In highly urbanized and industrialized areas, the demand for construction land is expanding, which should have an impact on the water environment. Taking the Yangtze River Delta (YRD) and considering chemical oxygen demand (COD) and ammonia nitrogen (NH₃-N) as characteristic pollutants, this study investigated the spatial–temporal characteristics of water pollutant emissions at the county level, optimized the spatial lag model (SLM) to estimate the spatial interaction of urban expansion and water pollutant emissions through direct and indirect effects. The results show that from 2011 to 2015, water pollutant emissions in the YRD decreased significantly and that the high-emissions pattern changed from a contiguous to a scattered distribution. The emissions of COD and NH₃-N in counties at various distances from the Yangtze River and coastline show a logarithmic curve relationship. The association between urban expansion and water pollutant emissions was significant and stable. In 2015, every 1% increase in the scale of urban expansion resulted in 0.299% and 0.340% increases in local COD and NH₃-N emissions, respectively, and emissions in the adjacent counties synchronously increased by 0.068% and 0.084%, respectively. The results show that to break the association and spatial interaction between urban expansion and water pollutant emissions and alleviate the environmental stress on the YRD, in addition to delimiting an urban expansion boundary and strictly restraining the scale of expansion, improvement in the regional environmental carrying capacity through urban water pollutant treatment facilities and pipe network construction is urgently needed.

In China, county-to-district conversion (CTDC) is a major form of administrative division adjustment (ADA) and an important element of national regime policy. In this study, the authors used the synthetic control method to analyze the economic growth effect and mechanism of the CTDC that occurred in Xiaoshan and Yuhang in 2001, and to explore the dialectical relationship between CTDC and city shrinkage. The research results are as follows. First, the economic growth following CTDC in Xiaoshan and Yuhang exhibited different patterns of variation. Economic growth in Xiaoshan increased progressively from 2002 to 2012, and the difference in annual growth of per capita GDP between the actual Xiaoshan and synthetic Xiaoshan reached 5.77%. However, the economic effect of CTDC had a time lag in Yuhang, with the growth curves of actual Yuhang and synthetic Yuhang largely coinciding in 2002–2006. Since 2007, the difference of annual growth rate was 2.2%. Second, the reduced shielding effect of administrative boundaries is the key to greater economic growth after CTDC. Thus, CTDC stimulates economic growth indirectly via urban integration planning, amplification of consumption and urban investment, and retention of financial and administrative power. Third, for Hangzhou, a city with strong development, CTDC will not cause economic recession. However, it is not suitable to add municipal districts to shrinking cities by CTDC, because it has the risk of aggravating the unbalanced urban development. Fourth, the study provides evidence for evaluating policy effects of CTDC and explains China's city shrinkage and urbanization from the perspective of ADA.