Newly-established free water-surface constructed wetland to treat agricultural waters in the low-lying Venetian plain: Performance on nitrogen and phosphorus removal

Highlights: • Agricultural water treatment of FWS CW increased over the 3-year monitoring. • Total nitrogen mass removal was up to 696 kg ha y⁻¹. • The wetland/catchment ratio of 7% could potentially serve a total of 83 ha. Constructed wetlands offer promising solutions for controlling nutrient pollution in agricultural systems with relatively low costs and energy inputs. In mainly central and northern Italy, semi-natural and reconstructed Free-Water Surface Constructed Wetlands (FWS CWs) are designed to treat nonpoint-source pollution from agricultural catchments. However, their performance depends on system design and time of establishment. This paper evaluates the efficiency of a recently established FWS CW to remove nonpoint-source nutrient pollution due to agricultural drainage in the low-lying Venetian plain (NE Italy). The system was established in 2014 by creating five consecutive sub-basins vegetated with macrophytes to restore a semi-natural wetland, and later monitored in terms of water quality parameters and nutrients removal over three consecutive agricultural seasons (2014–2016). Total (TN) and nitrate (N-NO₃) nitrogen concentrations showed peaks (16.37 and 15.31 mg l⁻¹ for TN and N-NO₃, respectively) in the various sub-basins during spring 2015, associated with fertilisation of surrounding croplands and intense rain events. Performance improved over the three years, with increasing median removals of TN (33.3–49.0%) and N-NO₃ (32.2–80.5%), corresponding to average mass of 1355 kg y⁻¹ and 1011 kg y⁻¹ for TN and N-NO₃. Concentrations of ammonium (N-NH₄) and orthophosphate (P-PO₄) were generally low (< 1 and < 0.3 mg l⁻¹ for N-NH₄ and P-PO₄, respectively), with average yearly mass removals of 50 kg for N-NH₄ and 9 kg for P-PO₄. According to the overall treatment performance, the FWS CW could treat a total area of about 30 ha with a wetland/catchment ratio of 7%. However, we expect that treatment efficiency will increase as a result of bank stabilisation and improvement of the aquatic environment, together with increases in surface vegetation.