Total Nitrogen Load Research Articles

Using a two-step downscaling method to assess the impact of climate change on total nitrogen load in a small basin

Climate data with high spatial and temporal resolution were an important basis for predicting regional climate and water environment changes. The current downscaling targets for GCM data were mainly medium-resolution (MR) reanalysis data, which were still coarse for small basins. We designed a two-step downscaling method in this study, aiming to perform 100 × resolution enhancements of GCM monthly temperature and precipitation data (i.e., target spatial resolution of 1 km × 1 km). We obtained a predictable set of MR climate data by adding a first-step downscaling process, which was then combined with high-resolution (HR) topographic data as auxiliary data in the second-step downscaling process to supplement the spatial and temporal details. The results showed that the performances of two-step downscaling method were better than the one-step downscaling method using only HR topographic data as auxiliary data. Then the two-step downscaling method was applied to downscale the future climate data of four scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5) during 2015–2099 for the target small basin. We fed the HR climate prediction data into a Bayesian water quality model, which had been successfully applied in this basin, to simulate the responses of hydrology and water quality to climate change. The results demonstrated that the downscaled climate data with high spatial resolution could be successfully used in the water quality model for spatially differentiated simulations of climate change, streamflow change and total nitrogen load change in a small basin.

A random forest approach to improve estimates of tributary nutrient loading

Estimating constituent loads from discrete water quality samples coupled with stream discharge measurements is critical for management of freshwater resources. Nutrient loads calculated based on discharge-concentration relationships form the basis of government nutrient load targets and scientific studies of the response of receiving waters to external loads. In this study, a new model is developed using random forests and applied to estimate concentrations and loads of total phosphorus, dissolved phosphorus, total nitrogen, and chloride, using data from 17 tributaries to Lake Champlain monitored from 1992 to 2021. I benchmark this model against one of the most widespread models currently used to estimate nutrient loads, Weighted Regressions on Time, Discharge, and Season (WRTDS). The random forest model outperformed both the base WRTDS model and an extension of the WRTDS model using Kalman filtering in the great majority of cases, likely due to the inclusion of rate-of-change in discharge and antecedent discharge over different leading windows as predictors, and to the flexibility of the random forest to model predictor-response relationships. The random forest also had useful visualization capabilities which provided important process insights. WRTDS remains a useful model for many applications, but this study represents a promising new approach for load estimation which can be applied easily to existing datasets, and which is easy to customize for different applications.

Shift in algal blooms from micro‐ to macroalgae around China with increasing eutrophication and climate change

AbstractBlooms of microalgal red tides and macroalgae (e.g., green and golden tides caused by Ulva and Sargassum) have caused widespread problems around China in recent years, but there is uncertainty around what triggers these blooms and how they interact. Here, we use 30 years of monitoring data to help answer these questions, focusing on the four main species of microalgae Prorocentrum donghaiense, Karenia mikimotoi, Noctiluca scintillans, and Skeletonema costatum) associated with red tides in the region. The frequency of red tides increased from 1991 to 2003 and then decreased until 2020, with S. costatum red tides exhibiting the highest rate of decrease. Green tides started to occur around China in 1999 and the frequency of green tides has since been on the increase. Golden tides were first reported to occur around China in 2012. The frequency of macroalgal blooms has a negative linear relationship with the frequency and coverage of red tides around China, and a positive correlation with total nitrogen and phosphorus loads as well as with atmospheric CO2 and sea surface temperature (SST). Increased outbreaks of macroalgal blooms are very likely due to worsening levels of eutrophication, combined with rising CO2 and SST, which contribute to the reduced frequency of red tides. The increasing grazing rate of microzooplankton also results in the decline in areas affected by red tides. This study shows a clear shift of algal blooms from microalgae to macroalgae around China over the past 30 years driven by the combination of eutrophication, climate change, and grazing stress, indicating a fundamental change in coastal systems in the region.

A pilot-scale SNAD-MBBR process for treating anaerobic digester liquor of swine wastewater: performance and microbial community.

In this pilot-scale study, simultaneous partial nitrification, anammox, and denitrification (SNAD) process was achieved successfully in a moving bed biofilm reactor (MBBR) for treating anaerobic digester liquor of swine wastewater. After 95days of operation, when the total nitrogen loading rate of SNAD-MBBR process was 1.09kg TN/m3/day, the total nitrogen removal rate could reach 0.87kg TN/m3/day, and the removal efficiencies of ammonium and total nitrogen were 92.0% and 79.7%, respectively. The optimum pH and temperature for SNAD-MBBR process were 8.5 and 35°C, respectively, and the optimum dissolved oxygen for SNAD1 and SNAD2 were 0.30 and 0.07mg/L, respectively. The 16S rRNA sequencing suggested that Candidatus Kuenenia, Candidatus Brocadia, Nitrosomonas, and Denitratisoma were the dominant nitrogen removal bacteria. Some of the co-existing bacteria (Truepera, Limnobacter, and Anaerolineaceae uncultured) promoted ammonium oxidation and guaranteed the growth of the anammox bacteria under adverse environmental conditions. Overall, this study demonstrated that the SNAD-MBBR process would be an energy-saving and cost-effective method for the removal of nitrogen from swine wastewater and provided important process parameters for stable operation of the full-scale SNAD process.

Response of streamflow and nutrient loads in a small temperate catchment subject to land use change

The aim of this study was to quantify the effect of land use change (LUC) implemented to meet nutrient load targets for a freshwater lake in New Zealand. We used the Soil and Water Assessment Tool (SWAT) model in combination with a non-parametric statistical test to determine whether afforestation of 15% of a subcatchment area was adequate to meet assigned nutrient load targets. A regional management authority set nutrient load targets of reduction in total nitrogen (TN) by 0.9 t yr−1 and reduction in total phosphorus (TP) by 0.05 t yr−1 to avoid eutrophication in the receiving waters of a freshwater lake. The load reduction was designed to be achieved through 200 ha of LUC from pasture to trees. Analysis of nutrient loads before, during, and following LUC shows that a 15% increase in forest cover decreased the annual flow (7.2%), TP load (33.3%), and TN load (13.1%). As flow and water quality observations were discrete and at irregular intervals, we used a parametric test and the SWAT model as different lines of evidence to demonstrate the effect of afforestation on flow and water quality. Policymakers concerned with decisions about LUC to improve the quality of receiving waters can benefit from applying our findings and using a statistical and numerical modelling framework to evaluate the adequacy of land use change to support improvements in water quality.

Evaluation of nitrous oxide emission during ammonia retention from simulated industrial wastewater by microaerobic activated sludge process

Considering the reciprocating processes of nitrogen gas (N2) fixation to ammonia (NH4-N) and NH4-N removal to N2 through nitrification and denitrification during wastewater treatment, a microaerobic activated sludge process (MAS) is proposed in this study as a pretreatment to retain NH4-N from high-strength nitrogenous wastewater for further NH4-N recovery through membrane technology, that is, inhibit nitrification, with sufficient removal of total organic carbon (TOC). With DO and pH control, the 3-reactor bench-scale MAS systems successfully realized an NH4-N retention rate of over 80 %, with TOC removal rates of over 90 %. In addition, the emissions of carbon dioxide (CO2) and nitrous oxide (N2O) during MAS were evaluated. The total N2O emissions were 407 and 475 mg-N/day when pH was controlled at 6.2 (S1) and 6.8 (S2), respectively, with average emission factors to total nitrogen load over 2 % in both systems. Also, the global warming potential of N2O is one order of magnitude larger than that of CO2, indicating the significance of N2O in the MAS process. Therefore, the mechanisms of N2O emission from each reactor were investigated. The first reactor, where most of the TOC was adsorbed, emitted only 1.98 % (S1) and 2.43 % (S2) of the total N2O emissions through the denitrification of nitrite and nitrate (NOx) from the return sludge. The second reactor emitted 79.9 % (S1) and 69.0 % (S2) of the total N2O with the emission rates the same order of magnitude as the NOx production rates. Multiple pathways were considered to contribute to the high N2O emissions, and biotic NH2OH oxidation was one potential pathway at pH 6.2. Finally, the third reactor emitted 9.98 % (S1) and 16.8 % (S2) of the total N2O by nitrifier denitrification. Overall, this study showed that the large N2O emissions under nitrification-inhibiting conditions of the MAS process owed to the incomplete nitrification under acidic conditions and large abundances of denitrifiers. On the other hand, the lower N2O emissions at pH 6.2 evidenced the potential N2O mitigation under slightly more acidic conditions, underlining the necessity of further study on N2O mitigation when adapting to the trend of NH4-N recovery.

Important Role of Overland Flows and Tile Field Pathways in Nutrient Transport.

Nitrogen and phosphorus pollution is of great concern to aquatic life and human well-being. While most of these nutrients are applied to the landscape, little is known about the complex interplay among nutrient applications, transport attenuation processes, and coastal loads. Here, we enhance and apply the Spatially Explicit Nutrient Source Estimate and Flux model (SENSEflux) to simulate the total annual nitrogen and phosphorus loads from the US Great Lakes Basin to the coastline, identify nutrient delivery hotspots, and estimate the relative contributions of different sources and pathways at a high resolution (120 m). In addition to in-stream uptake, the main novelty of this model is that SENSEflux explicitly describes nutrient attenuation through four distinct pathways that are seldom described jointly in other models: runoff from tile-drained agricultural fields, overland runoff, groundwater flow, and septic plumes within groundwater. Our analysis shows that agricultural sources are dominant for both total nitrogen (TN) (58%) and total phosphorus (TP) (46%) deliveries to the Great Lakes. In addition, this study reveals that the surface pathways (sum of overland flow and tile field drainage) dominate nutrient delivery, transporting 66% of the TN and 76% of the TP loads to the US Great Lakes coastline. Importantly, this study provides the first basin-wide estimates of both nonseptic groundwater (TN: 26%; TP: 5%) and septic-plume groundwater (TN: 4%; TP: 2%) deliveries of nutrients to the lakes. This work provides valuable information for environmental managers to target efforts to reduce nutrient loads to the Great Lakes, which could be transferred to other regions worldwide that are facing similar nutrient management challenges.

Nitrogen and Phosphorus Loading Characteristics of Agricultural Non-Point Sources in the Tuojiang River Basin

Agricultural non-point source (ANPS) pollution has emerged as a significant factor influencing water quality within watersheds. Understanding the spatial distribution and composition of ANPS is crucial for effective river water quality management. Based on the statistical data of 28 districts and counties in the Tuojiang River Basin (TJRB), the load distribution characteristics of total nitrogen (TN) and total phosphorus (TP) from ANPS were studied in this work by using the pollutant discharge coefficient method. In 2018, ANPS contributed 60,888.92 tons of TN and 20,085.98 tons of TP to the TJRB. By 2019, the TN load had decreased to 57,155.44 tons, while the TP load increased to 21,659.91 tons. Spatially, TN and TP loads follow a pattern of being lowest in the upstream, intermediate in the downstream, and highest in the middle reaches. Planting sources emerged as the primary contributors to TN and TP loads from ANPS in the TJRB, accounting for 61.43% and 77.39%, respectively. Rural living sources made a lesser contribution, at 20.23% for TN and 9.15% for TP, while poultry and livestock farming sources accounted for 18.34% of TN and 13.46% of TP loads. The analysis of grey water footprint (GWF) and water pollution level (WPL) revealed that TN and TP loads continued to exert significant pressure on the TJRB’s water environment throughout the study period. These findings offer valuable insights for enhancing water quality management in the TJRB.

Extracellular polymeric substances and microbial community shift during the start-up of a single-stage partial nitritation/anammox process.

A sequencing batch reactor (SBR) was operated to investigate variations of extracellular polymeric substances (EPS) and microbial community during the start-up of the single-stage partial nitritation/anammox (SPN/A) process at intermittent aeration mode. The SPN/A system was successfully started on day 34, and the nitrogen removal efficiency and total nitrogen loading rate were 82.29% and 0.31 kg N/(m3 ·d), respectively. Furthermore, the relationship between the protein secondary structures and microbial aggregation was strongly related. The α-helix/ (β-sheet + random coil) ratios increased obviously from 0.20 ± 0.03 to 0.23 ± 0.01, with the sludge aggregation mean size increased from 56 to 107 μm during the start-up of SPN/A. During the start-up of SPN/A, Candidatus Kuenenia was the primary anammox bacteria, whereas Nitrospira was the main functional bacteria of nitrite-oxidizing bacteria. Correlation between the microbial community and EPS components was performed. The EPS and microbial community played important roles in keeping stable nitrogen removal and the formation of sludge granules.

Discharge dynamics of agricultural diffuse pollution under different rainfall patterns in the middle Yangtze river

Rainfall plays a crucial role in influencing the loss of agricultural diffuse pollution. The middle Yangtze River region is well-know for its humid climate and numerous agricultural activities. Thus, this study quantitatively analyzed the concentration and distribution of nitrogen (N) and phosphorus (P) load and loss in a major tributary of the middle Yangtze River under different rainfall patterns by using sampling analysis and SWAT model simulation. The total nitrogen (TN) and nitrate-nitrogen (NO3−) concentrations were 1.604–3.574 and 0.830–2.556 mg/L, respectively. The total phosphorous (TP) and Soluble Reactive Phosphorus (SRP) were 2–148 and 2–104 μg/L, respectively. The modeling results demonstrated that higher rainfall intensity led to greater load and loss flux of diffuse pollutant at the outlet. Organic nitrogen (ORGN) is the main nitrogen form transported from the subbasin to the reach, while organic phosphorus (ORGP) and inorganic phosphorus (INORGP) were transported at similar amounts. Under the condition of conventional rainfall, the outlet reaches mainly transported NO3−, and ORGN gradually increased when rainstorm events occurred. The ratio of INORGP to ORGP was relatively stable. During extreme rainfall event, rainfall is the dominant element of agricultural diffuse pollution. A strong positive correlation exists between rainfall intensity and pollution loss during rainstorms. Storm rain events were the main source of TN and TP losses. Few storm rain days generated pollutants that accounted for a large proportion of the total loss, and their impact on TP loss was significantly greater than that of TN. The influence of storm rain on TN is mainly the increase in runoff, while TP is sensitive to the runoff and sediment transport promoted by rainfall. By setting different precipitation scenarios, it was confirmed that under the same rainfall amount, short-term storm rain has the most significant impact on the TN load, whereas TP load may be influenced more by the combined effects of rainfall duration and intensity. Therefore, to reduce the impact of agricultural diffuse pollution, it is important to take targeted measures for the rainstorm days.

Contribution of water erosion to organic carbon and total nitrogen loads in agricultural discharge from boreal mineral soils

While organic carbon (OC) in agricultural mineral soils is widely studied in terms of soil carbon sequestration and gaseous emissions, discharge-induced OC loss from soil is still poorly understood and estimations of boreal soil OC loads within water erosion are lacking. Loss of organic matter from arable soils is a concern for surface water quality, climate change and soil productivity. The main aim of this study was to quantify the role of water erosion in total OC and nitrogen (N) loads exported in agricultural discharge from boreal mineral soils under various cultivation practices. Surface water and subsurface drainage were collected near-continually over 2 years in two clayey and one sandy soil in Finland. Eroded sediment was mechanically separated by centrifugation from all discharge samples to detect sediment OC% and N% by dry-combustion method. Dissolved OC and N concentrations in selected discharge samples were measured with high-temperature catalytic oxidation of unfiltered supernatant. A multiple linear regression model was used to study the significant factors affecting dissolved, sediment and total OC loads. In the clayey soils, the sediment OC (2–24 kg ha−1 y−1) and N (0.2–1.1 kg ha−1 y−1) export accounted for up to 35 % and 20 % of the annual discharge-induced total loads of OC (19–85 kg ha−1) and N (2–8 kg ha−1), respectively. In the sandy soil, erosion was negligible and dissolved loads of 17–35 kg OC ha−1 y−1and 4–7 kg N ha−1 y−1 were detected. Subsurface drainage exported most of the sediment-associated OC and N loads from clayey soils. For the total OC loads, the distribution varied between the discharge routes, while the total N loads were mostly exported in subsurface drainage in both soil types. Sediment OC and N exports were related to soil plowing and discharge intensity, while dissolved OC loss was promoted by high surface soil OC%. Our results also indicated that a single cultivation practice may affect sediment and dissolved loads in opposite ways. These findings can be used to complement carbon budget estimations for mineral agricultural soils, and to assess soil management effects on terrestrial organic matter loading to boreal surface waters.

Nitrogen removal characteristics of anammox biofilm and analyses of the dynamic network structure of functional microbes under gradient nitrogen stress

The universal porous gel/vertical fixed bed-anaerobic ammonia oxidation (UPG/VFB-A) is a complex efficient denitrification symbiotic system of microorganisms for leachate. However, the different gradient nitrogen loading lead to dynamic correlation and difference in biofilm formation remain unclear. Therefore, the UPG/VFB-A was operated for 300 days under gradient total nitrogen loading rate (TNLR) conditions, and its biofilm biomass characteristics, bacterial population succession and functional denitrification gene dynamic network were investigated to achieve strong impact resistance and recovery of the reactor for landfill leachate. The results showed that the total nitrogen removal efficiency (TNRE) was 92.4 ± 2.7% and the nitrogen removal (NR) up to 487.0 ± 20.5 mg/L. The system was highly resistant and resilient to high concentrations of FA (11.68 mg/L) and FNA (0.0467 mg/L) with total nitrogen removal rate (TNRR) up to 1.12 kg/m3/d. Candidatus Kuenenia (from 0.09% to 17.2%) with high tolerance to FA and FNA was superior to Candidatus Brocadia (from 9.7% to 4.7%) as the main AnAOB. Meanwhile, the gradient nitrogen had a significant effect on the abundance of genes related to nitrogen conversion, and the relative abundance of typical anammox genes hzs and hdh was highest at a feedwater load of 1.34 kg/m3/d, reaching 20.7% and 16.5%, respectively. This study can provide guidance for anammox treatment of landfill leachate in practical engineering.

Study on the planning and influential factors of the safe width of riparian buffer zones in the upper and middle reaches of the Ziwu River, China.

In this study, we employed the random forest model to identify the riparian buffer zone in the upper and middle reaches of the Ziwu River, used the Soil and Water Assessment Tool (SWAT) to simulate and calculate the nonpoint source pollution load in the riparian buffer zone, and used empirical formulas to estimate the pollutant concentration when surface runoff passes the edge of the riparian buffer zone. Moreover, through correlation analysis, we identified the main factors that affect the safe width of the riparian buffer zone. By combining these factors with the characteristic parameters of the riparian buffer zone and the water quality demand, we analyzed and calculated the safe width of the riparian buffer zone. Our findings are as follows: ① the simulated values of the SWAT model were highly consistent with the measured values. Specifically, the calibration and verification results of the hydrological station achieved Ens ≥ 0.65, RE < ± 15%, and R2 ≥ 0.85, while the overall total nitrogen and total phosphorus loads achieved Ens ≥ 0.65, RE < ± 15%, and R2 > 0.65. ② We found that the total nitrogen (TN) and total phosphorus (TP) loads in the riparian buffer zone gradually increased from upstream to downstream. Among these loads, the normal season had the largest TN and TP concentrations reaching the edge of the riparian buffer zone, while the dry season had the minimum concentrations. ③ The factors affecting the safe width of the riparian buffer zone included the connectivity, slope of the buffer zone, cultivated land area, and regional population density. For the effective protection of water quality, it is recommended that the upstream, midstream, and downstream buffer zones be at least 77.9m, 33.37m, and 60.25m wide, respectively.

National-scale nitrogen loading from the Finnish agricultural fields has decreased since the 1990s

The national scale nutrient load modelling system VEMALA-ICECREAM was used to simulate agricultural total nitrogen(TN) loading and its trends for all Finnish watersheds for the period from 1990–2019. Across Finland, agricultural TN loading (ATNL) has decreased from 17.4 kg ha-1 a-1 to 14.4 kg ha-1 a-1 (moving 10-year averages) since the 1990s. The main driver of the decrease in simulated ATNL is a reduction in mineral fertilizer use, which has decreased the N surplus in the soils. The TN leached fraction, however, did not show a trend but did have high annual variability due to variations in runoff; this corresponds to an average of 14.4% of the TN applied. The ATNL was considerably higher in the Archipelago Sea catchment compared to other Finnish Baltic Sea sub-catchments, with the lowest ATNL found in the Vuoksi catchment in Eastern Finland. The highest decrease of ATNL was simulated for Vuoksi and Gulf of Finland catchments. In the Bothnian Sea, Bothnian Bay and Archipelago Sea catchments, the decreasing trend of ATNL was smaller but still significant, with the exception of the Quark catchment, where there was no significant change. The differences in decreasing trends between regions can be explained by the heterogeneity of catchment characteristics, hydrology and agricultural practices in different regions.

Enhanced nitrogen removal performance of microorganisms to low C/N ratio black-odorous water by coupling with iron-carbon micro-electrolysis

Black-odorous water was caused by urbanization, industrial sewage and domestic sewage. Black-odorous water with low carbon nitrogen ratio (C/N) may inhibit the activity of microorganisms due to the lack of C source during microbial treatment. In this study, iron-carbon micro-electrolysis (ICME) was used to couple nitrifying and denitrifying bacteria. The external factors affecting microbial activity were optimized through experiments, and the effect of low C/N wastewater was studied. The optimal external factors were temperature of 30 ℃, C/N of 3, DO of 3 mg/L and iron-carbon dosage of 150 g/L. Coupled with ICME, it was found that the removal rate of nitrate nitrogen in water increased by 50.74 %, indicating that ICME could make up for the deficiency of microbial C source and improve the removal rate of nitrate nitrogen in low C/N black-odorous water. The improvement of removal rate had much to do with ICME's role as electron donor to provide electrons for denitrification. In addition, the performance of wastewater treatment by this process is tested. The maximum load of ammonia nitrogen, COD, nitrate and total nitrogen was increased to 0.1928, 1.7732, 0.2691 and 0.5181 kg/(m3·d), respectively. High-throughput sequencing results showed that the species of dominant plant population changed after ICME coupling, but the abundance increased significantly, which further indicated that ICME played a synergic role in microbial nitrogen removal, which was related to the release of trace iron by ICME. This study can provide some guidance for the treatment of black-odorous water with high ammonia nitrogen and low C/N.

Monitoring, Modeling and Planning Best Management Practices (BMPs) in the Atwood and Tappan Lake Watersheds with Stakeholders Engagements

This study was conducted in the Atwood and Tappan Lakes watersheds of the Tuscarawas basin of Ohio. The flow, total nitrogen (TN), and total phosphorus (TP) loadings were monitored with the help of local stakeholders for a few years at various locations of the watershed to develop the Soil and Water Assessment Tool (SWAT). The multi-site SWAT model calibration and validation were accomplished with a reasonable model performance. In the next step, the scenario analysis was conducted in the SWAT model using various BMPs, including vegetative filter strips, grass waterways, fertilizer reduction, crop rotation, and cover crops to evaluate their performance in reducing TN and TP from the watershed. While BMPS in many studies are decided based on researchers’ intuition, these BMPs were selected based on active consultation with the local stakeholders, who were engaged in the reduction of TN and TP loadings from the watersheds. Since the SWAT model calibration for TN and TP was not as good as the hydrologic model calibration, various scenarios of TN and TP reduction using BMPs were investigated for several years using both calibrated and uncalibrated SWAT models. We examined all the BMPs in 12 sub-watersheds of the Atwood and 10 sub-watersheds of the Tappan Lake watershed. The analysis indicated that the management practices of cover crops (rye) in combination with grass waterways with a 10% fertilizer reduction could minimize the TN and TP loading by as much as 88%, without significantly compromising the agricultural yield. However, a 10% fertilizer reduction without any BMPs could reduce TN and TP by just 9%. The cover crop (rye) including 10% fertilizer reduction with grass waterways seemed to be the most effective in reducing TN and TP, whereas the implementation of a filter strip led to a 70% reduction and was the next effective BMPs in reducing TN and TP loadings. In general, TN losses were reduced by 8% to 53%, while TP losses were reduced by 7% to 88%, depending on the BMPs used. By and large, the TN and TP reduction achieved through the calibrated model was not significantly different from the uncalibrated model, even though the reduction using the calibrated model was slightly higher for all scenarios than that of the uncalibrated model. The TN and TP loadings were highly sensitive to cattle grazing. When just 50% of the cattle were permitted to graze, the model predicted that there would be a 40% increase in total nitrogen and a 70% increase in total phosphorus in both watersheds. Our investigation revealed that monitoring the watershed at a small sub-watershed scale and calibrating the SWAT model for nitrogen and phosphorus is delicate.

Reduction of pollution loads based on sewage treatment projects around a lake: A case study on Erhai Lake.

Erhai Lake is a highland freshwater lake in Dali, China. Rapid tourism development has generated large amounts of pollutants. Since 2015, six wastewater treatment plants (WWTPs) have been built to treat wastewater collected through sewage interception projects. In this study, reductions in the pollution load of wastewater from different sources were evaluated by considering the effects of groundwater leakage, microbial degradation, and rainfall-runoff. The results showed that the systems reduced the chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) loads by 85, 83, and 85%, respectively. Discharge wastewater has the characteristics of a low concentration of domestic sewage discharge, large tourist sewage discharge, and high concentration of livestock wastewater. Due to the high groundwater level, there is groundwater infiltration in the influent water of WWTPs, which dilutes the pollutant concentration of the influent and, therefore, results in a lower treatment efficiency. Further treatment of tailwater also reduced the pollution loads discharged into the lake as well as the COD and TN by 10.25 and 22.90%, respectively. The results indicate that groundwater infiltration in the sewer network system is the primary target to be addressed in future developments.

In-situ retention of nitrogen, phosphorus in agricultural drainage and soil nutrients by biochar at different temperatures and the effects on soil microbial response

This study conducted a two-year experiment to investigate the impacts of biochar with various temperatures (350 °C, 500 °C, and 650 °C), on the reduction of pollutants in agricultural runoff and the enhancement of soil fertility. The results showed that the biochar significantly reduced the concentrations of total nitrogen and total phosphorus in farmland runoff. Moreover, higher-temperature biochar demonstrated greater efficacy in decreasing pollutants in farmland drainage. Treatment with RB650 resulted in a reduction of the total nitrogen and total phosphorus output load by 29.31–30.67 % and 21.92–25.21 %, respectively, compared to RB350. Furthermore, biochar exhibited substantial enhancements in soil fertility. This was supported by heightened soil organic matter content, increased availability of nutrients, and a noteworthy (P < 0.05) upsurge in pH, organic matter, total nitrogen, and total phosphorus content observed in the second year following the application of biochar. Biochar has the potential to enhance soil enzyme activity and affect microbial community composition, thereby facilitating nutrient cycling. The findings illustrated the regenerative and recyclable characteristics of biochar's adsorption activity throughout crop growth. This process enables sustained improvement in soil nutrient retention capacity and fertility. Thus, it emphasizes the potential of biochar as an in-situ model for nutrient retention and recycling, offering an effective approach to mitigate agricultural non-point source (NPS) pollution and enhance soil fertility.

Stressors affecting the ecological status of temporary rivers in the Mediterranean region

Temporary rivers are widespread in the Mediterranean region and impose a challenge for the implementation of the Water Framework Directive (WFD) and other environmental regulations. Surprisingly, an overarching analysis of their ecological status and the stressors affecting them is yet missing. We compiled data on the ecological status of 1504 temporary rivers in seven European Mediterranean region countries and related their ecological status (1) to publicly available data on pressures from the European WISE-WFD dataset, and (2) to seven more specific stressors modelled on a sub-catchment scale. More than 50 % of the temporary water bodies in the Mediterranean countries reached good or even high ecological status. In general, status classes derived from phytobenthos and macrophyte assessment were higher than those derived from the assessment of benthic invertebrates or fish. Of the more generally defined pressures reported to the WISE-WFD database, the most relevant for temporary rivers were ‘diffuse agricultural’ and ‘point urban waste water’. Of the modelled more specific stressors, agricultural land use best explained overall ecological status, followed by total nitrogen load, and urban land use, while toxic substances, total phosphorus load and hydrological stressors were less relevant. However, stressors differed in relevance, with total nitrogen being most important for macrophytes, and agricultural land use for phytobenthos, benthic invertebrates and fish. For macrophytes, ecological quality increased with stressor intensity. The results underline the overarching effect of land use intensity for the ecological status of temporary water bodies. However, assessment results do not sufficiently reflect hydrological stress, most likely as the biological indicators used to evaluate these systems were designed for perennial water bodies and thus mainly target land use and nutrient impacts. We conclude that biomonitoring systems need to be updated or newly developed to better account for the specific situation of temporary water bodies.

Nitrogen mass balance and uptake velocity for eutrophic reservoirs in the Brazilian semiarid region.

The nitrogen (N) cycle from the catchment to the downstream reservoir is complex, particularly the quantification of N losses. However, in order to assess the nitrogen impact in a reservoir ecosystem, simplified models may be applicable regarding the TN load production and the magnitude of lake TN removal. This study presented a methodology to perform and validate a TN mass balance to further calibrate a simplified coefficient for TN losses (vf.) in 29 tropical semiarid reservoirs. The study reservoirs were highly productive ecosystems with an average total nitrogen (TN) concentration, accounting for all measurements in all reservoirs, ranging from 0.59 to 3.84 mg L-1. Regarding the production of TN load, the median values ranged from 4.35 to 2,499.43 t year-1 with median of 80.34 t year-1. The TN loads were estimated through an annual mass balance over a 24-year period. The median of the estimates was compared with reference values obtained by using the export modelling coefficient. The correlation between the median estimated and reference loads resulted in satisfactory agreement (r2 0.88) and reinforced the reliability of the mass balance alternative. From the validated TN loads, the TN uptake velocity (vf) was estimated for all reservoirs (44.9 ± 20.1 m year-1) and could be described as a general function of the water residence time. The reservoirs of the study region have demonstrated higher vf than temperate lakes and reservoirs and similar vf with Latin America/Caribbean ones. As expected, reservoirs of warmer climates tend to present intensified N loss processes compared to lakes and reservoirs of temperate regions. The methodology proposed in the present study can be used to potentially improve water quality management in tropical semiarid reservoirs.