Effect of national COVID-19 control measures on water quality in China using an improved dual difference-in-differences method

The impact of COVID-19 on the natural environment, particularly the aquatic environment, is a major concern. However, there is a lack of national-level research on the impact of COVID-19 on the aquatic environment. This study analysed the impacts of COVID-19 on water quality using an improved dual spatial-temporal Difference-in-Differences Method (DDiD) models in cities with and without formal lockdown policies all over the China. The study found that the effectiveness of national COVID-19 control measures varied spatially for different types of pollutants. The lockdown measure resulted in a 0.052-point decline in TN concentration and a 0.056-point decline in CODMn concentration, while the non-formal lockdown measures led to a 0.2-point increase in TN concentration and a 0.016-point decline in CODMn. Spatial-temporal analysis revealed that COVID-19 and control measures immediately reduced TN concentration. The decrease effect of non-formal lockdown measures was lower than that of the lockdown measure and caused an opposite overall effect of TN concentration change. The impact of COVID-19 and control measures on CODMn concentration had a time lag effect, as it maintained or slightly increased local CODMn concentration once the lockdown measure was implemented, and then decreased after 4 weeks. Heterogeneity analysis showed that the reduction effect on TN was more significant in cities that rely more on industrial activities, while the increase effect on CODMn was more significant in cities that rely more on general and domestic services industries, revealing the diversity of water pollution sources. Overall, the impact of the COVID-19 pandemic on water quality is not as significant as that on the atmospheric environment due to the diversity of pollution sources. The study partially proves that China has achieved excellent point source control for water pollution, but there is little potential reduction of water pollution at point sources, especially industrial sources. The best way to improve the aquatic environment is still to enhance its capacity by increasing water flow and storage ability and reducing the amount of discharge from every pollution source by improving production levels and pollution treatment ratios, especially for nonpoint sources.

Spatial responses of water quality to river density and connectivity alterations on the Taihu Plain

With the advancement of urbanization, the structure and connectivity of river networks have been changed by the interference of human activities, resulting in a series of water environment problems. Numerous studies have indicated that river networks are associated with water quality; unfortunately, few studies have revealed the contributions of the structure and connectivity of river networks to variations in water quality. Taking one water conservancy region with dense and braided rivers on the Taihu Plain as an example, we depicted the spatial aggregations of water quality using the Getis-Ord Gi* index, quantified the variations in polluted regions using the standard deviational ellipse method, and quantified the influence of river density and connectivity on water quality during the different seasons. The results showed that (1) the water quality during the flood season was better than that during the non-flood season, especially in the western region; (2) the spatial aggregations of most water quality indicators were higher and the polluted regions increased in size during the flood period compared to the non-flood period; and (3) the relative contribution rates of the river density and connectivity exhibited mean values of 62.5% (61.2%) and 37.5% (38.8%) in the flood (non-flood) period. Our results provide theoretical support for enhancing water environment management.