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

Wetlands are an effective edge-of-field conservation practice for reducing agricultural nitrate-nitrogen (NO3-N) loads, but their removal performance varies with hydrologic conditions and other factors difficult to capture with traditional grab sampling schemes. We quantified NO3-N retention in a large Iowa constructed wetland using high-frequency (15-min) in situ NO3-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 NO3-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 NO3-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, NO3-N mass retention was highest in Jun when NO3-N loading was highest, while retention efficiency - the percent of the incoming NO3-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 NO3-N dynamics not possible with lower-frequency sampling. Extrapolating the May-Nov 3-yr average wetland NO3-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 NO3-N load by 45%, indicating the sizable investment in wetland construction and restoration needed to achieve Gulf of Mexico Hypoxia water quality goals.