Trends in nitrate levels in Iowa's community water systems (2000–2022): Characteristics of systems vulnerable to maximum contaminant level exceedances and future regulatory scenarios

This study examines trends in nitrate contamination in Iowa's community water systems (CWS) from 2000 to 2022, focusing on the characteristics of CWS that are most vulnerable to elevated nitrate levels and those likely to be impacted by a lower maximum contaminant level (MCL). Using Safe Drinking Water Act (SDWA) compliance data for CWS currently without nitrate removal, we analyzed nitrate levels across CWS types, source water type, well characteristics, and geography. Results show that large CWS serving >100,000 people frequently exceed 5 mg-N/L due to their reliance on surface water that is vulnerable to non-point source pollution. Small systems (<10,000 consumers) often exhibit episodic spikes in nitrate, often during spring and early summer, coinciding with fertilizer use and rainfall-driven leaching. Shallow and pre-1990 wells were disproportionately affected. Geospatial mapping analysis identified nitrate hotspots in agriculturally intensive regions. A future MCL based on an annual average of 5 mg/L-N would only affect ∼25 CWS annually, far fewer than those impacted under a scenario where any instance above 5 mg/L-N would be a violation. These data-driven findings support future policy for nitrate regulation and drinking water protection.