Methodological issues associated with isotopic hydrograph separations (IHSs) in built-up environments are explored using results from the 1990 spring melt in a suburban basin in Peterborough, Ontario, Canada. The heterogeneous nature of suburban environments complicates the selection of appropriate isotopic signatures for event and pre-event waters. Near-stream groundwater 6l8O sampled from wells was poorly mixed, such that the pre-event water signature was best characterized by 6l8O in pre-melt baseflow or discharge from a headwater spring. The event water signature during snowmelt can be characterized using 6l80 in the pre-melt snowpack, surface runoff samples or meltwater from lysimeters. However, the use of snowpack 6 l 8 0 may be inappropriate in suburban basins where meltwater from thin snowcover may exhibit pronounced res onses to 6 l 8 0 in rainfall contributions. Intensive sampling of the spatial variability of runoff or meltwater 6' 0 may be required to characterize the average event water signature adequately. Rainfall 6l8O provided an appropriate event water signal during a large rain on snow event, and differences between this IHS and one generated using an event water signature that included meltwater contributions from snow-covered surfaces were within the uncertainty attributable to the analytical error in 6l80 values. Event water supplied 55-63% of the peak discharge and 48-58% of total runoff from the basin during the melt, which is consistent with the fraction of the basin that has been developed. These results contrast with IHSs conducted in forested basins that suggest that stormflow is dominated by pre-event water contributions.

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