A catchment study of sources and sinks of nutrients and sediments in south-east Australia

- Point sources were important sources of nutrients on a dairy farm.
- Farm tracks contribute sediments and particulates to waterways.
- A farm dam in this study removed P but was net neutral for N removal.
- Implementing BMPs in this catchment needs to limit point sources.

SummaryA study of nutrient and sediment generation was carried out on a dairy farm in south eastern Australia over a period of two years. The study collected nitrogen (N), phosphorus (P) and total suspended sediment (TSS) samples from three headwater subcatchments totalling 4.4 km2 in area, which had streamflow measurement sites located at their outlets. In addition to these three sites, samples were collected from farm tracks, the discharge from a farm dam (water storage pond) and a rainfall sampler. Over 20 runoff events were monitored in addition to background flows in the catchment. The data enabled nutrient exports to be estimated from the total catchment and two of the three subcatchments. The atmospheric nutrient loads were also estimated and found to be similar to other studies in the region, and were low compared with the nutrient export. The nutrient load estimates indicated that a large proportion of the catchment Total Nitrogen (TN) and Total Phosphorus (TP) loads originated from critical source areas on the farm associated with dairy operations, and that the catchment TN and TP export rates were high compared to other studies. The sediment data also indicated that TP concentrations were closely related to TSS in samples collected from surface runoff. Catchment TN loads in baseflow were high relative to the total load, probably due to indirect effluent and direct groundwater discharges into watercourses, with high concentrations of soluble N (nitrate plus nitrite and ammonium). The farm dam was found to trap a high percentage (>60%) of the influent TP load; however, the retention of TN by the dam was seasonally varying and close to net neutral.