Estimating nitrate-nitrogen retention in a large constructed wetland using high-frequency, continuous monitoring and hydrologic modeling

Abstract Wetlands are an effective edge-of-field conservation practice for reducing agricultural nitrate-nitrogen (NO 3 -N) loads, but their removal performance varies with hydrologic conditions and other factors difficult to capture with traditional grab sampling schemes. We quantified NO 3 -N retention in a large Iowa constructed wetland using high-frequency (15-min) in situ NO 3 -N sensors and a physically-based hydrologic model that estimated discharge. Monitoring from May–Nov over a 3-yr period (2014–16) indicated the wetland reduced incoming NO 3 -N concentrations 49% and loads by an estimated 61 kg day −1 (0.48 g m −2  day −1 based on wetland area removal). Monthly and seasonal (May–Nov) wetland retention performance were significantly influenced by hydrologic conditions, as NO 3 -N concentration reductions ranged from 23% in a year that received nearly 50% more seasonal precipitation than average (2016) to 59–65% in years that received average seasonal precipitation (2014–15). On a monthly basis, NO 3 -N mass retention was highest in Jun when NO 3 -N loading was highest, while retention efficiency – the percent of the incoming NO 3 -N load retained by the wetland – was highest in Jul and Aug when water temperature and hydraulic residence time were higher. The high-frequency monitoring captured NO 3 -N dynamics not possible with lower-frequency sampling. Extrapolating the May–Nov 3-yr average wetland NO 3 -N retention estimated in this study to a much larger scale, over 5600 wetlands treating more than 60% of Iowa’s area and totaling an estimated $1.5 billion in design and construction would be required to reduce the state’s baseline NO 3 -N load by 45%, indicating the sizable investment in wetland construction and restoration needed to achieve Gulf of Mexico Hypoxia water quality goals.