Assessment of nutrient and sediment loads in the Yarra River Catchment

Excessive loadings of non-point source pollution from different landuse activities cause eutrophication in rivers and creeks, and are a major concern to water resource managers. Catchment-scale management programs have been proven to be efficient in reducing water pollution from landuse activities. Understanding the connection between landuse activities and water quality is important for developing these management programs. However, the lack of knowledge of nutrient and sediment mass loads and their correlation with landuses are major impediments in applying these management programs. The main objective of this study was to assess nutrient and sediment loads at various sections of the Yarra River catchment in Victoria, Australia for the period of 1998-2009, and to investigate the correlation between these loads and different landuse types. The specific objectives were to: (1) establish whether there is a statistically significant difference between concentrations in water quality samples representing two flow regimes i.e. baseflow conditions and surface runoff events, (2) estimate pollutant mass load: nutrients and sediment at various sections of the catchment, and (3) identify the significant and major sources of pollutant loads. A generic methodology was applied to achieve the above objectives using long-term in-stream water quality data and other readily available tools, which is also applicable to other catchments containing different landuses. The proposed methods and techniques addressed the issues of selecting water quality stations, catchment subdivision, identification of major landuse types, analysis of different flow regimes, and suitable methods to estimate loadings from the catchments containing different landuses. Although the methodology does not indicate the specific mechanisms causing surface runoff and pollutant transport, the data-based model is simple to apply and leads to good results, particularly in circumstances where limited time and data are available for assessment. In this study, 12 water quality stations were selected representing the Yarra River and its major tributaries of which 5 stations were on the main stem of the Yarra River. The dominant landuse type in the tributary stations was either agriculture or urban where as in the main stem stations; it was forest-agriculture mix type. Hydrograph separation allowed the separation of the water quality data into that collected during baseflow condition and surface runoff event. Baseflow conditions were defined as those water quality data collected when baseflow discharge was greater than or equal to 70% of mean daily discharge (total streamflow), and surface runoff events were defined as those data collected when baseflow discharge was less than 70% of mean daily discharge. The terms "baseflow conditions" and "surface runoff events" will be referred as "baseflow" and "events" respectively. At all water quality stations, the pollutant concentrations were significantly (p=0.01) greater in events than in baseflow. Total nitrogen (TN), total phosphorus (TP) and total suspended solid (TSS) loads delivered during events at all main stem stations were on average 49, 54 and 64 percent of the total loads respectively, where events were only on average 18 percent during the study period. On the other hand, the runoff loads for TN, TP and TSS at the tributary stations were on average 79, 81 and 91 percent respectively, where events were on average 31 percent only. In each case, larger portion of pollutant loads delivered by events evidenced that major pollutant loads were generated from agricultural and urban runoff (i.e., non-point sources). These results are encouraging especially given the pressing need to identify appropriate management practices to improve the water quality within the catchment. Overall, water quality and pollutant concentrations were influenced by flow regimes and landuse types, demonstrating the need for water quality monitoring programs to specifically target the collection of water quality samples during events and to characterize the distribution of constituent values in each flow condition of the Yarra River. The presence of increasing nutrient and sediment concentrations during events suggests that Yarra River catchment managers should focus on potential non-point sources that occur during events in streams.