SURFACE WATER QUALITY PATTERNS AT THE SUB-WATERSHED SCALE IN AGRICULTURAL STREAMS WITH VARIABLE RIPARIAN COVER

The presence of riparian zones adjacent to water bodies has been linked to improved surface water quality however there is little information available to describe their impact on stream water quality when large scales are considered. In particular there is a lack of data to describe stream water quality at this scale during storm and snow-melt events. In this study an integral event-based surface water quality analysis was conducted on two small watersheds in Southern Ontario near Elmira each with an area of approximately 12 km2. The two watersheds are adjacent to one another with similar land-use characteristics but very different riparian zone characteristics. One watershed exhibits almost no riparian cover, the other exhibits substantial mature wetland riparian cover. Water quality constituents examined include total phosphorus and total nitrogen as well as turbidity, total suspended solids and fixed suspended solids, anions and cations. Samples were taken using float-triggered auto-samplers for a two year period from May 2005 to May 2007 to capture event-based water quality responses. Flow rates were also measured at the outlets of each watershed to provide for contaminant flux calculations.

Preliminary results indicate that under equivalent flow conditions the suspended solids fluxes were larger in the watershed with heavy riparian cover. The timing of the peak nitrogen concentration during each event lagged the hydrological peak flow and was consistent with the presence of tile drainage in the area. Phosphorus concentration profiles were highly correlated to suspended sediment concentrations but also showed high first-flush concentrations following periods of bed dryness. The data represent a multi-year analysis and early results raise questions about the efficacy of mature riparian wetland as water quality-mitigating agent during storm events, especially with regard to sediment transport, at the sub-watershed scale. The field data is currently being compared with a hydrological water quality model WATFLOOD/AGNPS to aid in data interpretation.