Efficacy of an off-line constructed wetland for mitigating nitrogen loss from a pastoral catchment

Constructed wetlands are an established mitigation option for reducing contaminant loads from pastoral land in New Zealand. We assess the long-term performance of a 0.5 ha, off-line, horizontal surface flow wetland, installed on a dairy farm with the primary objective of nitrate removal. Over seven years the median removal rates for nitrate-nitrogen (NO₃–N), total nitrogen and total suspended solids were 45%, 33% and 74%, respectively. Whilst dissolved reactive phosphorus (DRP) was initially removed, over time the wetland progressed to a small exporter of DRP. Median NO₃–N mass removal was 417 kg yr⁻¹_, which is considerable for such a small wetland, and equivalent to 7% of total modelled ‘farm-scale’ losses. Nitrate removal was found to be seasonal and although it could be simulated using a conventional wetland model with Arrhenius temperature dependence, the resulting joint estimates of <i>k</i>₂₀ and <i>Θ</i> were uncertain. We estimate the NO₃–N removal cost-effectiveness of the wetland (over a 25-year lifespan) to be NZ$34 kg N⁻¹. We foresee opportunities for off-line constructed treatment wetlands to be an effective nitrate mitigation option in perennial drains and lowland streams in pastoral catchments, but also emphasise the need for on-going maintenance to ensure longevity of performance.