Fate Of Nitrate Research Articles

Comprehensive simulation of nitrate transport in coupled surface-subsurface hydrologic systems using the linked SWAT-MODFLOW-RT3D model

This paper presents SWAT-MODFLOW-RT3D, a model that couples the semi-distributed watershed model SWAT (Soil and Water Assessment Tool) with the groundwater flow model MODFLOW and the groundwater solute reactive transport model RT3D to simulate nitrate (NO3) fate and transport in a watershed system. The model is based on a recently developed SWAT-MODFLOW model, with RT3D now called as a subroutine within the MODFLOW code to provide a single, stand-alone model code. RT3D uses NO3 concentration of deep percolation water from SWAT and groundwater heads and flows from MODFLOW to simulate spatially-varying groundwater NO3 concentration and NO3 loading to/from streams, with the latter used by SWAT to route NO3 mass through the network. Model use is demonstrated through an application to the Sprague River Watershed (4100 km2) in Oregon. Other chemical species of interest can be included in the RT3D reaction module in applications of the model to other watersheds.

Evaluation of nitrate sources and the percent contribution of bacterial denitrification in hyporheic zone using isotope fractionation technique and multi-linear regression analysis

Denitrification has documented as a promising pathway to permanently remove nitrate from a system. Numerous studies have used the isotope fractionation technique (IFT) to evaluate the denitrification rate in the constructed wetlands (CWs), but the potential of IFT method to quantify the denitrification rate in hyporheic zone (saturated sediments beneath a stream) is still challenging. Thus, more studies are required to investigate that if measurements of the natural abundance of δ15N-NO3- and δ18O-NO3- (IFT) can be employed to calculate the fate of nitrate in hyporheic zone. Therefore, in this study, the possibility of the IFT to quantify the hyporheic-denitrification rate was investigated. Then, the results were verified by the combined application of the pre-established net Sediment N2 flux and multi-linear regression analysis (p < 0.01). Finally, the groundwater bacterial groups (Fecal coliform (FC) and Escherichia coli (EC)), and the mass balance isotope mixing model were used to investigate the dominant sources of hyporheic-nitrate. The IFT reveals that denitrification contributes 74.1% and 29.1% of the hyporheic-nitrate removal during dry and wet seasons, respectively. The multi-linear regression analysis, considering at 99% confidence interval (R2 = 92.1%; n = 44; p < 0.01), slightly overestimates the rate and the percent contribution of denitrification in the dry season (475.15 ± 101.18 μmol/m2d; 80.7%) and underestimates it during the wet season (205.072 ± 35.39 μmol/m2d; 24.01%). The analysis of EC and FC demonstrates that manure (41.9 ± 4.2%) and sewage (54.1 ± 8.9%) are the dominant contributors of the hyporheic-nitrate load. In addition, the results achieved by the analysis of the fecal bacterial indicators (EC and FC) were confirmed by NO3−/Cl− vs Cl− diagram. This study provides an alternative-initiative framework to accurately quantify the spatio-seasonal variations in the hyporheic-nitrate sources and hyporheic-denitrification rate that enables decision-makers to apply appropriate and targeted strategies to regulate nitrate load in river-aquifer systems.

Tracing the Fate of Atmospheric Nitrate in a Subalpine Watershed Using Δ17O.

Nitrogen is an essential nutrient for life on Earth, but in excess, it can lead to environmental issues (e.g., N saturation, loss of biodiversity, acidification of lakes, etc.). Understanding the nitrogen budget (i.e., inputs and outputs) is essential to evaluate the prospective decay of the ecosystem services (e.g., freshwater quality, erosion control, loss of high patrimonial-value plant species, etc.) that subalpine headwater catchments provide, especially as these ecosystems experience high atmospheric nitrogen deposition. Here, we use a multi-isotopic tracer (Δ17O, δ15N and δ18O) of nitrate in aerosols, snow, and streams to assess the fate of atmospherically deposited nitrate in the subalpine watershed of the Lautaret Pass (French Alps). We show that atmospheric N deposition contributes significantly to stream nitrate pool year-round, either by direct inputs (up to 35%) or by in situ nitrification of atmospheric ammonium (up to 35%). Snowmelt in particular leads to high exports of atmospheric nitrate, most likely fast enough to impede assimilation by surrounding ecosystems. Yet, in a context of climate change, with shorter snow seasons, and increasing nitrogen emissions, our results hint at possibly stronger ecological consequences of nitrogen atmospheric deposition in the close future.

Identification of Nitrate Sources and the Fate of Nitrate in Downstream Areas: A Case Study in the Taizi River Basin

A total of 14 samples were collected in May 2016(dry season)and August 2016 (wet season) in the downstream area of the Taizi River. △15 N-NO3- and △18 O-NO3- were determined using the azide method, and △18 O-H2O was determined using a CO2-H2O equilibration technique. To identify NO3- sources and transformations in the downstream area of Taizi River Basin, ion chromatography, Nessler's reagent spectrophotometry, the azide method, and CO2-H2O equilibration methods were utilized to determine the concentrations of NO3-, Cl-, NH4+-N, and isotopic compositions (△15 N and △18 O) of NO3- and the △18 O-H2O in surface water. The results showed that the NO3- was mainly derived from mixed sources. During the dry season, the nitrate in the surface water was derived from soil nitrogen, manure, and sewage in the upper reaches, and mainly derived from synthetic fertilizer, manure, and sewage in the middle and lower reaches of the Beisha River. The nitrate was mainly derived from manure and sewage in the Nansha River. The nitrate was mainly derived from soil nitrogen in the upper reaches, mainly derived from synthetic fertilizer, manure, and sewage in the middle reaches, and mainly derived from manure and sewage in the lower reaches of the Haicheng River. During the wet season, the nitrate sources in surface water were soil nitrogen, synthetic fertilizer, manure, and sewage in the Beisha River; synthetic fertilizer, manure, and sewage in the middle and lower reaches of the Haicheng River and the Nansha River; and soil nitrogen and synthetic fertilizer in the upper reaches of the Haicheng River. NO3- and NH4+-N concentrations decreased with increasing △15 N-NO3- from the dry season to the wet season, indicating that volatilization of NH4+-N and denitrification of NO3- might occur during the wet season. There is a slightly positive relationship between the reciprocal of the concentration of 1/ρ(NO3-) and △15 N-NO3- during the wet season, indicating that mixing processes occurred in surface water. The results will provide information on nitrate sources during seasonal variations in the plain areas.

The fate of nitrite and nitrate during anaerobic digestion

ABSTRACTAnaerobic digestion is widely used to produce renewable energy. However, the main drawback is the limited conversion efficiency of organic matter. Applying an advanced oxidation process as a digestate post-treatment is able to increase this conversion efficiency but will also lead to the oxidation of ammonium to nitrite or nitrate. In this lab-scale study, the fate of the latter in the digester was investigated. Nitrite and nitrate were therefore added in concentrations that could arise from rate-limiting ammonium concentrations (1.25–5 g L−1 N). The study clearly demonstrated that nitrite and nitrate were denitrified during the subsequent digestion process resulting in the formation of nitrogen gas. After a concentration-dependent adaptation period, in which some biogas was produced, the added nitrite was denitrified in amounts proportional to the amounts of electron donor present. This denitrification, however, strongly reduces the possibility that Anammox bacteria can develop. Nitrate was also denitrified in amounts proportional to the amounts of electron donor, but biogas production was not completely blocked in this case. Moreover, high concentrations of nitrite and nitrate inhibited their own denitrification. The methane formed was used as electron donor for the further denitrification of nitrate and nitrite when no other readily available electron donor was present. After addition of either nitrite or nitrate and their denitrification, the biogas production did not recover properly.

Aquatic Nitrate Retention at River Network Scales Across Flow Conditions Determined Using Nested In Situ Sensors

AbstractNonpoint pollution sources are strongly influenced by hydrology and are therefore sensitive to climate variability. Some pollutants entering aquatic ecosystems, e.g., nitrate, can be mitigated by in‐stream processes during transport through river networks. Whole river network nitrate retention is difficult to quantify with observations. High frequency, in situ nitrate sensors, deployed in nested locations within a single watershed, can improve estimates of both nonpoint inputs and aquatic retention at river network scales. We deployed a nested sensor network and associated sampling in the urbanizing Oyster River watershed in coastal New Hampshire, USA, to quantify storm event‐scale loading and retention at network scales. An end member analysis used the relative behavior of reactive nitrate and conservative chloride to infer river network fate of nitrate. In the headwater catchments, nitrate and chloride concentrations are both increasingly diluted with increasing storm size. At the mouth of the watershed, chloride is also diluted, but nitrate tended to increase. The end member analysis suggests that this pattern is the result of high retention during small storms (51–78%) that declines to zero during large storms. Although high frequency nitrate sensors did not alter estimates of fluxes over seasonal time periods compared to less frequent grab sampling, they provide the ability to estimate nitrate flux versus storm size at event scales that is critical for such analyses. Nested sensor networks can improve understanding of the controls of both loading and network scale retention, and therefore also improve management of nonpoint source pollution.

Evaluation of the Fate of Nitrate and Analysis of Shallow Soil Water using Geo-electrical Resistivity Survey

Evaluation of the fate of nitrate and analysis of shallow soil water in a tobacco plantation area were conducted using integration of soil properties and hydrogeochemical analysis, and geo-electrical resistivity methods, taking measurements four times within a three-month period. The sampling data were taken in two areas: a fertilized and a nonfertilized zone. Chemical fertilizer was introduced to the fertilized zone after the first data acquisition. Hydrogeohemical analysis of the soil water was conducted from the surface to a depth of 1 m at an interval of 25 cm. The results show that the cations in the soil water were quite comparable for each monitoring time. Conversely, relatively larger changes in anion content occurred at the surface until a depth of 1 m. In particular, the nitrate concentration reached its maximum level at about 67 days after fertilization and returned to its initial value approximately 195 days after fertilization. The geo-electrical resistivity profiles exhibited no indication of low resistivity values prior to fertilization near the surface. However, lower resistivity values were found in the fertilized zone at the second and third measurement. The result shows that the adjoining environment dissolved the nitrate concentration in the pore soil within the three-month time period.

Fate of 15N-labelled ammonium nitrate with or without the new nitrification inhibitor DMPSA in an irrigated maize crop

Nitrification inhibitors, originally proposed for nitrate leaching mitigation, are recommended as effective nitrous oxide (N2O) mitigation strategies. Several compounds have been trialled and used in the past including dicyandiamide (DCD) or 3,4-dimethylpyrazole phosphate (DMPP). Yet, little is known about the new nitrification inhibitor 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA). A field experiment using 15N single-labelled ammonium nitrate (15NH4NO3 or NH415NO3) was conducted to understand the effectiveness of DMPSA on a biochemical basis in an irrigated maize (Zea mays L.) crop. Gaseous fluxes, i.e. N2O, 15N2O, 15N2, methane (CH4), and carbon dioxide (CO2) were measured, as well as soil mineral N, yield components and 15N recovery in plant and soil. During the maize cropping period, the use of DMPSA significantly reduced cumulative N2O emissions (118 g N ha−1) compared to ammonium nitrate without inhibitor (231 g N ha−1). The 15N analyses revealed that most N2O losses (particularly during the emission peak) came from 15NH4NO3 (i.e. nitrification, nitrifier denitrification and/or coupled nitrification denitrification) rather than NH415NO3 in this calcareous low C-content soil. As expected, DMPSA decreased N2O losses from 15NH4+ oxidation, but an effect on non-target microorganisms was noticed, as shown by the significant reduction of respiration rates and N2O losses coming from 15NO3−. No significant effect of DMPSA on CH4 oxidation or 15N2 fluxes was observed. The DMPSA did not lead to a significant improvement of the dry weights of grain or biomass, although an increment of root biomass by 64% was found. This compound also tended to increase plant N recovery (average 67.8%) and to decrease soil N recovery (average 18.3%) but differences were not statistically significant. Conversely, the nitrification inhibitor significantly reduced the residual fertilizer-N in the 15–30 cm and 30–45 cm soil layers. The use of DMPSA was confirmed as a highly effective tool to reduce N2O emissions from irrigated crops in semi-arid areas.

Relationship between land-use and sources and fate of nitrate in groundwater in a typical recharge area of the North China Plain

Identification of different nitrate sources in groundwater is challenging in areas with diverse land use and multiple potential inputs. An area with mixed land-uses, typical of the piedmont-plain recharge area of the North China Plain, was selected to investigate different nitrate sources and the impact of land use on nitrate distribution in groundwater. Multiple environmental tracers were examined, including major ions, stable isotopes of water (δ2H-H2O, δ18O-H2O) and nitrate (δ15N-NO3− and δ18O-NO3−). Groundwater was sampled from four land-use types; natural vegetation (NV), farmland (FL), economic forestland (EF) and residential areas (RA). A mixing model using δ18O and Cl− concentrations showed that groundwater recharge predominantly comprises precipitation and lateral groundwater flow from areas of natural vegetation in the upper catchment, while irrigation return water and wastewater from septic tanks were major inputs in farmland and residential areas, respectively. Land use variation is the major contributing factor to different nitrate concentrations. In total, 80%, 49% and 86% of samples from RA, FL and EF, respectively exceeded the WHO standard (50mg/L NO3−), compared to 6.9% of samples from NV. Isotopes of δ15N-NO3− and δ18O-NO3− verified that nitrate in groundwater of the NV (with δ15N ranging from 1.7‰ to 4.7‰) was sourced from soil and precipitation. Examination of δ15N-NO3− vs δ18O-NO3− values along with multivariate statistical analysis (principle component and cluster analysis) helped identify sources with overlapping isotopic values in other land-use areas (where δ15N values range from 2.5‰ to 10.2‰). Manure and septic waste were dominant sources for most groundwater with high NO3− and Cl− concentrations in both farmland and residential areas. The lack of de-nitrification and fact that the area is a recharge zone for the North China Plain highlight the importance of controlling nitrate sources through careful application of manure and fertilizers, and control of septic leakage.

Long-term net transformation and quantitative molecular mechanisms of soil nitrogen during natural vegetation recovery of abandoned farmland on the Loess Plateau of China

The availability of nitrogen (N) can alter vegetation species composition and diversity in degraded ecosystems. A comprehensive understanding of the dynamic fate of ammonium (NH4+-N) and nitrate (NO3−-N) processing and the underlying mechanisms are still lacking, particularly in arid to semi-arid degraded ecosystems. We compared and quantified the changes in the rates of net ammonification (Ra), nitrification (Rn) and total mineralization (Rm) and the abundance of bacteria, archaea, and microbial genes related to N transformation on the northern Loess Plateau of China across a 40-year chronosequence of farmland undergoing spontaneous restoration. We found that Ra, Rn, and Rm decreased in grassland soils (0–30-y sites) of different ages and exhibited significant increases at the 40-y sites. The capabilities of the soil to deliver NH4+-N and NO3−-N were not a limiting factor during the growing season after 40years of vegetation recovery. Soil mineral nitrogen may be not suitable for predicting and assessing the long-term (approximately 40years) restoration success and progress. The abundance of functional N genes showed differences in sensitivity to natural vegetation recovery of abandoned farmland, which likely reflects the fact that the multi-pathways driven by N functional microbial communities had a large influence on the dynamic fate of NH4+-N and NO3−-N. Quantitative response relationships between net N transformation rates and microbial genes related to N transformation were established, and these relationships confirmed that different N transformation processes were strongly linked with certain N functional genes, and collaboratively contributed to N transformation as vegetation recovery progressed. Specifically, Ra was controlled by AOA-amoA, AOB-amoA, and nxrA; Rn was governed by napA, narG, nirK, nirS, and nosZ; and Rm was controlled by nifH, apr, AOA-amoA, AOB-amoA, nirS, and nirK.

Fate of water and nitrate using drainage water management on tile systems in east-central Illinois

Drainage water management (DWM) is a potential edge-of-field technique that is being studied as a method to improve soil water management in agricultural fields, which may reduce nitrate losses to surface waters during the non-growing season. Inline water level control structures were installed on two adjacent tile systems draining a 34ha field located in the Upper Salt Fork of the Vermillion River Watershed in central Illinois to evaluate DWM from 2011 through 2013. The overall objective of this study was to determine the effectiveness of DWM in reducing nitrate losses from fields in a corn and soybean production system in east-central Illinois, as well as to investigate the fate of the retained water. A paired watershed approach was used to determine the impact of DWM on tile flow and nitrate load compared to a control treatment or free drainage (FD) tile system. The entire 34 ha field was under a corn (Zea mays L.) and soybean (Glycine max L.) rotation with continuous no-till. During 2011 and 2012, DWM was able to greatly reduce tile flow compared to the FD tile system. However, based on runoff and nitrate yields from the entire field, there was no measureable reduction in nitrate loss and shallow ground-water wells showed little area of influence in the field (∼2ha). Water from the DWM tile system flowed laterally to the nearby FD tile system, increasing flow and nitrate loss from the FD system. In 2013, when both tiles were under DWM, water was retained and the water table level was increased in a larger area of the field (∼6ha). However, at the end of the experiment when the control stoplogs were lowered the retained water was discharged through the tiles lines with little apparent reduction (10%) in overall water and nitrate loss for the year. Measurements of tile and well nitrate concentrations suggested that nitrate was not denitrified in the shallow groundwater of the field during the three-year study. Nitrate losses were directly proportional to tile flow each year of the study. Retrofitting DWM on an existing tile system was found to have a limited water quality benefit.

Tracking the fate of nitrate through pulse-flow wetlands: A mesocosm scale 15N enrichment tracer study

Quantitative information about the fate of applied nitrate (NO3-N) in pulse-flow constructed wetlands is essential for designing wetland treatment systems and assessing their nitrogen removal services for agricultural and stormwater applications. Although many studies have documented NO3-N losses in wetlands, controlled experiments indicating the relative importance of different processes and N sinks are scarce. In the current study, 15NO3-N isotope enrichment tracer experiments were conducted in wetland mesocosms of two different wetland soil types at two realistic agricultural NO3-N source loads. The 15N label was traced from the source NO3-N into plant biomass, soil (including organic matter and ammonium), and N-gas constituents over 7–10day study periods. All sinks responded positively to higher NO3-N loading. Plant uptake exceeded denitrification 2–3 fold in the low NO3-N loading experiments, while both fates were nearly equivalent in the high loading experiments. One to two years later, soils largely retained the assimilated tracer N, whereas plants had lost much of it. Results demonstrated that plant and microbial assimilation in the soil (temporary N sinks) can exceed denitrification (permanentN loss) in pulse-flow environments and must be considered by wetland designers and managers for optimizing nitrogen removal potential.

Opening Opportunities for High-Resolution Isotope Analysis - Quantification of δ15NNO3 and δ18ONO3 in Diffusive Equilibrium in Thin-Film Passive Samplers.

The fate of nitrate transported across groundwater-surface water interfaces has been intensively studied in recent decades. The interfaces between aquifers and rivers or lakes have been identified as biogeochemical hotspots with steep redox gradients. However, a detailed understanding of the spatial heterogeneity and potential temporal variability of these hotspots, and the consequences for nitrogen processing, is still hindered by a paucity of adequate measurement techniques. A novel methodology is presented here, using Diffusive Equilibrium in Thin-film (DET) gels as high-spatial-resolution passive-samplers of δ15NNO3 and δ18ONO3 to investigate nitrogen cycling. Fractionation of δ15NNO3 and δ18ONO3 during diffusion of nitrate through the DET gel was determined using varying equilibrium times and nitrate concentrations. This demonstrated that nitrate isotopes of δ15NNO3 and δ18ONO3 do not fractionate when sampled with a DET gel. δ15NNO3 values from the DET gels ranged between 2.3 ± 0.2 and 2.7 ± 0.3‰ for a NO3- stock solution value of 2.7 ± 0.4‰, and δ18ONO3 values ranged between 18.3 ± 1.0 and 21.5 ± 0.8‰ for a NO3- stock solution of 19.7 ± 0.9‰. Nitrate recovery and isotope values were independent of equilibrium time and nitrate concentration. Additionally, an in situ study showed that nitrate concentration and isotopes provide unique, high-resolution data that enable improved understanding of nitrogen cycling in freshwater sediments.

Effect of wheat-maize straw return on the fate of nitrate in groundwater in the Huaihe River Basin, China

Straw return is becoming a routine practice in disposing of crop residues worldwide. However, the potential effect of such operation on the chemistry of local groundwater is not well documented. Here, shallow groundwater in an area where wheat-maize straw return is practiced was analyzed, and the seasonal changes in the nitrate concentration and the isotope compositions of NO3− and H2O were determined along two flow paths. Measured δD and δ18O in waters indicated that the groundwater was mainly recharged by atmospheric precipitation, while measured δ15N and δ18O in nitrate suggested that the sources for groundwater NO3− included urea fertilizer, soil nitrogen, and sewage/manure. Reduced NO3− concentrations coincided with an enrichment of organic matter in the groundwater of the straw return area, revealing an environmental condition that facilitates nitrate reduction, whereas increased δ15N-NO3− and δ18O-NO3− along the flow path suggested the occurrence of denitrification. Further analyses showed that, compared to the cases in the absence of straw return, as much as 80% and 90% of groundwater nitrate was removed in low and high water seasons in the straw return area, pointing to a potential positive effect of straw return to groundwater quality.

Impact of the electron donor on in situ microbial nitrate reduction in Opalinus Clay: results from the Mont Terri rock laboratory (Switzerland)

At the Mont Terri rock laboratory (Switzerland), an in situ experiment is being carried out to examine the fate of nitrate leaching from nitrate-containing bituminized radioactive waste, in a clay host rock for geological disposal. Such a release of nitrate may cause a geochemical perturbation of the clay, possibly affecting some of the favorable characteristics of the host rock. In this in situ experiment, combined transport and reactivity of nitrate is studied inside anoxic and water-saturated chambers in a borehole in the Opalinus Clay. Continuous circulation of the solution from the borehole to the surface equipment allows a regular sampling and online monitoring of its chemical composition. In this paper, in situ microbial nitrate reduction in the Opalinus Clay is discussed, in the presence or absence of additional electron donors relevant for the disposal concept and likely to be released from nitrate-containing bituminized radioactive waste: acetate (simulating bitumen degradation products) and H2 (originating from radiolysis and corrosion in the repository). The results of these tests indicate that—in case microorganisms would be active in the repository or the surrounding clay—microbial nitrate reduction can occur using electron donors naturally present in the clay (e.g. pyrite, dissolved organic matter). Nevertheless, non-reactive transport of nitrate in the clay is expected to be the main process. In contrast, when easily oxidizable electron donors would be available (e.g. acetate and H2), the microbial activity will be strongly stimulated. Both in the presence of H2 and acetate, nitrite and nitrogenous gases are predominantly produced, although some ammonium can also be formed when H2 is present. The reduction of nitrate in the clay could have an impact on the redox conditions in the pore-water and might also lead to a gas-related perturbation of the host rock, depending on the electron donor used during denitrification.

Quantitative study on the fate of residual soil nitrate in winter wheat based on a 15N-labeling method.

A considerable amount of surplus nitrogen (N), which primarily takes the form of nitrate, accumulates in the soil profile after harvesting crops from an intensive production system in the North China Plain. The residual soil nitrate (RSN) is a key factor that is included in the N recommendation algorithm. Quantifying the utilization and losses of RSN is a fundamental necessity for optimizing crop N management, improving N use efficiency, and reducing the impact derived from farmland N losses on the environment. In this study, a 15N-labeling method was introduced to study the fate of the RSN quantitatively during the winter wheat growing season by 15N tracer technique combined with a soil column study. A soil column with a 2 m height was vertically divided into 10 20-cm layers, and the RSN in each layer was individually labeled with a 15N tracer before the wheat was sown. The results indicated that approximately 17.68% of the crop N derived from RSN was located in the 0–2 m soil profile prior to wheat sowing. The wheat recovery proportions of RSN at various layers ranged from 0.21% to 33.46%. The percentages that still remained in the soil profile after the wheat harvest ranged from 47.08% to 75.44%, and 19.46–32.64% of the RSN was unaccounted for. Upward and downward movements in the RSN were observed, and the maximum upward and downward distances were 40 cm and 100 cm, respectively. In general, the 15N-labeling method contributes to a deeper understanding of the fates of the RSN. Considering the low crop recovery of the RSN from deep soil layers, water and N saving practices should be adopted during crop production.

Exploring impacts of vegetated buffer strips on nitrogen cycling using a spatially explicit hydro-biogeochemical modeling approach

Agriculture has been recognized as a major anthropogenic source of surplus loads of nitrogen in the environment. Losses of nitrate via subsurface pathways are severely threatening groundwater and surface waters. This study explored the capability of a coupled hydro-biogeochemical spatially explicit model, simulating nitrogen cycling in agricultural soils and the associated fate of excess nitrate subjected to vertical and lateral displacement towards water bodies. Different vegetated buffer strips (VBS) were tested for their nitrate retention capability and impacts on N2O and N2 emissions. The effectiveness of a VBS to remove nitrate by denitrification strongly depends on soil characteristics and hydrological flow paths. Simulated N2 emissions from VBS with high soil moisture were up to twenty-fold compared to VBS where groundwater levels were low. Simulated streamwater nitrate concentrations without VBS were 3.7 mg NO3− l−1 and showed a decrease to 0.1 mg NO3− l−1 for a 20 m VBS.

Fate of Ammonia and Nitrate From Small POTWs in Ohio Streams

Fate of Ammonia and Nitrate From Small POTWs in Ohio Streams

Sources and transformations of anthropogenic nitrogen along an urban river–estuarine continuum

Abstract. Urbanization has altered the fate and transport of anthropogenic nitrogen (N) in rivers and estuaries globally. This study evaluates the capacity of an urbanizing river–estuarine continuum to transform N inputs from the world's largest advanced (e.g., phosphorus and biological N removal) wastewater treatment facility. Effluent samples and surface water were collected monthly along the Potomac River estuary from Washington D.C. to the Chesapeake Bay over a distance of 150 km. In conjunction with box model mass balances, nitrate stable isotopes and mixing models were used to trace the fate of urban wastewater nitrate. Nitrate concentrations and δ15N-NO3− values were higher down-estuary from the Blue Plains wastewater outfall in Washington D.C. (2.25 ± 0.62 mg L−1 and 25.7 ± 2.9 ‰, respectively) compared to upper-estuary concentrations (1.0 ± 0.2 mg L−1 and 9.3 ± 1.4 ‰, respectively). Nitrate concentration then decreased rapidly within 30 km down-estuary (to 0.8 ± 0.2 mg L−1), corresponding to an increase in organic nitrogen and dissolved organic carbon, suggesting biotic uptake and organic transformation. TN loads declined down-estuary (from an annual average of 48 000 ± 5000 kg day−1 at the sewage treatment plant outfall to 23 000 ± 13 000 kg day−1 at the estuary mouth), with the greatest percentage decrease during summer and fall. Annually, there was a 70 ± 31 % loss in wastewater NO3− along the estuary, and 28 ± 6 % of urban wastewater TN inputs were exported to the Chesapeake Bay, with the greatest contribution of wastewater TN loads during the spring. Our results suggest that biological transformations along the urban river–estuary continuum can significantly transform wastewater N inputs from major cities globally, and more work is necessary to evaluate the potential of organic nitrogen and carbon to contribute to eutrophication and hypoxia.

How test vessel properties affect the fate of silver nitrate and sterically stabilized silver nanoparticles in two different test designs used for acute tests with Daphnia magna.

The relation between test conditions such as medium composition or pH on silver nanoparticle (AgNP) behavior and its link to toxicity is one of the major topics in nanoecotoxicological research in the last years. In addition, the adaptation of the ecotoxicological standard tests for nanomaterials is intensely discussed to increase comparability and reliability of results. Due to the limitation of test material production volumes and the need for high-throughput screening, miniaturization has been proposed for several test designs. In the present study, the effect of a miniaturization of the acute Daphnia immobilization test on AgNP behavior was investigated. For this purpose, available, adsorbed, and dissolved silver fractions were measured using AgNP and silver nitrate in the following two test designs: a standard test (ST) design and a miniaturized test (MT) design with reduced test volume and less animals. Despite the increase in surface area in relation to the test volume in MT, more AgNP attached to the ST vessel surface, so that in this case, exposure concentrations were significantly lower compared to the MT assessment. Ionic silver concentrations resulting from AgNP dissolution were similar in both test designs. The same was observed for ionic silver concentrations in silver nitrate (AgNO3) treatments, but adsorbed silver was also higher in ST treatments. Assessing the structure-activity relationships revealed that surface properties such as hydrophobicity, potential binding sites, or surface roughness were of higher importance than surface:volume ratios for both test substances.