Estimating nitrate 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 loads, but their removal performance varies with hydrologic conditions and other factors difficult to capture with traditional grab sampling schemes. We quantified nitrate retention in a large constructed Iowa wetland using high-frequency (15-min) monitoring and a physically-based hydrologic model that estimated discharge. The objectives were to quantify the wetland nitrate retention and compare to other studies, evaluate the factors important to wetland performance, compare insights gained from low- and high-frequency monitoring, and consider the broader implications of wetland nutrient removal as related to achieving water quality goals associated with the Iowa Nutrient Reduction Strategy and Gulf Hypoxia. Over a 3-yr monitoring period (May-Nov, 2014-2016), the wetland reduced incoming nitrate concentrations 49% and loads 60 kg/day (0.49 g/m²-wetland/day). Wetland performance was strongly influenced by hydrologic conditions, but variables describing NO_x-N supply and biological conditions were also important. Average concentration reductions ranged from 23% in a wet year (2016) to 59-65% in more average climatological years (2014-2015). On a monthly basis, mass retention was highest in Jun and retention efficiencies were highest in Jul and Aug. The high-frequency monitoring captured a greater amount of variability in NO_x-N concentrations and wetland retention processes compared to low-frequency monitoring schemes. The retention performance of this wetland indicates 4700~6800 wetlands totaling $1.2-1.8 billion in design and construction costs would be required to reduce the baseline nitrate load in Iowa by 45%, indicating a significant investment in conservation is needed to achieve Gulf Hypoxia water quality goals.