High Frequency Nitrate Concentrations Measured in Two Iowa Springs Fed by Intense Row Crop Agriculture

Nitratax sensors were deployed to measure high frequency nitrate concentrations discharging at two fish hatchery springs in northeast Iowa to assess groundwater vulnerabilities. At Big Spring in Clayton County, Ordovician-age carbonate rocks exhibit karst development and 10% of the watershed drains to sinkholes. Over a 2-year period, nitrate concentrations averaged 12.4 ± 0.94 mg/l and exhibited a pronounced decrease in concentration following runoff events. Near the Manchester hatchery in Delaware County, Silurian-age dolomite is overlain by variable sandy surficial materials. N sensor deployment revealed higher nitrate concentrations (averaging 17.4 ±1.9 mg/l), with concentrations increasing after rainfall periods before returning to high baseline values. The difference in nitrate concentration patterns observed at the two Iowa springs indicates differences in hydrologic pathways. While both springs are heavily impacted by agricultural-sourced nitrogen from row-crop dominated watersheds (>80%), decreases in nitrate at Big Spring are indicative of low N in runoff lost to sinkholes features diluting the groundwater baseflow signal. In contrast, much higher concentrations and increases in nitrate following wet periods at Manchester is indicative of systemic high nitrate levels throughout the aquifer system. Both spring areas will require consideration of their distinct nitrate delivery pathways to implement groundwater-based nutrient reduction strategies.