ANALYSIS OF STORM RUN-OFF SOURCES USING O-18 IN A HEADWATER SWAMP

A combination of hydrometric data and observations of natural isotope (oxygen- 1 8) variations in saturation over an flow and stream dischar ge were used to investigate the sources of storm run-off in a headwate r swamp located in a permanent groundwater discharge zone near Toronto , Canada. The results of a two-component hydrograph separation indicat ed that pre-event water formed 80-90% of the stream hydrograph volume for six of the seven storms analyzed in June-November 1990. However, t he instantaneous event water contribution showed considerable variabil ity, ranging from maximum values of 20-25% for four moderate intensity storms to 63% for a high intensity thunderstorm with a return period of two years. The relative contribution of event and pre-event water t o storm run-off from saturated areas and small streamlets within the s wamp was similar to the main outlet stream. The dominance of pre-event water during storms could be accounted for by the mixing of a small v olume of event water with a large pool of pre-event water on the surfa ce of permanently saturated areas within the swamp. Occasional storms of high intensity or long duration produced a greater shift towards an event water signature in the saturated areas and a larger event water contribution to the outlet stream hydrograph.