Modeling nutrient release in the Tai Lake basin of China: source identification and policy implications

Occurrence and driving forces of different nitrogen forms in the sediments of the grass and algae-type zones of Taihu Lake

Excessive nitrogen (N) load in sediments is at risk of release resulting in the degradation of grass-type lake ecosystems. At present, the occurrence characteristics of N forms and the driving forces of organic N (ON) hydrolysis in the sediments of Taihu Lake were still unclear. Here, 52 sampling sites in 7 lake areas in Taihu Lake were investigated to compare the spatial occurrence characteristics of the sedimentary free N (FN), exchangeable N (EN), acid hydrolyzable N (HN), and residual N (RN) and their associated driving forces. The results showed that the total N contents in the dry sediment ranged from 1811.56 to 5594.06 mg kg^-1, and the contribution was in the order of RN > HN > EN > FN. Spatially, RN and total organic carbon were significantly consistently influenced by dam construction and deposition algal residue. The HN concentration was high in the estuaries affected by N inputs from the rivers. The coupling relationship of spatial distribution between ON and N forms was revealed. The factors, i.e., algal residue deposition and terrigenous N inputs, were considered as the main driving forces stimulating the ON hydrolysis in the algae-type lake zones. It can be deduced that controlling terrigenous N inputs and sediment suspension may be the key to inhibiting the transformation from grass-type to algae-type lake ecosystem.

An Analytical Review of Different Approaches to Wastewater Discharge Standards with Particular Emphasis on Nutrients

Despite the implementation of strict legal standards concerning nutrient loads within wastewater discharges in all European Union (EU) Member States it was not possible to achieve good ecological and chemical water status by 2015 in all EU countries. The main reasons for this situation are the imperfections of the legislation tools regarding the standardization of wastewater quality and the methodology of determining the conditions for wastewater introduction into receivers. The study aims to review and analyze the currently existing in various countries legal regulations setting the standards for wastewater discharged into receivers, which were intended for surface water protection and eutrophication mitigation. Besides the EU effluent standards, the regional and national regulations in chosen EU Member States (e.g., Germany, Sweden, and Denmark) have been reviewed. Moreover, the Helsinki Commission recommendations for signatory countries within the Baltic Sea catchment and the approaches for wastewater quality standardization in non-EU countries (e.g., Russia, Belarus, Switzerland, China, USA, Canada, and Dubai) were assessed. The analysis of the reviewed legal regulations allowed to diversify the methodological approaches for setting effluent quality standards in different regions and countries and to assess the effectiveness of existing legal tools in the field of eutrophication mitigation with the consideration of the environmental and economic reasonability. The results suggest that the receiver-oriented policies used among others in Switzerland and China are the most reasonable in terms of eutrophication mitigation.

Application of Export Coefficient Model and QUAL2K for Water Environmental Management in a Rural Watershed

Water quality deterioration caused by excessive nutrient discharge from various point and non-point sources are a global challenge. Understanding the pollution sources and their respective contribution is the prerequisite for environmental planning, management and restoration. In this study, the influence of complex pollution sources on the water quality of the Dengsha River watershed in Dalian, China, was investigated. The export coefficient method was coupled with the QUAL2K water quality model to estimate the loads of ammonia nitrogen (NH4-N) and total phosphorus (TP) from different sources, and to explore their respective contributions. Results indicated that animal feedlot and crop production were major sources for NH4-N load, and crop production, soil erosion and animal feedlot are the largest three sources of TP load with an annual total contribution of 98.4%. The pollutant load exhibited an intra-annual variation mainly due to the seasonality of rainfall and anthropogenic agricultural activities. The overall waste assimilation capacity (WAC) is overloaded and suggestions for water pollution control and treatment regarding each pollution source were proposed. This study addressed a new application of QUAL2K model coupled with the export coefficient model for watershed managers towards a sustainable water environmental management, and can therefore be a reference example for other small and medium-sized rural watersheds.

Excess nutrient loads to Lake Taihu: Opportunities for nutrient reduction

Intensive agriculture and rapid urbanization have increased nutrient inputs to Lake Taihu in recent decades. This resulted in eutrophication. We aim to better understand the sources of river export of total dissolved nitrogen (TDN) and phosphorus (TDP) to Lake Taihu in relation to critical nutrient loads. We implemented the MARINA-Lake (Model to Assess River Inputs of Nutrients to seAs) model for Lake Taihu. The MARINA-Lake model quantifies river export of dissolved inorganic and organic N and P to the lake by source from sub-basins. Results from the PCLake model are used to identify to what extent river export of nutrients exceeds critical loads. We calculate that rivers exported 61 kton of TDN and 2 kton of TDP to Lake Taihu in 2012. More than half of these nutrients were from human activities (e.g., agriculture, urbanization) in Sub-basins I (north) and IV (south). Most of the nutrients were in dissolved inorganic forms. Diffuse sources contributed 90% to river export of TDN with a relatively large share of synthetic fertilizers. Point sources contributed 52% to river export of TDP with a relatively large share of sewage systems. The relative shares of diffuse and point sources varied greatly among nutrient forms and sub-basins. To meet critical loads, river export of TDN and TDP needs to be reduced by 46-92%, depending on the desired level of chlorophyll-a. There are different opportunities to meet the critical loads. Reducing N inputs from synthetic fertilizers and P from sewage systems may be sufficient to meet the least strict critical loads. A combination of reductions in diffuse and point sources is needed to meet the most strict critical loads. Combining improved nutrient use efficiencies and best available technologies in wastewater treatment may be an effective opportunity. Our study can support the formulation of effective solutions for lake restoration.

An improved export coefficient model to estimate non-point source phosphorus pollution risks under complex precipitation and terrain conditions

To control non-point source (NPS) pollution, it is important to estimate NPS pollution exports and identify sources of pollution. Precipitation and terrain have large impacts on the export and transport of NPS pollutants. We established an improved export coefficient model (IECM) to estimate the amount of agricultural and rural NPS total phosphorus (TP) exported from the Luanhe River Basin (LRB) in northern China. The TP concentrations of rivers from 35 selected catchments in the LRB were used to test the model's explanation capacity and accuracy. The simulation results showed that, in 2013, the average TP export was 57.20 t at the catchment scale. The mean TP export intensity in the LRB was 289.40 kg/km², which was much higher than those of other basins in China. In the LRB topographic regions, the TP export intensity was the highest in the south Yanshan Mountains and was followed by the plain area, the north Yanshan Mountains, and the Bashang Plateau. Among the three pollution categories, the contribution ratios to TP export were, from high to low, the rural population (59.44%), livestock husbandry (22.24%), and land-use types (18.32%). Among all ten pollution sources, the contribution ratios from the rural population (59.44%), pigs (14.40%), and arable land (10.52%) ranked as the top three sources. This study provides information that decision makers and planners can use to develop sustainable measures for the prevention and control of NPS pollution in semi-arid regions.

The cycle of nitrogen in river systems: sources, transformation, and flux

Nitrogen is a requisite and highly demanded element for living organisms on Earth. However, increasing human activities have greatly altered the global nitrogen cycle, especially in rivers and streams, resulting in eutrophication, formation of hypoxic zones, and increased production of N2O, a powerful greenhouse gas. This review focuses on three aspects of the nitrogen cycle in streams and rivers. We firstly introduce the distributions and concentrations of nitrogen compounds in streams and rivers as well as the techniques for tracing the sources of nitrogen pollution. Secondly, the overall picture of nitrogen transformations in rivers and streams conducted by organisms is described, especially focusing on the roles of suspended particle-water surfaces in overlying water, sediment-water interfaces, and riparian zones in the nitrogen cycle of streams and rivers. The coupling of nitrogen and other element (C, S, and Fe) cycles in streams and rivers is also briefly covered. Finally, we analyze the nitrogen budget of river systems as well as nitrogen loss as N2O and N2 through the fluvial network and give a summary of the effects and consequences of human activities and climate change on the riverine nitrogen cycle. In addition, future directions for the research on the nitrogen cycle in river systems are outlined.

Identifying external nutrient reduction requirements and potential in the hypereutrophic Lake Taihu Basin, China

Reducing external nutrient loads is the first step for controlling eutrophication. Here, we identified external nutrient reduction requirements and potential of strategies for achieving reductions to remediate a eutrophic water body, Lake Taihu, China. A mass balance approach based on the entire lake was used to identify nutrient reduction requirements; an empirical export coefficient approach was introduced to estimate the nutrient reduction potential of the overall program on integrated regulation of Taihu Lake Basin (hereafter referred to as the "Guideline"). Reduction requirements included external total nitrogen (TN) and total phosphorus (TP) loads, which should be reduced by 41-55 and 25-50%, respectively, to prevent nutrient accumulation in Lake Taihu and to meet the planned water quality targets. In 2010, which is the most seriously polluted calendar year during the 2008-2014 period, the nutrient reduction requirements were estimated to be 36,819 tons of N and 2442 tons of P, and the potential nutrient reduction strategies would reduce approximately 25,821 tons of N and 3024 tons of P. Since there is a net N remaining in the reduction requirements, it should be the focus and deserves more attention in identifying external nutrient reduction strategies. Moreover, abatement measures outlined in the Guideline with high P reduction potential required large monetary investments. Achieving TP reduction requirement using the cost-effective strategy costs about 80.24 million USD. The design of nutrient reduction strategies should be enacted according to regional and sectoral differences and the cost-effectiveness of abatement measures.

Spatiotemporal patterns and source attribution of nitrogen pollution in a typical headwater agricultural watershed in Southeastern China

Excessive nitrogen (N) discharge from agriculture causes widespread problems in aquatic ecosystems. Knowledge of spatiotemporal patterns and source attribution of N pollution is critical for nutrient management programs but is poorly studied in headwaters with various small water bodies and mini-point pollution sources. Taking a typical small watershed in the low mountains of Southeastern China as an example, N pollution and source attribution were studied for a multipond system around a village using the Hydrological Simulation Program-Fortran (HSPF) model. The results exhibited distinctive spatio-seasonal variations with an overall seriousness rank for the three indicators: total nitrogen (TN) > nitrate/nitrite nitrogen (NO_x^--N) > ammonia nitrogen (NH₃-N), according to the Chinese Surface Water Quality Standard. TN pollution was severe for the entire watershed, while NO_x^--N pollution was significant for ponds and ditches far from the village, and the NH₃-N concentrations were acceptable except for the ponds near the village in summer. Although food and cash crop production accounted for the largest source of N loads, we discovered that mini-point pollution sources, including animal feeding operations, rural residential sewage, and waste, together contributed as high as 47% of the TN and NH₃-N loads in ponds and ditches. So, apart from eco-fertilizer programs and concentrated animal feeding operations, the importance of environmental awareness building for resource management is highlighted for small farmers in headwater agricultural watersheds. As a first attempt to incorporate multipond systems into the process-based modeling of nonpoint source (NPS) pollution, this work can inform other hydro-environmental studies on scattered and small water bodies. The results are also useful to water quality improvement for entire river basins.

Scenario analysis of the impacts of socioeconomic development on phosphorous export and loading from the Dongting Lake watershed, China

Socioeconomic development in lake watersheds is closely related with lake nutrient pollution. As the second largest freshwater lake in China, the Dongting Lake has been experiencing an increase in nutrient loading and a growing risk of eutrophication. This study aimed to reveal the likely impacts of the socioeconomic development of the Dongting Lake watershed on the phosphorous pollution in the lake. We estimated the contributions from different sources and sub-watersheds to the total phosphorous (TP) export and loading from the Dongting Lake watershed under two most likely socioeconomic development scenarios. Moreover, we predicted the likely permissible and actual TP loadings to the Dongting Lake. Under both two scenarios, three secondary sub-watersheds-the upper and lower reaches of the Xiang River watershed and the Dongting Lake Area-are expected to dominate the contribution to the TP export from the Dongting Lake watershed in 2020. Three primary sub-watersheds-the Dongting Lake Area, the Xiang River, and the Yuan River watersheds-are predicted to be the major contributors to the TP loading from the entire watershed. The two scenarios are expected to have a slight difference in TP export and lake TP loading. Livestock husbandry is expected to be the predominant anthropogenic TP source in each of the sub-watersheds under both scenarios. Compared to 2010, permissible TP loading is not expected to increase but actual TP loading is predicted to grow significantly in 2020. Our study provides methodologies to identify the key sources and regions of lake nutrient loading from watersheds with complex socioeconomic context, and to reveal the potential influences of socioeconomic development on nutrient pollution in lake watersheds.

Sources, distribution and export coefficient of phosphorus in lowland polders of Lake Taihu Basin, China

Identifying phosphorus (P) sources, distribution and export from lowland polders is important for P pollution management, however, is challenging due to the high complexity of hydrological and P transport processes in lowland areas. In this study, the spatial pattern and temporal dynamics of P export coefficient (PEC) from all the 2539 polders in Lake Taihu Basin, China were estimated using a coupled P model for describing P dynamics in a polder system. The estimated amount of P export from polders in Lake Taihu Basin during 2013 was 1916.2 t/yr, with a spatially-averaged PEC of 1.8 kg/ha/yr. PEC had peak values (more than 4.0 kg/ha/yr) in the polders near/within the large cities, and was high during the rice-cropping season. Sensitivity analysis based on the coupled P model revealed that the sensitive factors controlling the PEC varied spatially and changed through time. Precipitation and air temperature were the most sensitive factors controlling PEC. Culvert controlling and fertilization were sensitive factors controlling PEC during some periods. This study demonstrated an estimation of PEC from 2539 polders in Lake Taihu Basin, and an identification of sensitive environmental factors affecting PEC. The investigation of polder P export in a watershed scale is helpful for water managers to learn the distribution of P sources, to identify key P sources, and thus to achieve best management practice in controlling P export from lowland areas.

Modelling of the estimated contributions of different sub-watersheds and sources to phosphorous export and loading from the Dongting Lake watershed, China

Considerable growth in the economy and population of the Dongting Lake watershed in Southern China has increased phosphorus loading to the lake and resulted in a growing risk of lake eutrophication. This study aimed to reveal the spatial pattern and sources of phosphorus export and loading from the watershed. We applied an export coefficient model and the Dillon-Rigler model to quantify contributions of different sub-watersheds and sources to the total phosphorus (TP) export and loading in 2010. Together, the upper and lower reaches of the Xiang River watershed and the Dongting Lake Area contributed 60.9% of the TP exported from the entire watershed. Livestock husbandry appeared to be the largest anthropogenic source of TP, contributing more than 50% of the TP exported from each secondary sub-watersheds. The actual TP loading to the lake in 2010 was 62.9% more than the permissible annual TP loading for compliance with the Class III water quality standard for lakes. Three primary sub-watersheds-the Dongting Lake Area, the Xiang River, and the Yuan River watersheds-contributed 91.2% of the total TP loading. As the largest contributor among all sources, livestock husbandry contributed nearly 50% of the TP loading from the Dongting Lake Area and more than 60% from each of the other primary sub-watersheds. This study provides a methodology to identify the key sources and locations of TP export and loading in large lake watersheds. The study can provide a reference for the decision-making for controlling P pollution in the Dongting Lake watershed.

Spatial Regression and Prediction of Water Quality in a Watershed with Complex Pollution Sources

Fast economic development, burgeoning population growth, and rapid urbanization have led to complex pollution sources contributing to water quality deterioration simultaneously in many developing countries including China. This paper explored the use of spatial regression to evaluate the impacts of watershed characteristics on ambient total nitrogen (TN) concentration in a heavily polluted watershed and make predictions across the region. Regression results have confirmed the substantial impact on TN concentration by a variety of point and non-point pollution sources. In addition, spatial regression has yielded better performance than ordinary regression in predicting TN concentrations. Due to its best performance in cross-validation, the river distance based spatial regression model was used to predict TN concentrations across the watershed. The prediction results have revealed a distinct pattern in the spatial distribution of TN concentrations and identified three critical sub-regions in priority for reducing TN loads. Our study results have indicated that spatial regression could potentially serve as an effective tool to facilitate water pollution control in watersheds under diverse physical and socio-economical conditions.

Sulfate radical-based oxidation of fluoroquinolone antibiotics: Kinetics, mechanisms and effects of natural water matrices

Widespread occurrence of fluoroquinolone antibiotics (FQs) in surface water, groundwater, soil and sediment has been reported and their remediation is essentially needed. Sulfate radical (SO₄^-) based advanced oxidation processes (SR-AOPs) are promising technologies for soil and groundwater remediation. In this study, the degradation kinetics, mechanisms, and effects of natural water matrices on heat-activated persulfate (PS) oxidation of FQs were systematically investigated. Experimental results clearly demonstrated that 92% of CIP was removed within 180 min (pH = 7, 60 °C). Higher temperature and lower pH facilitated the degradation of ciprofloxacin (CIP). The piperazine moiety of CIP was identified as the reactive site for SO₄^- attack by comparison with substructural analogs, flumequine (FLU) and 1-(2-fluorophenyl) piperazine (FPP). A comparison of the degradation of CIP, norfloxacin (NOR), enrofloxacin (ENR) and ofloxacin (OFL) confirmed that the presence of cyclopropane ring also influence the degradation of FQs. Water matrix significantly influenced the degradation of CIP and ENR, and the degradation rate followed the order of Milli-Q water (pH = 7) > groundwater > artificial seawater > artificial surface water > lake water. Degradation products of CIP in different water matrix were enriched by solid phase extraction (SPE) and then analyzed by liquid chromatography-electrospray ionization-triple quadrupole mass spectrometry (LC-ESI-MS/MS). Detailed transformation pathways of CIP were proposed and were compared with respect to different water matrices. Four transformation pathways including stepwise piperazine ring oxidation, OH/F substitution, hydroxylation, and cyclopropane ring cleavage were proposed for CIP degradation. Results clearly show that the water matrix influenced the degradation of FQs appreciably, a phenomenon that should be taken into consideration when applying SR-AOPs for remediation of soil and groundwater contaminated by FQs.

Spatiotemporal patterns and source attribution of nitrogen load in a river basin with complex pollution sources

Environmental problems such as eutrophication caused by excessive nutrient discharge are global challenges. There are complex pollution sources of nitrogen (N) discharge in many river basins worldwide. Knowledge of its pollution sources and their respective load contributions is essential to developing effective N pollution control strategies. N loads from all known anthropogenic pollution sources in the Upper Huai River basin of China were simulated with the process-based SWAT (Soil and Water Assessment Tool) model. The performances of SWAT driven by daily and hourly rainfall inputs were assessed and it was found that the one driven by hourly rainfall outperformed the one driven by daily rainfall in simulating both total nitrogen (TN) and ammonia nitrogen (NH4-N) loads. The hourly SWAT model was hence used to examine the spatiotemporal patterns of TN and NH4-N loads and their source attributions. TN load exhibited significant seasonal variations with the largest in summer and the smallest in spring. Despite its declining proportion of contribution downstream, crop production remained the largest contributor of TN load followed by septic tanks, concentrated animal feedlot operations (CAFOs), municipal sewage treatment plants, industries, and scattered animal feedlot operations (SAFOs). There was much less seasonal variation in NH4-N load. CAFOs remained the largest source of NH4-N load throughout the basin, while contributions from industries and municipal sewage treatment plants were more evident downstream. Our study results suggest the need to shift the focus of N load reduction from "end-of-pipe" sewage treatment to an integrated approach emphasizing stakeholder involvement and source prevention.

Variations in source apportionments of nutrient load among seasons and hydrological years in a semi-arid watershed: GWLF model results

Quantifying source apportionments of nutrient load and their variations among seasons and hydrological years can provide useful information for watershed nutrient load reduction programs. There are large seasonal and inter-annual variations in nutrient loads and their sources in semi-arid watersheds that have a monsoon climate. The Generalized Watershed Loading Function model was used to simulate monthly nutrient loads from 2004 to 2011 in the Liu River watershed, Northern China. Model results were used to investigate nutrient load contributions from different sources, temporal variations of source apportionments and the differences in the behavior of total nitrogen (TN) and total phosphorus (TP). Examination of source apportionments for different seasons showed that point sources were the main source of TN and TP in the non-flood season, whereas contributions from diffuse sources, such as rural runoff, soil erosion, and urban areas, were much higher in the flood season. Furthermore, results for three typical hydrological years showed that the contribution ratios of nutrient loads from point sources increased as streamflow decreased, while contribution ratios from rural runoff and urban area increased as streamflow increased. Further, there were significant differences between TN and TP sources on different time scales. Our findings suggest that priority actions and management measures should be changed for different time periods and hydrological conditions, and that different strategies should be used to reduce loads of nitrogen and phosphorus effectively.