Size‐based regulation and water quality: Evidence from the Iowa hog industry

The growing prevalence of animal feeding operations (AFOs) in the United States raises concerns among the public and regulators about their impact on local environmental quality. By linking historical regulatory records of AFOs in Iowa to downstream surface water pollution monitors, this paper studies the effects of the 2003 Clean Water Act regulations that targeted water pollution from the largest hog AFOs. The regulation decreased ammonia concentrations downstream of large hog AFOs by 6–8 percentage points. We find limited to no evidence of improvement for dissolved oxygen and phosphorus concentrations. Pollution reductions are largest during heavy precipitation months, consistent with the regulations reducing on‐site spills and nutrient runoff from local fields. However, we find that pollution increased downstream from mid‐sized hog AFOs, which were exempt from the updated regulations. Given the growth in the number of mid‐sized facilities relative to large AFOs, we estimate that the regulation had little discernible impact on overall water quality.