Quantifying the Nitrogen-Removal Performance of a Constructed Wetland Dominated by Diffuse Agricultural Groundwater Inflows Using a Linked Catchment–Wetland Model

Nitrogen loading from diffuse agricultural sources is a major water-quality problem worldwide. Constructed wetlands have been increasingly used to treat runoff and drainage from agricultural lands. However, the diffuse nature of nitrogen loading from farmlands often makes it challenging to trace flow pathways and measure the direct input loading to wetlands, and assess their nutrient-reduction performance. The Owl Farm wetland, Cambridge, New Zealand, receives inputs mainly from a subsurface drain and groundwater seepage. As it was not possible to directly measure wetland inflows, we used the Soil and Water Assessment Tool (SWAT) to estimate and partition the wetland inflow and nitrogen loading from the drain and seepage. A dynamic first-order tanks-in-series wetland model was linked with SWAT to evaluate the wetland capacity for nitrogen removal over a four-year period. The linked catchment–wetland model could simulate flow and nitrate load at the wetland outlet reasonably well with a Nash–Sutcliffe efficiency (NSE) of 0.7 and 0.76, respectively, suggesting that it provides a good representation of the hydrological and nitrogen processes in the upland catchment and the constructed wetland. We used two approaches, a mixed measurement-and-modelling-based approach and a process-based modelling approach to estimate the wetland efficiency of nitrogen removal. In both approaches, we found that the percentage load removal for nitrate-N and total N was related exponentially to the wetland outflow rate. Based on the process-based model estimates, the Owl Farm constructed wetland is very effective in removing nitrate-N with annual estimates of 55–80% (average 61%) removal. However, this capacity is very dynamic depending on the inflow from the catchment. The removal efficiency is very high at low flow and reduces when flow increases but is still maintained at around 20–40% during higher-flow periods. However, actual nitrogen-load removal in the wetland is greatest during high-flow periods when input loads are elevated. This study illustrates how a linked catchment–wetland modelling approach can be used to partition and quantify diffuse nitrogen input loads into wetlands from different types of runoff and to evaluate their subsequent reduction rates. The tool is particularly useful for situations where diffuse groundwater inflows, which are difficult to measure, are important nutrient sources