Residential pollutant discharge and spatial transfer of hotspot pollutants induced by population migration in China

Large-scale migration in China is anticipated to reshape pollutant discharge patterns due to urban–rural differences in water use and treatment, yet its impacts remain insufficiently quantified. Focusing on three wastewater-related indicators—chemical oxygen demand, total nitrogen, and total phosphorus, we established a county-to-county migration matrix incorporating three population types and pathways to quantify the spatial patterns of migration and subsequent environmental disturbances. Migration data were from China’s 2020 National Population Census, defining migrants as those living outside their household registration county for over six months. Results indicate that despite increasing pollutant generation, migration reduces national discharges by up to 10.3% due to improved treatment efficiency in urban areas, while simultaneously intensifying notable increases in discharge density across major urban centers. The asynchronous reduction shifts hotspots from chemical oxygen demand to nitrogen and phosphorus, with alternating dominance between nitrogen and phosphorus. Cost-benefit analysis indicates that rural infrastructure upgrades most effectively reduce chemical oxygen demand, urban investment benefits total phosphorus, and both approaches comparably address total nitrogen. Overall, rural wastewater investment offers the greatest cost-effectiveness nationwide. (Figure presented.)