Nitrate Concentration Trends in Iowa's Rivers, 1998 to 2012: What Challenges Await Nutrient Reduction Initiatives?

Abstract

Nitrate-nitrogen (NO-N) concentrations threaten water supplies and contribute to impairments of surface water resources. In this study, we analyzed concentration trends at 60 ambient river monitoring sites in Iowa for the years 1998 to 2012 to assess the presence of linear trends in the NO-N concentration data using a time-series method that accounted for temporal correlation and combined the trend information from individual sites into an assessment of the state-wide rate of change in river NO-N concentrations. Forty-six of the sites had sufficient records for trend analysis. Study results indicated that 37 out of 46 sites (80%) did not have statistically significant trends over the monitoring period ( > 0.1). Six monitoring sites in western Iowa had statistically significant increasing trends ( < 0.05), and three additional sites located in western and southern Iowa showed nominally significant increasing trends ( < 0.1). The rate of statistically significant increases ranged from 0.15 to 0.33 mg L yr. Aggregated across the state, the overall trend of NO-N concentrations in Iowa rivers is increasing, with an average and median rate of 0.05 and 0.03 mg L yr, respectively. Increasing concentration is likely associated with increasing trends in fertilizer sales and animal production, but better tracking is needed to establish a definitive relation. Reducing NO-N concentrations using conservation practices is a major focus of the recently proposed Iowa Nutrient Reduction Strategy, and our study provides an important milestone preceding implementation of the strategy.