Picking Where to Measure: Sorting vs. Siting in Water Quality Monitoring
Abstract
Across sectors from education to health, spatial patterns in resource allocation and risk exposure reflect both household sorting and policy-driven siting. In environmental justice, sorting occurs when disadvantaged groups settle in lower-amenity areas due to housing costs, while siting refers to differential monitoring and regulatory responses.The U.S. Clean Water Act (CWA) mandates water quality monitoring but leaves placement to state discretion, creating potential inequality through demographic differences in monitoring and regulatory oversight. We provide the first comparison of randomly and strategically placed monitors, isolating policy-driven disparities from population traits.
Our analysis combines high-resolution demographic and water quality data across the U.S. Demographics come from the Census Bureau’s Gridded Environmental Impacts Frame, providing race, ethnicity, income, and population at a 0.01-degree resolution. We classify grid cells as near random monitors, strategic monitors, or unmonitored streams. Monitoring data come from two sources: the National Rivers and Streams Assessment (NRSA), with 2,000 randomly placed sites sampled every five years, and the Water Quality Portal (WQP), with roughly 20,000 agency-placed sites from 1989–2019 reflecting regulatory priorities. NRSA captures sorting alone; WQP reflects both sorting and siting.
We estimate census-tract-level models of monitor placement and density as functions of demographic composition for both monitor types. Random sites identify sorting, while differences between random and strategic models identify siting. Controls include river miles, stream order, population, tract area, and county and year fixed effects.
We find demographic differences in both random and strategic placement, with monitors less likely in tracts with higher shares of Hispanic or Black residents and more likely in predominantly White tracts. Effects are larger for strategic monitors, with 60–96\% of the total effect attributable to siting, indicating under-monitoring in disadvantaged communities. This has implications for water quality regulation and CWA implementation, making monitoring bias a key concern for equitable environmental governance.