Nutrient behaviour in urban drainages

Urbanisation has altered the natural landscape, removing features that previously acted to retain, transform and process nitrogen and phosphorus within catchments. In Melbourne the past legacy of traditional urban design aimed at efficient drainage of stormwater has resulted in higher stormwater volumes with high pollutant concentrations. Excessive concentrations of such nutrients in urban runoff can have an adverse impact on the ecological health of receiving waters. The aims of this study are to identify the catchment characteristics and mechanisms that influence nitrogen and phosphorus concentrations in dry and wet weather, and to recommend the most appropriate treatment technologies to remediate the situation. The study used existing data sourced from previous researchers on a number of Melbourne catchments. Standard statistical methods were used to gain insights into concentrations, compositions and relationships between catchment characteristics and nutrient concentrations. Catchment land use is found to have an impact on dry weather average nitrogen concentrations with residential concentrations (e. g. mean TN=3.2 mg/L, mean NOx=1.868 mg/L) significantly higher than in industrial catchments (mean TN=1.06 mg/L, mean NOx=0.296 mg/L). However, this disparity between land uses could not be evaluated for wet weather due to the lack of data. In dry weather, the catchment characteristics of catchment (human) population and area both positively influence most nutrients, since they are measures of nutrient loading. Some nitrogen species are affected by the hydraulic conductivity of the catchment’s underlying soils, suggesting nitrogen leaching and subsurface flow pathways may be critical in supplying nitrogen oxides and other dissolved nutrient species during dry weather. Older catchments, compared among the urban catchments studied, discharge higher levels of phosphorus and dissolved nitrogen, most likely as a result of degraded and leaking wastewater infrastructure, whereas steeper catchments deliver higher concentrations of ammonia, suggesting retention times may limit the transformation of organic nitrogen and ammonia to nitrate and nitrite. Nitrogen loading is not only generated from current anthropogenic activities but appears to be complicated by past legacies of land use, where significant long-term leaching of oxidised forms of nitrogen occurred. Unfortunately, inadequate historical land use data were available to test this hypothesis. In wet weather, catchment population influences nitrogen and phosphorus species, especially the dissolved forms. Curiously, catchment area influences nitrogen oxides and dissolved nitrogen, with higher concentrations observed for larger catchments. The percentage of impervious surfaces influences dissolved nitrogen and total phosphorus. The hydraulic conductivity of soil and bedrock influences dissolved nitrogen, with more permeable soils producing higher concentrations, whereas surprisingly, the catchment slope negatively influences total nitrogen concentrations. A plausible reason is that slope affects water retention time. A steeper slope provides a quicker runoff of rainwater containing less total nitrogen, whereas a flatter slope, with longer retention time, provides more baseflow seepage from soils. A factor possibly explaining the higher nitrogen oxide concentrations seen in dry weather is that in the process of urbanization, when stormwater drainage networks are built, (especially for buildings and roads), the ground is effectively disturbed and drained. This results in the water table usually being lowered, hence producing a thicker aerobic soil zone that promotes the nitrification process. In wet weather, in contrast, nitrogen oxide concentrations are much lower. In wet weather, particulate nitrogen sourced from soil surface runoff and impervious surfaces is a significant component of stormwater. The average proportion of nitrogen oxides to total nitrogen found in Melbourne catchments in stormwater (in surface runoff) is similar to the national average for stormwater of urban catchments in the USA, probably due to similarities in urban development. Based on knowledge of nutrient concentrations and their composition, appropriate treatment technologies can be planned for their removal. The processes targeting removal of particulates and soluble nutrients involve physical and biological processes. It is recommended to apply methods to settle, detain, reduce and assimilate nutrients focussing on particulate removal in stormwater, and dissolved nitrogen, especially nitrogen oxides, in dry weather flows. The main processes for the removal of dissolved nutrients are anaerobic processes and biotic assimilation. The sources of nitrogen, both natural and anthropogenic, are varied and dispersed throughout the urban catchment. They include both surface sources (e.g. sediment and organic matter) and sub-surface sources (e.g. nitrogen inputs from previous land uses, and leaks from wastewater infrastructure). It is clear that the management of nutrients remains a critical challenge for stormwater managers. In order to address the problem, appropriate treatment technologies should focus particularly on the removal of particulate nitrogen during wet weather flows, given that it comprises 35% of total nitrogen in stormwater under these conditions. During dry weather, the focus should be on the treatment of nitrogen oxides, which make up 64% of total nitrogen in inter-event flows. Ideally, to treat wet weather flows from catchments (where space allows), it is recommended to construct wetlands which are capable of removing sediments, particulate organic matter, and dissolved nutrients. To treat dry weather flows from catchments, it is recommended to build in-line clastic filters or “filter barriers”, which are capable of removing dissolved nutrients. These could be built with a range of substrates combined with woodchips, (incorporating reducing cells), and planted with an assortment of sedges and macrophytes. Thus there are methods for treating urban drainages throughout the year.