Nitrate Concentration Profiles Research Articles

An inverse model to correct for the effects of post-depositional processing on ice-core nitrate and its isotopes: model framework and applications at Summit, Greenland, and Dome C, Antarctica

Abstract. Comprehensive evaluation of the effects of post-depositional processing is a prerequisite for appropriately interpreting ice-core records of nitrate concentration and isotopes. In this study, we developed an inverse model that uses archived snow/ice-core nitrate signals to reconstruct primary nitrate flux (i.e., the deposition flux of nitrate to surface snow that originates from long-range transport or stratospheric input) and its isotopes (δ15N and Δ17O). The model was then applied to two polar sites, Summit, Greenland, and Dome C, Antarctica, using measured snowpack nitrate concentration and isotope profiles in the top few meters. At Summit, the model successfully reproduced the observed atmospheric δ15N(NO3-) and Δ17O(NO3-) and their seasonality. The model was also able to reasonably reproduce the observed snowpack nitrate profiles at Dome C as well as the skin layer and atmospheric δ15N(NO3-) and Δ17O(NO3-) at the annual scale. The calculated Fpri at Summit was 6.9 × 10−6 kgN m2 a−1, and the calculated Δ17O(NO3-) of Fpri is consistent with atmospheric observations in the Northern Hemisphere. However, the calculated δ15N(NO3-) of Fpri displays an opposite seasonal pattern to atmospheric observations in the northern mid-latitudes, but it is consistent with observations in two Arctic coastal sites. The calculated Fpri at Dome C varies from 1.5 to 2.2 × 10−6 kgN m−2 a−1, with δ15N(NO3-) of Fpri varying from 6.2 ‰ to 29.3 ‰ and Δ17O(NO3-) of Fpri varying from 48.8 ‰ to 52.6 ‰. The calculated Fpri at Dome C is close to the previous estimated stratospheric denitrification flux in Antarctica, and the high δ15N(NO3-) and Δ17O(NO3-) of Fpri at Dome C also point towards the dominant role of stratospheric origin of primary nitrate to Dome C.

Impact of Air Refreshing and Cloud Ice Uptake Limitations on Vertical Profiles and Wet Depositions of Nitrate, Ammonium, and Sulfate

AbstractThe impacts of cloud mixing and uptake on wet scavenging are not adequately resolved in global models which can lead to an overestimation of the removal of water‐soluble gases and aerosols from the atmosphere. To address this issue, we develop and implement novel parameterizations to consider the impacts of these processes. Our analysis of vertical profiles of nitric acid, inorganic nitrate, ammonium, and sulfate concentrations during the Atmospheric Tomography Mission periods indicates that air refreshing limitation has a significant impact above 800 hPa, while cloud ice uptake limitation plays an important role above 500 hPa. Incorporating these two processes resulted in a reduction of wet depositions of these species across source regions and a slight increase in their downwind regions. Wet depositions of nitrate, ammonium, and sulfate were reduced in source regions by 22.7%, 8.4%, and 8.3%, respectively and increased in downwind regions by 10.1%, 7.0%, and 4.3%, respectively.

In situ temperature-compensated ultraviolet spectrophotometry to estimate nitrate and chloride concentrations in estuarine seawater with different salinity and composition

Various methods have been proposed for in situ measurement of nitrate concentrations from the ultraviolet (UV) absorbance spectrum of seawater with stable salinity and constituents. However, salinity and temperature affect the UV absorption spectrum of seawater. In sea areas with large variability in salinity and water temperature, accurate nitrate ion concentration measurements remain challenging. We performed in situ measurements of nitrate, chloride, and bromide in estuarine seawater with different salinity compositions and applied water temperature compensation. First, the impact of water temperature on the UV absorbance of chloride, bromide, and nitrate was experimentally investigated and represented in a mathematical model. Next, chloride, bromide, and nitrate concentrations were estimated by suppressing the impact of residual components from the UV absorbance spectra of seawater using principal component regression (PCR). Hence, the chloride, bromide, and nitrate concentrations were determined by measuring the UV absorbance spectrum of seawater alone, without measuring water temperature and electrical conductivity. The proposed method was more accurate (±1.39 μM below 100 μM and ±0.90 μM below 20 μM) than the conventional method (±2.35 μM below 100 μM and ±1.88 μM below 20 μM) and PCR without water temperature compensation (±3.67 μM). In a field study, an in situ UV spectrophotometer with water temperature compensation was used to measure depth profiles of nitrate concentrations in estuarine seawater. We successfully measured the depth profiles of low chloride and high nitrate concentrations in the surface layer as well as high chloride and low nitrate concentrations in the lower layer. The proposed method enables in situ measurements of nitrate concentrations in waters with either stable or highly variable salinity and composition. Unlike conventional chemical analysis, our method can describe detailed spatiotemporal variations in nitrate concentrations.

Novel insight into ammonium, phosphate, and iron(II) dynamics in the sediment porewater of a constructed wetland under artificial aeration through the diffusive equilibrium in thin films technique

The analysis of porewater concentrations in constructed wetland sediments could help to understand biogeochemical processes, the sources and sinks of nutrients, and their effect on overlying water quality. In this study, we measured high-resolution porewater concentration profiles of ammonium (NH4+-N), nitrate (NO3N), phosphate (PO43--P), and ferrous iron (Fe(II)) in-situ in the Laratinga constructed wetland in Mount Barker (South Australia) using diffusive equilibration in thin films (DET) techniques. Measurements were taken under light and dark conditions, and non-aerated and aerated conditions to determine the effect on sediment porewater nutrient concentrations. Baseline surface water nutrient concentrations (NH4+-N > 36 mg L-1, PO43--P > 0.43 mg L-1) greatly exceeded water quality guideline criteria. Aeration of the water column alleviated night-time hypoxic conditions (i.e. dissolved oxygen increased from a minimum of 0.7 mg L-1 to a minimum of 4 mg L-1), and increased the redox potential in the sediment. Significant differences were present for NH4+-N, PO43--P, and Fe(II) concentrations with depth in the sediment. Ammonium concentrations in the sediment reduced under aerated conditions, presumably due to enhanced nitrification. However it was observed that PO43--P and Fe(II) concentrations increased significantly with aeration, especially under dark conditions, and were strongly correlated (R2>0.8). This was not what was hypothesised and points to complex interactions between Fe and P in the sediment. Nitrate concentrations in the sediment were below the detection limit (<0.9 mg L-1) which suggests limited nitrification-denitrification is occurring. Overall the results suggest that DET techniques are useful tools for quantifying porewater concentrations of nutrients in constructed wetlands under various environmental conditions.

An improved 1D reactive Bürger-Diehl settler model for secondary settling tank denitrification.

An improved 1D reactive settler model is pursued in order to increase the understanding of reactive settling processes and obtain a better prediction of the nitrogen mass balance in wastewater treatment systems. The developed model is based on the 1D Bürger-Diehl settler model with compression function and the Activated Sludge Model No. 1 biological reactions. Specific attention was paid in the model development phase to optimal selection of settling velocity functions and integration of the correct clarifier geometry. A unique measurement campaign was carried out with different operational scenarios to quantify the denitrification in a secondary settling tank. A detailed step-wise calibration effort demonstrated that by choosing an appropriate settling velocity function (power-law structure) and considering the true clarifier geometry allows to accurately capture the biomass concentration profile, total sludge mass, sludge blanket height, and the reaction rates. The resulting model is able to accurately describe total suspended solids (TSS) and nitrate concentration profiles throughout a settling tank under different operational conditions. As such the model can be applied in further scenario analysis and system optimization. PRACTITIONER POINTS: A unique measurement campaign was carried out to obtain detailed data for a reactive settler model development. A 1-D reactive settler model is developed based on the Bürger-Diehl framework including ASM1 biokinetics and the clarifier geometry. An extensive calibration and model selection effort was performed. The model accurately predicts measured concentration profiles in the settling tank. The developed model can be integrated in a plant-wide model to properly calculate the nitrogen mass balance of a WRRF.

Development and Evaluation of E3SM‐MOSAIC: Spatial Distributions and Radiative Effects of Nitrate Aerosol

AbstractNitrate aerosol plays an important role in affecting regional air quality as well as Earth's climate. However, it is not well represented or even neglected in many global climate models. In this study, we couple the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) module with the four‐mode version of the Modal Aerosol Module (MAM4) in DOE's Energy Exascale Earth System Model version 2 (E3SMv2) to simulate nitrate aerosol and its radiative effects. We find that nitrate aerosol simulated by E3SMv2‐MAM4‐MOSAIC is sensitive to the treatment of gaseous HNO3 transfer to/from interstitial particles related to accommodation coefficients of HNO3 () on dust and non‐dust particles. We compare three different treatments of HNO3 transfer: (a) a treatment (MTC_SLOW) that uses a low in the mass transfer coefficient (MTC) calculation; (b) a dust‐weighted MTC treatment (MTC_WGT) that uses a high on non‐dust particles; and (c) a dust‐weighted MTC treatment that also splits coarse mode aerosols into the coarse dust and sea salt sub‐modes in MOSAIC (MTC_SPLC). MTC_WGT and MTC_SPLC increase the global annual mean (2005–2014) nitrate burden from 0.096 (MTC_SLOW) to 0.237 and 0.185 Tg N, respectively, mostly in the coarse mode. MTC_WGT and MTC_SPLC also produce stronger nitrate direct radiative forcing (−0.048 and −0.051 W m−2, respectively) and indirect forcing (−0.33 and −0.35 W m−2, respectively) than MTC_SLOW (−0.021 and −0.24 W m−2). MTC_WGT and MTC_SPLC improve nitrate surface concentrations over remote oceans based on limited observations and vertical profiles of nitrate concentrations against aircraft measurements below 400 hPa.

Development of a predictive kinetic model with statistically analyzed parameters for Donnan Dialysis process

Donnan Dialysis (DD) utilizes ion exchange membranes that allow the selective transport of target ions from a feed solution to a concentrated receiver solution. The kinetics of DD involves boundary layer diffusion (BLD), diffusional membrane transport (MD), and a mixed regime where both mechanisms are active. The objective of the research was to develop an automated computational model that can predict nitrate, bicarbonate, and sulfate concentration profiles and transport mechanisms in DD processes. Experiments were conducted in a batch dialyzer in sole- and multi-component feed solutions and NaCl receiver solutions.Training of the model included numerical extraction of the BLD and MD coefficients from the experimental data; determination of the statistically significant parameters that affect the DD transport; development of correlations between these parameters and the kinetic coefficients; and mapping of the ratio between the boundary layer resistance and the membrane resistance to determine the transport mechanism.The predictive model, included the kinetic equations, i.e., the Nernst-Planck formulation, mass balances, and electro-neutrality, the correlations, and the mapping of the transport mechanism. The model was verified by the experimental data used for the model training and validated by independent datasets. Very good fits between the predicted and experimental concentration profiles were obtained, and the ability to determine the transport mechanism was demonstrated.

Nitrate transport in the chalk vadose zone in Picardy, France

Abstract Since the 1980s, nitrate has been shown to be present in soils and the vadose zone of various types of geological materials years after fertilizer application. In chalk, where the vadose zone is thick, nitrate storage can be considerable and its transport time towards groundwater can be lengthy. In this context, evaluation of the impact of changes in agricultural practices on groundwater quality remains a major question. Improvement of groundwater quality can in certain cases be greatly delayed after the implementation of environmental agricultural practices. The principal objective of this study is to improve our knowledge of when changes in agricultural practices will have a noticeable effect on groundwater quality. To meet this objective, nitrate concentration profiles were performed in agricultural plots in Picardy (France). A crop marker event was used to calculate the transport velocity of water and associated solutes. This method is useful when other tracers (such as tritium or chloride) cannot be used. Estimated velocities range from 0.51 to 0.54 m a −1 ; these values are similar to those described in similar chalk aquifers.

Developing an artificial neural network model for predicting the growth of Chlorella sorokiniana in a photobioreactor

Microalgae have been long considered as a potential source of biofuel. Species such as Chlorella sorokiniana can store large amounts of carbohydrates and lipids which can be used to produce biofuels. This paper demonstrates a method for developing an artificial neural network model which can predict C. sorokiniana growth in a photobioreactor. The data used for training the model came from cultivation experiments conducted at the National Cheng Kung University in Taiwan. A feedforward backpropagation ANN model with three inputs (i.e. aeration rate, biomass concentration, and nitrate concentration) and two targets (i.e. biomass concentration and nitrate concentration after 24 hours) was used for this study. Using MATLAB, multiple configurations of this ANN model were created and tested by varying the number of neurons and hidden layers and the training algorithm. Models were initially assessed in terms of their mean square error (MSE) and training performance plots. The models were then further assessed based on their simulation capabilities. After setting the initial biomass and nitrate concentration and aeration profile, the model can already predict the daily biomass and nitrate concentration of C. sorokiniana for the whole cultivation period. The final model selected has one (1) hidden layer and four (4) hidden neurons and it was trained using the Bayesian regularization backpropagation algorithm. For the final selected model, the calculated mean absolute percentage error (MAPE) for the predicted daily biomass and nitrate concentration were all below 7.59% and 3.68% respectively. Thus, the simulation results showed that the final model can accurately predict C. sorokiniana growth at varying aeration profiles. For future studies, this model can be used to determine the aeration profile that can maximize C. sorokiniana growth in a photobioreactor while minimizing aeration costs.

Evaluation of nutrient beneficial management practices on nitrate loading to groundwater in a Southern Ontario agricultural landscape

Evaluation of the performance of agricultural Beneficial Management Practices (BMPs) intended to protect groundwater resources that may be impacted by the leaching of excess agricultural nutrients is both essential and problematic. Many field-monitoring techniques are hampered by the substantial lag time that often exists between when the BMP is implemented and when a related impact on the groundwater quality might be observed. As a result, agricultural nitrogen models that are adapted to site-specific field conditions are often utilized in concert with field observations to provide estimates of BMP performance. In the current work, the Root Zone Water Quality Model was used to evaluate the long-term reduction of nitrate loading as a result of regional nutrient reduction BMP implementation across agricultural fields located within a municipal well field capture zone. Soil nitrate concentration and soil moisture content profiles were collected from a series of monitoring locations. These data, in conjunction with a heuristic optimization algorithm, were used to calibrate and validate the model. Validation results showed that the simulated moisture content profiles matched very well with the observed profiles, and that the simulated soil nitrate concentration was in general agreement with field observations except for the highly reactive and transient rooting zone. The calibrated model was used to investigate a series of potential nutrient reduction BMP scenarios. Results indicated that the annual nitrate loading varied both spatially and temporally relative to the subsurface conditions and the agricultural land management. The overall results indicate that BMP effectiveness needs to be investigated over relatively long time periods and that short-term, point-scale field measurements may not provide sufficient information to evaluate BMP performance. The results obtained through the integration of field data and an agriculture nitrogen model indicates that this approach can be highly beneficial to assess or evaluate the potential long-term performance of agricultural BMPs.

Seasonal and Spatial Variation in the Location and Reactivity of a Nitrate‐Contaminated Groundwater Discharge Zone in a Lakebed

AbstractGroundwater discharge delivering anthropogenic N from surrounding watersheds can impact lake nutrient budgets. However, upgradient groundwater processes and changing dynamics in N biogeochemistry at the groundwater‐lake interface are complex. In this study, seasonal water‐level variations in a groundwater flow‐through lake altered discharge patterns of a wastewater‐derived groundwater contaminant plume, thereby affecting biogeochemical processes controlling N transport. Pore water collected 15 cm under the lakebed along transects perpendicular to shore varied from oxic to anoxic with increasing nitrate concentrations (10–75 μM) and corresponding gradients in nitrite and nitrous oxide. Pore water depth profiles of nitrate concentrations and stable isotopic compositions largely reflected upgradient groundwater N sources and N cycle processes, with minor additional nitrate reduction in the near‐surface lakebed sediments. Potential denitrification rates determined in laboratory microcosms were 10–100 times higher in near‐surface sediments (0–5 cm) than in deeper sediments (5–30 cm) and were correlated with sediment carbon content and abundance of denitrification genes (nirS, nosZI, and nosZII). Potential anammox‐driven N2 production was detectable in deeper anoxic sediments. Injection of bromide and nitrite in the lake sediments showed that the highest net nitrite consumption rates were within the top 10 cm. However, short transit times owing to rapid upward pore water velocities (4–5 cm hr−1) limited removal of the contaminant nitrate transiting through the sediments. Results demonstrate that local hydrologic and biogeochemical processes at the point of discharge affect the distribution and discharge rate of N through lakebed sediments, but processes in the upgradient groundwater can be more important for affecting N speciation and concentration.

Seasonality of Dissolved Organic Carbon in the Upper Northeast Pacific Ocean

AbstractFrom seasonal cruises in the NE Pacific Ocean during 2017, we (1) determined dissolved organic carbon concentrations; (2) calculated net community production (NCP) from nitrate drawdown; and (3) established relationships between NCP and seasonal dissolved organic carbon (DOC) accumulation in the upper 75 m. The fraction of NCP that accumulated as DOC, hereafter referred to as the net dissolved production ratio, was calculated for several stations during spring and summer. The net dissolved production ratio was about 0.26 at the oceanic station Ocean Station Papa during different seasons and years. Using nitrate concentration profiles obtained from Bio‐Argo floats during 2009–2018 operating near Ocean Station Papa, we calculated NCP at high temporal resolution and then applied the 0.26 constant in order to (4) estimate DOC variability for the 9‐year period. We found strong seasonality near Ocean Station Papa, with NCP maxima during summers ranging from 0.3 to 2.9 mol C/m2 and surface DOC concentrations estimated from 56 μmol/kg in winters to 73 μmol/kg in summers. There was a 10‐fold interannual variability in the seasonally accumulated inventory of DOC, ranging from 0.078 to 0.75 mol C/m2. This study reinforces the value of deploying floats equipped with chemical sensors in order to better understand marine biogeochemical cycles, especially when high resolution data cannot be obtained otherwise. Given that ~26% of NCP accumulates as DOC in the central Gulf of Alaska, the remaining balance of ~74% is available for export as sinking biogenic particles.

Understanding the Dynamics of the Oxic‐Anoxic Interface in the Black Sea

AbstractThe Black Sea, the largest semienclosed anoxic basin on Earth, can be considered as an excellent natural laboratory for oxic and anoxic biogeochemical processes. The suboxic zone, a thin interface between oxic and anoxic waters, still remains poorly understood because it has been undersampled. This has led to alternative concepts regarding the underlying processes that create it. Existing hypotheses suggest that the interface originates either by isopycnal intrusions that introduce oxygen or the dynamics of manganese redox cycling that are associated with the sinking of particles or chemosynthetic bacteria. Here we reexamine these concepts using high‐resolution oxygen, sulfide, nitrate, and particle concentration profiles obtained with sensors deployed on profiling floats. Our results show an extremely stable structure in density space over the entire basin with the exception of areas near the Bosporus plume and in the southern areas dominated by coastal anticyclones. The absence of large‐scale horizontal intrusive signatures in the open‐sea supports a hypothesis prioritizing the role of biogeochemical processes.

PHBV polymer supported denitrification system efficiently treated high nitrate concentration wastewater: Denitrification performance, microbial community structure evolution and key denitrifying bacteria

Biodegradable polymer supported denitrification (BPD) system shows good denitrification performance for the wastewater with low nitrate concentrations. In this study, a BPD system using Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) polymer as carbon source was developed to treat the wastewater with high nitrate concentrations. The denitrification performance, utilization ratio of PHBV polymers, and microbial community structure evolution and key denitrifying bacteria were comprehensively studied. Results indicated that an average nitrate removal efficiency of 99% could be achieved with an influent NO3−-N concentration of 100 mg L−1 and a hydraulic retention time (HRT) of 7.25 h. Mass balance model predicted that 80% of the PHBV polymers were consumed by denitrifying bacteria, close to 72% consumption in real condition, suggesting the model might be useful for PHBV polymers management in BPD system. Further, the bacterial community structures varied along the bioreactor profile, which closely linked to the concentration profiles of nitrate and ammonia. Metatranscriptomic analysis identified the key denitrifying bacteria as Comamonas, Acidovorax and Dechloromonas. The PHBV supported denitrification system developed in this study shows potential for removal of high concentration of nitrate from wastewater.

Evaluation of Mechanistic Models for Nitrate Removal in Woodchip Bioreactors.

Woodchip bioreactors (WBRs) are increasingly being applied to remove nitrate from runoff. In this study, replicate columns with aged woodchips were subjected to a range of measured flow rates and influent nitrate concentrations with an artificial stormwater matrix. Dissolved oxygen (DO), nitrate, and dissolved organic carbon (DOC) were measured along the length of the columns. A multispecies reactive transport model with Michaelis-Menten kinetics was developed to explain the concentration profiles of DO, nitrate, and DOC. Four additional models were developed based on simplifying assumptions, and all five models were tested for their ability to predict nitrate concentrations in the experimental columns. Global sensitivity analysis and constrained optimization determined the set of parameters that minimized the root-mean-squared error (RMSE) between the model and the experimental data. A k-fold validation test revealed no statistical difference in RMSE for predicting nitrate concentrations between a zero-order model and the other multispecies reactive transport models tested. Additionally, the multispecies reactive transport models demonstrated no significant differences in predicting DO and DOC concentrations. These results suggest that denitrification in an aged woodchip bioreactor at constant temperature can effectively be modeled using zero-order kinetics when nitrate concentrations are >2 mg-N L-1. A multispecies model may be used if predicting DOC or DO concentrations is desired.

The magnitude of the snow-sourced reactive nitrogen flux to the boundary layer in the Uintah Basin, Utah, USA

Abstract. Reactive nitrogen (Nr = NO, NO2, HONO) and volatile organic carbon emissions from oil and gas extraction activities play a major role in wintertime ground-level ozone exceedance events of up to 140 ppb in the Uintah Basin in eastern Utah. Such events occur only when the ground is snow covered, due to the impacts of snow on the stability and depth of the boundary layer and ultraviolet actinic flux at the surface. Recycling of reactive nitrogen from the photolysis of snow nitrate has been observed in polar and mid-latitude snow, but snow-sourced reactive nitrogen fluxes in mid-latitude regions have not yet been quantified in the field. Here we present vertical profiles of snow nitrate concentration and nitrogen isotopes (δ15N) collected during the Uintah Basin Winter Ozone Study 2014 (UBWOS 2014), along with observations of insoluble light-absorbing impurities, radiation equivalent mean ice grain radii, and snow density that determine snow optical properties. We use the snow optical properties and nitrate concentrations to calculate ultraviolet actinic flux in snow and the production of Nr from the photolysis of snow nitrate. The observed δ15N(NO3−) is used to constrain modeled fractional loss of snow nitrate in a snow chemistry column model, and thus the source of Nr to the overlying boundary layer. Snow-surface δ15N(NO3−) measurements range from −5 to 10 ‰ and suggest that the local nitrate burden in the Uintah Basin is dominated by primary emissions from anthropogenic sources, except during fresh snowfall events, where remote NOx sources from beyond the basin are dominant. Modeled daily averaged snow-sourced Nr fluxes range from 5.6 to 71 × 107 molec cm−2 s−1 over the course of the field campaign, with a maximum noontime value of 3.1 × 109 molec cm−2 s−1. The top-down emission estimate of primary, anthropogenic NOx in Uintah and Duchesne counties is at least 300 times higher than the estimated snow NOx emissions presented in this study. Our results suggest that snow-sourced reactive nitrogen fluxes are minor contributors to the Nr boundary layer budget in the highly polluted Uintah Basin boundary layer during winter 2014.

Regional patterns in current and future export production in the central Arctic Ocean quantified from nitrate fluxes

AbstractDue to severe nutrient and light limitation, the central Arctic Ocean has been characterized as a region of low primary productivity, with high retention of carbon in the surface waters. Using an in‐depth analysis of published and new measurements of turbulent microstructure and high‐resolution profiles of nitrate concentration, we reassess the vertical supply of nitrate to the Polar Mixed Layer and the associated export of particulate organic matter across the nitracline. We estimate annual export production to be approximately 1.5–3 g C m−2, but regional differences in both current and future potential of export production are large, with the eastern Arctic being least constrained by vertical nutrient supply and the western Arctic the most. Future changes in export production are assessed using a 1‐D budget model; increases in the Atlantic sector are possibly compensated by decreases in the rest of the central Arctic Ocean such that the net change might be insignificant.

Disentangling the influence of hydroclimatic patterns and agricultural management on river nitrate dynamics from sub-hourly to decadal time scales

Despite extensive efforts to reduce nitrate transfer in agricultural areas, limited response is often observed in the nitrate concentration in rivers. To investigate the reasons for this limited response, nitrate dynamics in a 100km2 agricultural catchment in eastern Germany was analysed from sub-hourly to decadal time-scales. Sub-hourly analysis of storm event dynamics during a typical hydrological year (2005–2006) was performed to identify periods of the year with high leaching risk and to link the latter to agricultural management practices in the catchment. Dynamic Harmonic Regression analysis of a 32-year (1982–2014) record of nitrate and discharge revealed that i) the long-term trend in nitrate concentration was closely related to that in discharge, suggesting that large-scale weather and climate patterns were masking the effect of improved nitrogen management on nitrate trends; ii) a persistent seasonal pattern with winter concentration maxima and summer minima could be observed, which was interpreted in terms of a dynamic nitrate concentration profile in the soil and subsoil; and iii) the catchment progressively changed from chemodynamic to more chemostatic behaviour over the three decades of study, which is a sign of long-term homogenisation of nitrate concentrations distribution over depth. This study shows that detailed physical understanding of nitrate dynamics across time scales can be obtained only through combined analysis of long-term records and high-resolution sensor data. Hence, a joint effort is advocated between environmental authorities, who usually perform long-term monitoring, and scientific programmes, which usually perform high-resolution monitoring.

Vertical fluxes of nitrate in the seasonal nitracline of the Atlantic sector of the Arctic Ocean

AbstractThis study compiles colocated oceanic observations of high‐resolution vertical profiles of nitrate concentration and turbulent microstructure around the Svalbard shelf slope, covering both the permanently ice‐free Fram Strait and the pack ice north of Svalbard. The authors present an overview over the seasonal evolution of the distribution of nitrate and its relation to upper ocean stratification. The average upward turbulent diffusive nitrate flux across the seasonal nitracline during the Arctic summer season is derived, with average values of 0.3 and 0.7 mmol m−2 d−1 for stations with and without ice cover, respectively. The increase under ice‐free conditions is attributed to different patterns of stratification under sea ice versus open water. The nitrate flux obtained from microstructure measurements lacked a seasonal signal. However, bottle incubations indicate that August nitrate uptake was reduced by more than an order of magnitude relative to the May values. It remains inconclusive whether the new production was limited by an unidentified factor other than supply in late summer, or the uptake was underestimated by the incubation method.

Nitrate contamination of groundwater in the western Po Plain (Italy): the effects of groundwater and surface water interactions

This study aims to investigate the physical and chemical effects of interactions between groundwater and surface water (GW–SW)—particularly in streams—on nitrate contamination. The effects of GW–SW interactions are briefly reviewed, with a particular emphasis on processes and environments that influence increases or decreases in nitrate concentration. Then, this paper analyses nitrate concentrations in groundwater and surface water in the western Po plain (Northwestern Italy); this analysis includes the nitrate concentration profiles across the shallow aquifer and intersecting the main streams on the plain. The investigation highlights how the concentration trends are similar, even when nitrate levels in rivers and groundwater are not comparable. The maximum nitrate concentrations in the surface water were generally measured in areas with high-nitrate levels in groundwater. An analysis of the nitrate concentration profiles highlighted the mutual influences of GW–SW. The most important streams on the plain (the Po River and Stura di Demonte River), both of them gaining streams, seem to reduce the nitrate concentrations of groundwater at a study scale. The proposed conceptual model indicates how the near-stream environment (the riparian zone, wetlands, hyporheic zone and shallow organic-rich soils in the near-stream environment) and the groundwater flow systems in shallow and deep aquifers, from the recharge zone to the streams, could dramatically affect the nitrate concentrations.