Modeling the non-point source pollution risks by combing pollutant sources, precipitation, and landscape structure

Analysis and estimation of nonpoint source pollution under different land use in Anjiagou watershed, Gansu, China

Agriculture-related nonpoint source pollution has been a worldwide issue in the past few decades. Estimating pollutant sources at the basin scale remains a challenge due to the complexity of pollutant transport mechanisms affected by land use, variable climatic condition, geological formation, and lack of long-term observation data. This study was based on the long-term plot observational data of surface runoff and water quality and used principal component analysis and other statistical analyses to analyze the variation of water quality affected by different land uses (cropland, tree forest, shrub, grassland, and wildland). The mean concentration method with the local calibrated/verified SWAT (soil and water assessment tool) model was used to quantify the load of nonpoint source (NPS) pollutants on slope areas under different land uses in the Anjiagou Watershed. Our research results determined permanganate index (COD_Mn), ammonia nitrogen (NH₃-N), total nitrogen (TN), fluoride (F^-), nitrite nitrogen (NO₂-N), total phosphorus (TP), and hexavalent chromium (Cr⁶⁺) as the significant pollutants while 5-day biochemical oxygen demand (BOD₅) was identified to be below the water quality standards of Grade V (water for agricultural and general landscape use) in the studied watershed. Pollutants were discharged through either hillslope at a total rate of 2.4 kg ha^-1, accounting for 67.6% of the total, or through waterway channels (32.4%). The pollutant concentrations were from 23.5 mg L^-1 to 37.4 mg L^-1, varying with pollutants and land uses. All examined water quality indicators exceeded the minimum safety standards defined by the regulations of the Gansu provincial government by averaging 3.5 times higher than the safety threshold. The pollutants from hillslopes exceeded water quality standards by a factor of 3.4-4.4 times compared with from the waterway channel by 1.9. Implementing soil and water conservation measures can mitigate pollutants to some extent, particularly during the process of highland runoff converging into waterways. At the watershed level, between 33 and 38% of the runoff and pollutants were discharged from croplands, between 59 and 66% from forest land, < 2% from grassland, and 1% from wildland. This study also demonstrates a simple but novel method to estimate NPS pollutants using long-term plot observations in conjunction with SWAT models, which can be used in other watersheds with similar conditions.

Identification method and application of critical load contribution areas based on river retention effect

For the entire watershed, the critical source areas (CSAs) and the critical load contribution areas (CLCAs) are two completely different concepts. The CLCAs can reflect the impact of river retention effects on pollutant transmission. In this study, an integrated modelling approach was developed for those complex watersheds by combining two models: MECM (modified export coefficient model) and SWAT (Soil and Water Assessment Tool). A case study was performed in a typical rural area-Miyun Reservoir watershed, China. The simulated results indicated that anthropogenic pollution is the main source of pollutants in most townships, including livestock breeding, rural activities, and crop cultivation. It spreads upstream with the outlet of the basin as the center, and the transport efficiency decays regularly, so the location of the pollution source is closely related to its transport efficiency. The river retention effect has a significant retardation effect on the transportation of pollutants, more than half of the pollutant load will be deposited in the river network. Generally, the CLCAs are concentrated in the area where the transport efficiency and pollutant load are relatively high, which is quite different from the spatial distribution of the CSAs. The research results fully excavated the transmission path and process of pollutants, especially the process of river migration, which helps to improve the scientific configuration of management practices.

Monitoring the Landscape Pattern and Characteristics of Non-Point Source Pollution in a Mountainous River Basin

This study aimed to assess the relationship between the landscape patterns and non-point source (NPS) pollution distribution in Qixia County, China. The sub-basin classification was conducted based on a digital elevation model and Landsat8 satellite images. Water samples were collected from each sub-basin, andtheir water quality during the wet and dry seasons was estimated. The correlation between the landscape indices and water pollution indicators was determined by Pearson analysis. The location-weighted landscape contrast index (LWLCI) was calculated based on the "source-sink" theory. Qixia was further divided into five sections based on the LWLCI score to illustrate the potential risk of NPS pollution. The results showed that the water quality in Qixia County was generally good. Cultivated land, orchards, construction areas, and unused land were positively correlated with the water pollution index and weredesignated as the "source" landscape categories, while forests, grasslands, and water bodies, which were negatively correlated with water pollution, were the "sink" landscapes; the LWCI was high in 36.94% of the study area. In these areas, measures such as increasing vegetation buffer zones are necessary to decrease the sediment and nutrient loads carried by precipitation.

Teleconnection between climate oscillations and riverine nutrient dynamics in Southeast China based on wavelet analysis

Nonpoint source (NPS) pollution is mainly driven by hydrological processes; climate oscillation can affect regional water cycle processes. However, the relationship between climate oscillation and NPS pollution is still unclear, which increases the difficulty of water quality prediction and management. In this study, Mann-Kendall test and wavelet transform were adopted to investigate the teleconnection between ENSO (El Niño-Southern Oscillation) phenomenon and riverine NPS load dynamics in an agricultural watershed of Southeast China from 2003 to 2016. Results showed that annual precipitation, streamflow, and riverine nutrient load increased significantly during the study period. The change point for long-term riverine TN load and TP load appeared in 2009 and 2007, respectively. Rainfall, streamflow, nutrient loads, and Niño 3.4 sea temperature (SST) shared a common periodicity of 10-16 months. The southern oscillation index (SOI) and Niño 3.4 SST shared a common periodicity of 28-36 months. Moreover, Niño 3.4 SST showed a positive correlation with riverine nutrient loads at a periodicity of 10-16 months, while SOI showed a weakly negative correlation with riverine nutrient loads at a periodicity of 28-36 months. These findings indicate that the increasing frequency of warm ENSO events would enhance the risk of nutrient export to rivers in Southeast China and more attention should be paid to large-scale climate oscillation in the prediction of agricultural nutrient pollution and management of water quality in agricultural watersheds.

Landscape Ecological Risk Assessment under Multiple Indicators

Rapid urbanization and intensification of human activities increases the risk of disturbance of ecological systems via multiple sources, with consequences for regional ecological security and health. Landscape ecological risk assessment (LERA) is an effective way to identify and allocate risk to resources. We used the north and south Qinling Mountain area as a case study to analyze the spatial heterogeneity of landscape ecological risk using a potential- connectedness-resilience three-dimensional (PCR 3D) framework based on an integrated and dynamic risk assessment concept from adaptive cycle theory. We explored factors driving the risks with a spatial model GeoDetector. The results show that the comprehensive landscape ecological risk was north–south polarized and dominated by low and moderate risk levels (90.13% of total risk) across the whole study area. The high-risk area was centered on the Weihe plain north of the Qinling Mountains (NQL), while low-risk areas accounted for 86.87% of the total area and were prevalent across the south of the study area. The areas with high potential and connectedness risks were centered in the Xi’an–Xianyang urban agglomeration and those with high-resilience risk were in the upper reaches of the Hanjiang River. The vast majority of the area to the south of the Qinling Mountains (SQL) is at low risk. In terms of driving forces, population density and vegetation coverage (NDVI) are the primary factors affecting landscape ecological risk. Our findings suggest that anthropogenic activity is the primary cause of landscape ecological risks in the study area and regional socioeconomic exploitation and environmental conservation need to be rebalanced to achieve sustainability for the social ecosystem. The PCR 3D LERA framework employed in this study can be used to inform landscape ecological health and security and to optimize socioeconomic progress at regional scales.

Catchment versus Riparian Buffers: Which Land Use Spatial Scales Have the Greatest Ability to Explain Water Quality Changes in a Typical Temperate Watershed?

Identifying the multi-scale spatial relationship between land use and water quality is critical for determining the priorities and key areas of river management. To more accurately identify the scale effect of land-use patterns on water quality and quantitatively distinguish the difference in the impacts of land-use composition and configuration on water quality, we used 94 sites to extract the upstream catchment and riparian buffer zone with different widths. The results showed that the ability of land use variables with different buffer widths to explain water quality differed slightly from the ability of these variables at the catchment scale, and the joint explanatory ability of land use composition and configuration was greater than that of each individually. The patch density and landscape shape index of cultivated land, shrubland, and built-up land in the buffer area close to the water bodies were the main factors for the increase in the concentration of total nitrogen, nitrate nitrogen, total phosphorus, and suspended solids. As the width of the buffer increased, the role of the percent of land use increased. Our research indicates that water quality management needs to adopt a multi-scale perspective and focus on key local areas while coordinating at a broader scale.

Identification and Regulation of Critical Source Areas of Non-Point Source Pollution in Medium and Small Watersheds Based on Source-Sink Theory

The identification and regulation of the critical source areas (CSAs) of non-point source (NPS) pollution have been proven as economical and effective ways to control such pollution in watersheds. However, the traditional models for the identification of CSAs have complex operation processes, and comprehensive systematic methods for the regulation of CSAs are still lacking. This study systematically developed a new methodological framework for the identification and regulation of CSAs in medium and small watersheds based on source-sink theory, which included the following: (1) a grid-based CSAs identification model involving the evaluation of the rationality of the source-sink landscape pattern and three geographical factors (landscape slope, relative elevation, and the distance from the river), and identifying CSAs by the calculation and division of the integrated grid pollution index (IGPI); (2) a comprehensive CSAs regulation strategy that was formulated based on three landscape levels/regulation intensities—including the optimization of the overall source-sink landscape pattern, the conversion of the landscape type or landscape combination, and local optimization for single source landscape—to meet various regulatory intensity requirements in watersheds. The Jiulong River watershed in Fujian Province of China was taken as a case study. The results indicate that: (1) the identified CSAs of the Jiulong River watershed covered 656.91 km2, equivalent to 4.44% of the watershed, and through adopting multiple-intensity regulation measures for 10 key control zones that had spatially concentrated high values of the IGPI among the CSAs, the watershed IGPIs were predicted to be generally reduced and the area of CSAs was predicted to decrease by 23.84% (31.43% in Zhangzhou, the major city in the watershed); (2) the identification model can identify the CSAs with easy data access and simple operation, and the utilization of neighborhood impact analysis makes the grid-based research more scientific in the evaluation of the rationality of the source-sink landscape pattern; (3) the application of multi-scale landscape planning framework and the principle of source-sink landscape pattern regulation make the CSAs regulation strategy systematic and cost-effective, and the provision of different intensity regulation strategies makes the regulation strategy easy to implement and relatively lower cost. The proposed methodological framework can provide technical support for governments to quickly and accurately identify the CSAs of NPS pollution and effectively control such CSAs in medium and small watersheds.

Modeling field scale nitrogen non-point source pollution (NPS) fate and transport: Influences from land management practices and climate

The use of nitrogen (N) fertilizer marked the start of modern agriculture that boosted food production to help alleviate food shortages across the globe but at the cost of severe environmental issues and critical stress to the agroecosystem. This paper was aimed at determining the fate and transport of nitrite and ammonia under future climate projections by adapting the recommended land management practices that are supposed to reduce nitrate N in surface water to state government target. To accomplish these objectives, a fully-distributed physical-based hydrologic model, MIKE SHE, and a hydrodynamic river model, MIKE 11, were coupled with MIKE ECO-Lab to simulate the fate and transport of different forms of N in the agro-ecosystem in the Upper Sangamon River Basin (USRB). Twelve (12) combinations of land management and climate projections were simulated to evaluate the N fate and transport in the USRB from 2020 to 2050. Under the current land management, the nitrate concentration in surface water was expected to exceed the EPA limit of 10 ppm up to 2.5% of the days in the simulation period. Regulating the fertilizer application rates to approximately 50% of the current rate will ensure this limit will not be exceeded in the future. Implementing cover cropping alone can potentially decrease nitrate N concentrations by 33% in surface water under dry climate and in the saturated zone under future projections. By combining the cover cropping and regulated application rate management, the nitrate N concentration in the saturated zone was expected to decrease by 67% compared with historic baseline. The modeling framework developed and used in this study can help evaluate the effectiveness of different management schemes aimed at reducing future nutrient load in our surface water and groundwater.

Discussing on "source-sink" landscape theory and phytoremediation for non-point source pollution control in China

Water pollution is exacerbated due to irrational human activities in China. Restoring and rebuilding river basin ecosystems are major ecological strategies at present. Controlling the non-point source pollution (NPSP) by reasonable management of land use in the basin and phytoremediation of contaminated waters is the optimum approach. Thus, it is significant to study on the relationship that between landscape change and the aquatic environment, as well as further to analyze on the combined effect of the landscape and water quality. This paper describes the application and development of the "source-sink" landscape theory in China, and the role of the theory in controlling NPSP. From this perspective, a landscape capable of generating NPSP would be a "source" landscape, such as farmland, while another capable of preventing NPSP would be a "sink" landscape, such as forests and wetland. Applying the source-sink landscape theory, it is possible to exert the ecological benefits of the landscape while playing the esthetic value of the landscape. Also, the purification mechanism of plants in contaminated water is discussed. Besides, it is vital that research on water body restoration should focus not only on single discipline but also on integration and coordination between various ones such as ecology, environmental science, and geography to jointly push up researches related to water body phytoremediation. Hopefully, this paper could help to control water pollution from a new perspective, also to improve water environment and benefit human lives.

Assessment and analysis of agricultural non-point source pollution loads in China: 1978-2017

China's successful agriculture development has resulted in public concerned environmental problems. However, continuous and detailed data about Chinese agricultural non-point source pollution (ANPSP) loads are lacking. To assess and analyze Chinese ANPSP loads from 1978 to 2017, an inventory analysis was performed, and a socioeconomic and spatiotemporal analysis in the scale of provinces was conducted. The results showed that the pollution loads of chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) increased by 91.0%, 196.2% and 244.1%, respectively, and their variation underwent a free development stage, reform promotion stage, market regulation stage and policy incentive stage. The results of the pollution source analysis showed that over the past 40 years, the total percent contribution to COD by livestock and poultry breeding (LPB) and rural household waste (RHW) accounted for 83.1%-96.6%, the total percent contribution to TN by mineral fertilizers (MF) and LPB accounted for 72.3%-80.8%, and the total percent contribution to TP by LPB, RHW and MF accounted for 69.1%-88.6%. In addition, Shandong, Guangdong, Sichuan, and Henan were the top producers of ANPSP loads, and their COD, TN, and TP loads accounted for approximately 32%, 30%, and 35% of the national totals, respectively. The discharge intensity of COD, TN and TP decreased by 79.2%, 67.8%, and 62.6%, respectively. The discharge intensity exhibited a phasic feature that aligned with the national economic plan in the temporal scale and was closely related to the agricultural conditions in the spatial scale.