Sediment-Nitrogen (N) connectivity: suspended sediments in streams as N exporters and reactors for denitrification and assimilatory N uptake during storms

Nitrogen (N) pollution in riverine ecosystems has substantial environmental, economic, and policy consequences. Various riverine N removal processes include permanent dissimilatory sinks such as denitrification (Uden) and temporary assimilatory sink such as microbial N uptake (Uassim). Both processes have been extensively evaluated in benthic sediments but only sparsely in the water column, particularly for storm flows producing high suspended sediment (SS) concentrations. Stormflows also increase the sediment bound N (Sed-N) export, and in turn, the overall N exports from watersheds. The balance between N removal by Uden and Uassim vs. Sed-N export has not been studied and is a key knowledge gap. We assessed the magnitude of Uden and Uassim against stormflow Sed-N exports for multiple storm events of varying magnitude and across two drainage areas (750 ha and 15,330 ha) in a mixed landuse mid-Atlantic US watershed. We asked: How do the Uden and Uassim sinks compare with Sed-N exports and how do these N fluxes vary across the drainage areas for sampled storms on the rising and falling limbs of the discharge hydrograph? Mean Uden and Uassim as % of the Sed-N exports ranged between 0.1–40% and 0.6–22%, respectively. Storm event Uassim fluxes were generally slightly lower than the corresponding Uden fluxes. Similarly, comparable but slightly higher Uden fluxes were observed for the second order vs. the fourth order stream, while Uassim fluxes were slightly higher in the fourth-order stream. Both of these N sinks were higher on the falling vs. rising limbs of the hydrograph. This suggests that while the N sinks are not trivial, sediment bound N exports during large stormflows will likely overshadow any gains in N removal by SS associated denitrification. Understanding these N source-sink dynamics for storm events is critical for accurate watershed nutrient modeling and for better pollution mitigation strategies for downstream aquatic ecosystems. These results are especially important within the context of climate change as extreme hydrological events including storms are becoming more and more frequent

Impact of Riparian Grass Filter Strips on Surface-Water Quality

The effectiveness of natural riparian grass filter strips in removing sediment and agricultural chemicals from surface runoff was studied using no-tillage and conventional-tillage erosion plots. Runoff from the tillage plots was directed onto 4.57, 9.14, and 13.72 m (15, 30, and 45 ft.) length filter strips, where the inflow and outflow concentrations and sediment size distributions were measured. Trapping efficiencies for sediment and agricultural chemicals typically ranged near or above 90 percent, mainly because of high infiltration rates. The filters also significantly reduced peak discharge concentrations, which reduced the impact of sediment and agricultural chemicals on receiving surface waters

Ghosts of Landuse Past: Legacy Effects of Milldams for Riparian Nitrogen (N) Processing and Water Quality Functions

Milldams and their legacies have significantly influenced fluvial processes and geomorphology. However, less is known about their effects on riparian zone hydrology, biogeochemistry, and water quality. Here, we discuss the potential effects of existing and breached milldams on riparian nitrogen (N) processing through multiple competing hypotheses and observations from complementary studies. Competing hypotheses characterize riparian zone processes that remove (sink) or release (source) N. Elevated groundwater levels and reducing soil conditions upstream of milldams suggest that riparian zones above dams could be hotspots for N removal via denitrification and plant N uptake. On the other hand, dam removals and subsequent drops in stream and riparian groundwater levels result in drained, oxic soils which could increase soil nitrification and decrease riparian plant uptake due to groundwater bypassing the root zone. Whether dam removals would result in a net increase or decrease of N in riparian groundwaters is unknown and needs to be investigated. While nitrification, denitrification, and plant N uptake have typically received the most attention in riparian studies, other N cycle processes such as dissimilatory nitrate reduction to ammonium (DNRA) need to be considered. We also propose a novel concept of riparian discontinuum, which highlights the hydrologic and biogeochemical discontinuities introduced in riparian zones by anthropogenic structures such as milldams. Understanding and quantifying how milldams and similar structures influence the net source or sink behavior of riparian zones is urgently needed for guiding watershed management practices and for informed decision making with regard to dam removals

Variation of organic matter quantity and quality in streams at Critical Zone Observatory watersheds

The quantity and chemical composition of dissolved organic matter (DOM) in surface waters influence ecosystem processes and anthropogenic use of freshwater. However, despite the importance of understanding spatial and temporal patterns in DOM, measures of DOM quality are not routinely included as part of large-scale ecosystem monitoring programs and variations in analytical procedures can introduce artifacts. In this study, we used consistent sampling and analytical methods to meet the objective of defining variability in DOM quantity and quality and other measures of water quality in streamflow issuing from small forested watersheds located within five Critical Zone Observatory sites representing contrasting environmental conditions. Results show distinct separations among sites as a function of water quality constituents. Relationships among rates of atmospheric deposition, water quality conditions, and stream DOM quantity and quality are consistent with the notion that areas with relatively high rates of atmospheric nitrogen and sulfur deposition and high concentrations of divalent cations result in selective transport of DOM derived from microbial sources, including in-stream microbial phototrophs. We suggest that the critical zone as a whole strongly influences the origin, composition, and fate of DOM in streams. This study highlights the value of consistent DOM characterization methods included as part of long-term monitoring programs for improving our understanding of interactions among ecosystem processes as controls on DOM biogeochemistry

A LONG-TERM, WATERSHED-SCALE, EVALUATION OF THE IMPACTS OF ANIMAL WASTE BMPs ON INDICATOR BACTERIA CONCENTRATIONS1

ABSTRACT: Driven by increasing concerns about bacterial pollution from agricultural sources, states such as Virginia have initiated cost sharing programs that encourage the use of animal waste best management practices (BMPs) to control this pollution. Although a few studies have shown that waste management BMPs are effective at the field scale, their effectiveness at the watershed scale and over the long term is unknown. The focus of this research was to evaluate the effectiveness of BMPs in reducing bacterial pollution at the watershed scale and over the long term. To accomplish this goal, a 1,163 ha watershed located in the Piedmont region of Virginia was monitored over a ten-year period. Fecal coliforms (FC) and fecal streptococci (FS) were measured as indicators of bacterial pollution. A pre-BMP versus post-BMP design was adopted. Major BMPs implemented were manure storage facilities, stream fencing, water troughs, and nutrient management. Seasonal Kendall trend analysis revealed a significant decreasing trend during the post-BMP period for FC concentrations at the watershed outlet, but not at the subwatershed level. Implementation of BMPs also resulted in a significant reduction in the geometric mean of FS concentrations. FC concentrations in streamflow at the watershed outlet exceeded the Virginia primary standard 86 and 74 percent of the time during pre-BMP and post-BMP periods, respectively. Corresponding exceedances for the secondary standard were 50 and 41 percent. Violations decreased only slightly during the post-BMP period. The findings of this study suggest that although BMP implementation can be expected to accomplish some improvement in water quality, BMP implementation alone may not ensure compliance with current water quality standards. (KEY TERMS: nonpoint source pollution; watershed management; water quality; bacterial pollution; fecal coliform; fecal streptococcus; BMP.) INTRODUCTION Contamination from bacterial sources has been identified as the third leading cause of pollution in the nation's rivers, after siltation and nutrients (USEPA, 1999). Pollution from bacteria accounts for nearly 79,820 impaired river miles or 12 percent of the total river miles surveyed in the United States (USEPA, 1999). In Virginia, fecal contamination of surface waters is the leading pollution problem, and agriculture has been cited as the largest contributor of this pollutant (USEPA, 1999). Land application of animal wastes and runoff from livestock facilities are the major agricultural practices contributing to bacterial pollution Although several studies and reviews have evaluated bacterial pollution from agricultural land

Molecular fingerprinting of particulate organic matter as a new tool for its source apportionment: changes along a headwater drainage in coarse, medium and fine particles as a function of rainfalls

International audienceTracking the sources of particulate organic matter (POM) exported from catchments is important to understand the transfer of energy from soils to oceans. The suitability of investigating the molecular composition of POM by thermally assisted hydrolysis and methylation using tetramethylammonium hydroxide directly coupled to gas chromatography and mass spectrometry is presented. The results of this molecular-fingerprint approach were compared with previously published elemental (% C, % N) and isotopic data (δ13C, δ15N) acquired in a nested headwater catchment in the Piedmont region, eastern United States of America (12 and 79 ha). The concordance between these results highlights the effectiveness of this molecular tool as a valuable method for source fingerprinting of POM. It emphasizes litter as the main source of exported POM at the upstream location (80±14 %), with an increasing proportion of streambed (SBed) sediment remobilization downstream (42 ± 29 %), specifically during events characterized by high rainfall amounts. At the upstream location, the source of POM seems to be controlled by the maximum and median hourly rainfall intensity. An added value of this method is to directly investigate chemical biomarkers and to mine their distributions in terms of biogeochemical functioning of an ecosystem. In this catchment, the distribution of plant-derived biomarkers characterizing lignin, cutin and suberin inputs were similar in SBed and litter, while the proportion of microbial markers was 4 times higher in SBed than in litter. These results indicate that SBed OM was largely from plant litter that has been processed by the aquatic microbial community

Evolution of the source of storm particulate organic matter exported along forested nested catchment. ? Impact of granulometry and climate

International audienceParticulate organic matter (POM) plays an important biogeochemical role towards ecology, ecotoxicology andcarbon cycle. Moreover POM within the fluvial suspended sediment load during infrequent high flows cancomprise a larger portion of long-term flux than dissolved species. It is well documented that storm events thatconstituted only 10-20% of the year contributed to >80% of POC exports. But the origin and composition ofPOM transferred during those hot moments remained unclear. In order to improve our knowledge on this topicwe explore the variability in storm event-transported sediments’ POM content and source down a continuum ofcatchment drainage locations.Wetland, upland and forest O horizons, litter, river banks and bed sediments were analyzed for their contentin organic C, isotopic (13C) and molecular (thermochemiolysis-gas chromatography-mass spectrometry)fingerprints. The isotopic and molecular fingerprints recorded in suspended and deposited (differentiated into fine,medium and coarse particles) sediments sampled during different storm events down a continuum of catchmentdrainage locations (12 and 79 ha).This study highlights compositional differences between the catchment size (12 versus 79 ha), the particlesize of deposited sediment (fine versus medium versus coarse) and the sampling time during a storm event (risinglimb versus peak flow versus falling limb). Two sampling strategies were used. Suspended sediments sampled ata specific time during flood events allow evaluating changes along the hydrograph, while deposited sedimentsthat integrate the entire event allow making comparisons with drainage scale. For deposited sediments, theproportion of OM coming from the endmembers wetland, litter and Forest O horizon decreases from the 12hato the 79ha catchment, which exhibited a higher proportion of OM coming from stream bed sediment and riverbanks. For both catchments, from fine to coarse particles, the influence of stream bed sediments and river banksdecreases while the influence of Forest O horizon increases. For suspended sediments, the evolution during stormevents were opposite in the 12ha and the 79ha catchments. In the 12ha catchment, during the rising limb of thehydrograph, POM seems to be inherited from stream bed sediments and river banks, while from the rising limbto the peak flow, the influence of litter and/or wetland increases. This influence decreases during the falling limb.The opposite trend was observed in the 79ha catchment, with an increasing contribution of stream bed sedimentsto the OM exported during a storm event.What is the information to take away? First POM transferred in headwater catchments has multiple sources.Secondly, the combination of those sources is different along the size continuum of particles. Then, down acontinuum of catchment drainage locations, the combination of sources changes both along the size continuumand during storm events. This information is critical for identifying the various drivers and mechanisms behindPOM transport and for understanding the impacts of POM on aquatic metabolism and downstream water qualit

FE-2018-00549R2 data

Data used in FE-2018-00549R2. Data covers all figures. Excel workbook contains readme and detailed key to all column headers/variables

Streambank Legacy Sediments in Surface Waters: Phosphorus Sources or Sinks?

Streambank legacy sediments can contribute substantial amounts of sediments to Mid-Atlantic waterways. However, there is uncertainty about the sediment-bound P inputs and the fate of legacy sediment P in surface waters. We compared legacy sediment P concentrations against other streambank sediments and upland soils and evaluated a variety of P indices to determine if legacy sediments are a source or sink of P to surface waters. Legacy sediments were collected from 15 streambanks in the mid-Atlantic USA. Total P and M3P concentrations and % degree of phosphorus saturation (DPS) values for legacy sediments were lower than those for upland soils. % DPS values for legacy sediments were below the water quality threshold for P leaching. Phosphorus sorption index (PSI) values for legacy sediments indicated a large capacity for P sorption. On the other hand, equilibrium phosphorus concentration (EPC0) for legacy sediments suggested that they could be a source or a sink depending on stream water P concentrations. Anoxic conditions resulted in a greater release of P from legacy sediments compared to oxic conditions. These results suggest that legacy sediment P behavior could be highly variable and watershed models will need to account for this variability to reliably quantify the source-sink behavior of legacy sediments in surface waters

Influence of relict milldams on riparian sediment biogeochemistry

Purpose: Riparian zones are important modifiers of nutrient flux between terrestrial and aquatic ecosystems. However, dams alter riparian zones—trapping fine-grained, organic matter-rich sediment and creating poorly mixed, low oxygen conditions—thereby affecting sediment biogeochemistry in poorly understood ways. Methods: We characterized the impact of two relict US mid-Atlantic milldams (one from a primarily agricultural watershed and one from a mixed land use/urban watershed) on spatial patterns of bioavailable element concentrations (Mehlich-3 extractable P, K, Ca, Mg, Mn, Zn, Cu, Fe, B, S, and Na) in sediments upstream and downstream of milldams, with depth, and along transects running parallel and perpendicular to the stream. Results: Element concentrations were not clearly correlated with grain size or organic matter content and, although generally higher, were not significantly more concentrated in upstream riparian sediments when similar (shallow, variably saturated) depths were compared. Pronounced differences were observed: upstream of milldams, sediment concentrations of Ca and Mg were highest in variably saturated shallow sediments, while Fe and Mn were highest in deeper, continuously saturated, low-oxygen sediments. Additionally, data was significantly different by milldam site, a result of differences in land-use histories (e.g., road salt and fertilizer application/runoff) and dominant bedrock geology. Conclusion: Overall, results highlight the combined importance of milldams (and associated influences on groundwater hydrology and sediment redox conditions) and external drivers (other land-use legacies and bedrock geology) in influencing spatial patterns of bioavailable elements in riparian sediments