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

ABSTRACT

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.