Evaluation of Variation in Nitrate Concentration Levels in the Raccoon River Watershed in Iowa

Abstract

The Raccoon River Watershed in Iowa has received considerable attention in the recent past due to frequent detections of nitrate concentrations above the federal drinking water standard. This paper econometrically investigates the determinants of variation of nitrate concentrations in the Raccoon River. The analysis relies on a generalized autoregressive conditional heteroscedastic process to model the serial dependence of volatility of the monthly nitrate concentrations in the Raccoon River. Monthly nitrate concentration data from Des Moines Water Works at Van Meter from 1992 to 2008 are used in the study. We found no statistically significant increasing trend in nitrate concentrations over the study period. There are substantial intra‐annual variations in nitrate concentrations, and we noted a very strong seasonal pattern. Variations in rainfall and temperature contribute more to the monthly variation in nitrate concentration than do the changes in nitrogen application rates.

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

“…The results of this study indicate that at the scale of an individual river, most sites in Iowa (80%) had no statistically significant (p < 0.1) trend in concentration from approximately 1998 to 2012. The lack of trends observed in 37 rivers located throughout Iowa is consistent with studies that indicated no significant trends in concentrations observed in the Raccoon River from 1972 to 2000 (Schilling and Lutz, 2004) or from 1992 to 2008 ( Jayasinghe et al, 2012) and in other rivers located in the central United States region from 1985 to 1999 (Battaglin et al, 2010) or 1993to 2003(Sprague and Lorenz, 2009. Recent stability in concentration trends over the last decade is contrary to conditions observed throughout the 20th century in Iowa, where concentrations increased 3-fold since the early 1940s (IDNR-GSB, 2001) and up to 6-fold since the early 1900s (Schilling, 2005).…”

“…Recent stability in concentration trends over the last decade is contrary to conditions observed throughout the 20th century in Iowa, where concentrations increased 3‐fold since the early 1940s (IDNR‐GSB, 2001) and up to 6‐fold since the early 1900s (Schilling, 2005). The lack of trends in many Iowa rivers is consistent with the idea that the agricultural landscape has stabilized in the last few decades as wholesale changes, including land use conversions, conservation adoption, and tile drainage, have slowed (Schilling, 2005; Jayasinghe et al, 2012; Jones and Schilling, 2011). For example, as one measure of landscape stability, the total acres of harvested corn and soybeans in Iowa varied within a very narrow range (22,210,000–22,930,000 acres) from 1997 to 2010 (Iowa Agricultural Statistics, 2012).…”

“…Schilling and Lutz (2004) used nonparametric tests to analyze a 28‐yr record of monthly NO 3 –N concentrations from the Raccoon River in Iowa and found no statistically significant trends ( p < 0.1). Jayasinghe et al (2012) used an autoregressive model of monthly NO 3 –N concentration patterns in the same watershed for the 1992 to 2008 period and similarly found no statistically significant time trend ( p < 0.1). Sprague and Lorenz (2009) examined flow‐weighted concentrations and observed values in their assessment of regional nutrient trends in U.S. rivers.…”

The environmental effects of crop price increases: Nitrogen losses in the U.S. Corn Belt

“…However, the timing of nitrate-nitrogen concentration in surface water varies substantially across space and crop years due to weather (Jayasinghe et al, 2012) and other biophysical factors (Green et al, 2014). Thus, it would be difficult to obtain meaningful estimates of the effect of crop prices on observed nitrate-nitrogen concentrations, especially if one were to model the dynamics and spatial heterogeneity.…”

Decadal surface water quality trends under variable climate, land use, and hydrogeochemical setting in Iowa, USA

“…A time series analysis of 1998–2012 data from 46 river monitoring sites in Iowa found that most rivers did not have statistically significant trends of concentrations, but found increasing trends in some western basins and a slight increasing trend (0.05 mg‐N L −1 yr −1 ) in the average across all rivers [ Li et al ., ]. A study of the Raccoon River reported no trend in concentrations from 1992 to 2008 [ Jayasinghe et al ., ]. The M.P.C.A .…”