Results from the Big Spring basin water quality monitoring and demonstration projects, Iowa, USA

Agricultural practices, hydrology, and water quality of the 267-km(2) Big S pring groundwater drainage basin in Clayton County, Iowa, have been monitor ed since 1981. Land use is agricultural; nitrate-nitrogen (-N) and herbicid es are the resulting contaminants in groundwater and surface water. Ordovic ian Galena Group carbonate rocks comprise the main aquifer in the basin. Re charge to this karstic aquifer is by infiltration, augmented by sinkhole-ca ptured runoff. Groundwater is discharged at Big Spring, where quantity and quality of the discharge are monitored. Monitoring has shown a threefold increase in groundwater nitrate-N concentr ations from the 1960s to the early 1980s. The nitrate-N discharged from the basin typically is equivalent to over one-third of the nitrogen fertilizer applied, with larger losses during wetter years. Atrazine is present in gr oundwater all year; however, contaminant concentrations in the groundwater respond directly to recharge events, and unique chemical signatures of infi ltration versus runoff recharge are detectable in the discharge from Big Sp ring. Education and demonstration efforts have reduced nitrogen fertilizer applic ation rates by one-third since 1981. Relating declines in nitrate and pesti cide concentrations to inputs of nitrogen fertilizer and pesticides at Big Spring is problematic. Annual recharge has varied five-fold during monitori ng, overshadowing any water-quality improvements resulting from incremental ly decreased inputs.