The atrazine concentration in surface water reflects the regional discharge level. Therefore, we collected 180 sets of atrazine in surface water from the relevant literature worldwide (Table S4). The global average level of atrazine in surface water was 430 ng/L. The atrazine concentrations in different countries varied by up to three orders of magnitude, with developed crop-farming countries, such as the United States, Australia, and China, exerting high concentrations (Fig. 5A). In addition to crop farming, sugarcane is an atrazine-consuming source, and countries, including Brazil and Myanmar, have harmful atrazine concentrations in surface water. In the subregions, the highest atrazine level, 552 ng/L, was found in the Asia-Pacific region, followed by North America and Latin America (Fig. 5B). Notably, the regions with the largest number of studies were typically the areas most polluted with atrazine. Europe banned atrazine application in the 1990s, and its atrazine levels in surface waters have decreased17. The average concentration of atrazine in the surface waters of China is 535 ng/L, which is higher than the global average.
Atrazine exposure in surface waters is closely related to the use of herbicides. Global herbicide usage reached 1 million tons in 2004, increased by 1.2-fold from 1990 to 201229, and leveled off between 1.37 and 1.43 million tons (Fig. 5C). Notably, agriculture in China is a crucial part of the global agricultural industry. Herbicide use in China increased to 141,244 tons in 2014, an increase of 135% from 1990. In 2015, to mitigate the risk of diffuse pollution, the Chinese government launched a zero-growth strategy for pesticides14. This policy led to a staged decline in pesticide use, especially the 10% decrease in 2017–2018 (Fig. 5D). The YRW in China also attained this goal: with a 13.6% decline in herbicide use in 2018–2019. The application of the zero-growth policy effectively reduced herbicide usage, mitigating atrazine levels in surface water. The dataset for China in the literature was divided into two groups for comparison: before (n = 18) and after (n = 11) implementation of the zero-growth policy (Figure S3). The average atrazine level before the implementation of the policy was 259 ng/L, which decreased to 105 ng/L thereafter. As well as in the YRW from 2014 to 2019, the average concentration decreased from 154.8 ng/L to 99.1 ng/L15.
Spatiotemporal variations and driving factors of atrazine were abundant in small agricultural watersheds. In small watersheds, atrazine discharge levels are affected by the atrazine application intensity, precipitation, soil type, and slope, the main factors considered in determining the ratio of atrazine to total nitrogen loss7. Atrazine concentrations were the highest during the first few runoff events after application and decreased rapidly thereafter, with a 78% loss within 1 month of application30. However, atrazine showed excellent persistence and can be detected in the aqueous environment 20 years after concentration decay17. Regarding spatial distribution, herbicide concentrations in tributaries with intensive agricultural activities are usually higher than those in the mainstreams31,32. The average tributary concentration in this study was 9.1 ng/L, which is significantly higher than the average concentration of 5.8 ng/L in the mainstream. However, when the scope expands to the large watershed, the discharge loads of atrazine are mainly obscured by high spatiotemporal heterogeneity33. The pesticide discharge areas are mainly located in intensive agricultural areas. In contrast with the tributary confluence in traditional irrigation areas, dispersing into an irrigation network from the mainstream in gravity-fed irrigation areas shortens the transport pathway. The dispersed channel form evolves into a vast irrigated farmland area; thus, atrazine can be directly discharged to the mainstream34. Per our field observations, there was a 1.4-fold increase in atrazine concentration as the Yellow River water flowed through the agricultural farming area. This finding suggests that agricultural structures with short migration distance have a great risk of pesticide diffuse pollution.
Chemical loads, products of water discharge, and chemical concentrations are good indicators of chemical loss over time35. The modified export coefficient method provides reliable discharge loads for atrazine in watersheds11. The spatial discharge loads of atrazine were generally consistent with the concentration patterns. The two major parameters of the export coefficient method, λ and E, reflect the dynamics of precipitation and herbicide application intensity, respectively. Before the implementation of the pesticide reduction policy, precipitation controlled the interannual discharge load of atrazine within the watershed. During this period, the interannual variation in precipitation was the most sensitive factor (Fig. 4). The discharge load of atrazine was greater in wet years than that in dry years36. Typically, increased precipitation enhances the frequency of runoff events during wet years and farmland soil erosion, promoting atrazine discharge loads in large vulnerable watersheds. In addition, increasingly wet environments in a large watershed and on a global scale are accompanied by increasing precipitation variability1; thus, instances of drought and flood increase profoundly affect pesticide migration behavior and environmental risks at the watershed scale and deteriorate the process of the pesticide reduction policy.
Some anthropogenic efforts have mitigated diffuse pollution through regulatory management16. Long-term water quality monitoring of the Yellow River showed that agricultural chemical reduction made the largest contribution to improved water quality when considering precipitation, runoff, evaporation, reservoir storage, and anthropogenic discharge. Mechanistic evidence through Fourier spectra and wavelet analysis showed that anthropogenic activities exceeded most natural biogeochemical processes at all spatial scales of the watershed. Systematic changes in the spectral properties of water flow have been associated with anthropogenic activities37. Since the government launched a pesticide reduction policy in 2015, increasing pesticide efficiency and reducing pesticide usage have mitigated the intensity of diffuse pollution in the YRW region. Projections based on the current regulations suggest that chemical pesticides, including atrazine, will be completely replaced by biopesticides by 205012, and the diffuse pollution from chemical pesticides will be gradually eliminated. Although unusual increases in precipitation affect human efforts to optimize environmental pollution issues, we are confident in the on-going environmental optimization policies.
Notably, this study has several limitations. First of all, the spatial heterogeneity of atrazine application data affects the accuracy of the estimation38. The mismatch in spatial resolution between the atrazine application database and the watershed's inflow coefficient increased the uncertainty of the estimation results. We acknowledge that the estimated atrazine discharge loads values remained significant uncertainty; however, this uncertainty did not affect the main points in this study, as the spatial resolution's uncertainty did not propagate to the temporal trend variations. The development of a regular monitoring system is required to provide the basic data on the migration behavior of pollutants in the watershed. In addition, the pesticide reduction policy was launched for 8 years and the available data could only trance for 5 year, which needs continue to follow up the policy impact. Although this study is a regional simulation, the changes in precipitation caused by climate change are a global issue that affects us all1. Reducing the use of conventional chemical pesticides is also an inevitable path for sustainable agriculture12,39. Exploring the process of environmental changes under the dual influence of climate change and human policies is beneficial for adjusting current policies and contemplating future development directions.
The pesticide reduction policy requires ongoing effort. Our comparison of the atrazine concentrations calculated from the annual atrazine loads and runoff discharge in the watershed demonstrated that the average atrazine concentration from 2014 to 2017 exceeded the surface water atrazine concentration limit of 3 µg/L (Figure S4). Excess atrazine concentration can damage the ecological environment, especially for algae, fish, and frogs18. Because of the spatial variability in the entire watershed and the high exposure to atrazine in agricultural areas32, we speculated that atrazine levels would exceed the surface water concentration limits in intensively cultivated areas during the first few runoff events after its application40. Therefore, the current water quality levels require continuously political efforts to mitigate the potential risk of atrazine. Current estimations of the atrazine discharge load are useful for understanding the dynamic patterns of pesticide diffuse pollution in the dual effects of climate change and policy regulations and for providing knowledge for future policy development. The scale gap in refined modeling of diffuse pollutants in watersheds needs to be bridged in the future to better support government decision-making.