Modelling the removal of nitrogen and sediment by a constructed wetland system in north Queensland, Australia

Abstract

This study of a multi-component 2.5 ha constructed wetland system near Mackay, north Queensland was established to determine the efficacy of these types of wetlands for improving water quality entering the Great Barrier Reef lagoon. To do this a water balance model was developed using water depth measurements made in the wetland inlet basin, between 25 December 2018 and 30 September 2020. The water balance model allowed daily values of run-in and drainage to be calculated, which were combined with a nitrogen balance model to derive a value for the amount of total nitrogen (TN) removed by the wetland of 83 kg ha−1 year−1, or 37% of the nitrogen input to the wetland. The nitrogen balance model used a mean inflow TN concentration of 5060 (± 1734) μg L−1 derived from TN concentration measurements (automatic and grab) made during four high flow events. The largest losses of nitrogen where via drainage with 49% leaving as outflow and 14% as groundwater seepage. Outflow carried 38% of the sediment and particulate nitrogen (PN) out of the wetland, which means that 62% settled to the bottom of the wetland. Dissolved nitrogen (DN) removal in Bakers Creek is mainly (89%) due to the relatively long residence time of water in its biodiversity pond component, the largest and deepest part of this wetland system which always contained water, even during the dry season. This illustrates the value of large, permanent deep pools in a wetland system, whether they are constructed or natural. Wetlands constructed downstream of areas generating sediments and nutrients should therefore be able to improve the quality of water entering the Great Barrier Reef lagoon. However, to do this they need to retain water for significant amounts of time, which they may do in comparatively low rainfall areas. In higher rainfall areas open, flow through wetlands may not be able to do this, so wetlands constructed in these regions need to be designed to retain water in order to have significant N removal capacity. Proper monitoring of future constructed wetlands is vital in order to derive robust design criteria for systems that can remove significant nitrogen from agricultural run-off water.