Evaluating Minnesota's Fall Fertilizer Application Restriction: Effects on Waterway Nitrogen Pollution

Abstract

Fertilizer is a leading source of nitrogen pollution in waterways, yet its application is generally unregulated throughout the United States. Minnesota began restricting when fertilizer can be applied to fields with the passage of the Groundwater Protection Rule in 2019, which bans fertilizer application in the fall months and on frozen soils in areas with geologies vulnerable to nitrate leaching and adjacent to municipal public wells with high nitrate levels. There is limited research evaluating the efficacy of the fertilizer application restriction under the Groundwater Protection Rule, and we assess its impact on nitrate contamination by evaluating nitrate concentrations in surface waters. Using a synthetic difference-in-differences model, we compare surface water nitrate concentrations in basins impacted by the Groundwater Protection Rule to the concentrations in basins unaffected by the Groundwater Protection Rule. We find that the Groundwater Protection Rule decreases annual mean surface water nitrate concentrations by 12.4%, and that this effect is statistically significant even after accounting for pollution spillovers. We find that the Groundwater Protection Rule's effect is heterogeneous, with the largest reductions in downstream basins and no measurable effect in headwaters.