CATCHMENT-WIDE IMPACTS ON WATER-QUALITY - THE USE OF SNAPSHOT SAMPLING DURING STABLE FLOW

Water quality is usually monitored on a regular basis at only a small number of locations in a catchment, generally focused at the catchment outlet. This integrates the effect of all the point and non-point sou rce processes occurring throughout the catchment. However, effective c atchment management requires data which identify major sources and pro cesses. As part of a wider study aimed at providing technical informat ion for the development of integrated catchment management plans for a 5000 km(2) catchment in south eastern Australia, a 'snapshot' of wate r quality was undertaken during stable summer flow conditions. These l ow flow conditions exist for long periods so water quality at these fl ow levels is an important constraint on the health of in-stream biolog ical communities. Over a 4 day period, a study of the low flow water q uality characteristics throughout the Latrobe River catchment was unde rtaken. Sixty-four sites were chosen to enable a longitudinal profile of water quality to be established. All tributary junctions and sites along major tributaries, as well as all major industrial inputs were i ncluded. Samples were analysed for a range of parameters including tot al suspended solids concentration, pH, dissolved oxygen, electrical co nductivity, turbidity, Bow rate and water temperature. Filtered and un filtered samples were taken from 27 sites along the main stream and tr ibutary confluences for analysis of total N, NH4, oxidised N, total P and dissolved reactive P concentrations. The data are used to illustra te the utility of this sampling methodology for establishing specific sources and estimating non-point source loads of phosphorous, total su spended solids and total dissolved solids. The methodology enabled sev eral new insights into system behaviour including quantification of un known point discharges, identification of key in-stream sources of sus pended material and the extent to which biological activity (phytoplan kton growth) affects water quality. The costs and benefits of the samp ling exercise are reviewed.