanding of runoff generation in large watersheds greater than 1000 km2 remains poor. Small-scale runoff mechanisms are well documented in the literature including: overland flow, subsurface runoff, bypass flow, unsaturated flow, etc. Yet, despite the great variety in runoff mechanisms observed at the hillslope scale, chemical fluxes derived from runoff often become damped as the observed watershed scale increases. Hydrologists continue to explain this behavior using small-scale runoff mechanisms which are often related to shallow subsurface features or processes despite residence time studies which indicate that chemicals can be temporally persistent in watersheds. In doing so, we ignore the importance of deep groundwater contributions. As a matter of fact, recent studies seem to reject the "increasing deep groundwater" hypothesis whereby stream chemistry would continue to increase if the contributions from deep groundwater continue to increase and have instead embraced an "integration process hypothesis". This hypothesis suggests that variations in stream chemistry follow the central limit theorem as scale increases; large scatter at small scales and convergence on a median value as scale increases. A critical problem exists with the integration process hypothesis with regards to the contributions of deep groundwater to streamflow generation and its effect on residence times. For example, if a conservative chemical constituent in deep groundwater approaches some invariant concentration as scale increases, then it would follow that a conservative environmental tracer would approach an invariant apparent age since both processes reflect mixing. This implies that the apparent age of streamflow will become scale invariant. Current research in a large alpine watershed of the San Juan Mountains of southern Colorado suggests otherwise. Deep groundwater components in this watershed appear to be important controls on cation concentrations in stream water as a consequence of the increased time for rock/water interactions. Furthermore, Carbon-14 and stable isotope data indicate that residence times in these watersheds may be underestimated. The deep groundwater component continues to be ignored in watershed hydrology and it may in fact be the primary control on streamflow generation and stream chemistry in similar alpine watersheds of the American Southwest.