The use of wetlands for the control of non-point source pollution

There is increasing interest in the use of wetlands to intercept nutrients in diffuse run-off from rural catchments. However, the scientific basis for this strategy is far from secure. While research in several countries provides support for this approach, there is a general lack of rigorous data sets of nutrient balances showing the real effect of such wetlands on the quality of run-off emanating from rural catchments. Research being conducted on two natural and one constructed wetlands in south-eastern Australia will contribute to filling this gap. In each of these three wetlands, volume of inflow and outflow is being measured at 15 minute intervals. Automatic water samplers connected to the flow measurement device are measured and are triggered to take samples at appropriate intervals during run-off events. All these water samples are analysed chemically and the total loads of selected chemicals entering and leaving a wetland are calculated for several storm events over the winter and spring period during 1993. Results for Total Nitrogen and Total Phosphorus show that during winter there is a net release of these nutrients from the linear wetlands with greater flows resulting in greater flushing. However, in spring and early summer there was a net retention of nutrients in the wetlands despite similar hydrological loadings. These results were affected by the size of the wetland relative to the catchment (and therefore retention time), land use of the catchment, any intrusion of ground water and the nature of the wetland in terms of its shape and vegetation.