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.

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

“…For example, low discharges around 2000 and high discharges around 2010 tend to create a general trend of increasing annual mean (uncorrected for discharge) concentrations. Similar positive trends were observed by Li et al [2013] for some basins in Western Iowa based on time series analysis of raw NO 2 3 data. In contrast, the smooth curves in Figure 4 are 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Year 0 2,000,000 4,000,000 6,000,000 flow-normalized (FN) concentration values computed as described in section 3.3.…”

“…Similar positive trends were observed by Li et al . [] for some basins in Western Iowa based on time series analysis of raw data. In contrast, the smooth curves in Figure are flow‐normalized (FN) concentration values computed as described in section .…”

“…A study of trends in N concentration and flux in Mississippi River drainages between 1980 and 2008 indicated mixed results, with decreasing trends of N concentration and flux in the Iowa River at Wapello and increasing trends in the Missouri River at Hermman, MO and the Mississippi River at Clinton [ Sprague et al ., ]. 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 ., ].…”

Iowa Statewide Stream Nitrate Load Calculated Using In Situ Sensor Network

“…Measurements of NO x ‐N concentration were collected on an average of 225 days per site, with a range of 64–330 days for the 48 IIHR sites, and a range of 233–366 days for the 12 USGS sites. Daily average concentrations ranged from 0.5 mg/L (South Chequest Creek near Douds) to 15.8 mg/L (Floyd River near James), consistent with previous studies of Iowa rivers (Kalkhoff et al ., ; Sprague et al ., ; Li et al ., ). The average concentration of all the sites in aggregate was 8.7 mg/L.…”

Reducing Nitrogen Export from the Corn Belt to the Gulf of Mexico: Agricultural Strategies for Remediating Hypoxia

“…It is therefore not surprising that, despite significant investments in nitrogen‐management practices, nitrogen loads in the main stem of the Mississippi River continue to increase (Sprague et al ., ; MPCA, ). Nitrogen concentrations have also been documented to be rising in springs and streams in Minnesota (MPCA, ) and in several rivers in Iowa (Li et al ., ). These observations suggest a need for a more comprehensive and systematic approach to agricultural conservation that more effectively reduces, intercepts, and treats nitrogen losses at field, farm, and watershed scales.…”