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

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 COD_Mn concentration, while the non-formal lockdown measures led to a 0.2-point increase in TN concentration and a 0.016-point decline in COD_Mn. 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 COD_Mn concentration had a time lag effect, as it maintained or slightly increased local COD_Mn 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 COD_Mn 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.