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Impacts of urban hillslope development and agriculture on hydrology and water quality in the Chilliwack Creek watershed, British Columbia MacDonald, Jennifer Rae


The Lower Fraser Valley (LFV) has one of the most rapidly growing urban populations in Canada, and as a result water pollution problems associated with non-point source (NPS) pollution from urban expansion and agricultural intensification are increasing rapidly in this region. At the same time, the increasing demand for housing combined with the protection of agricultural land in the valley has pushed development onto the hillslopes. The transition from natural forest cover to impervious surfaces alters the hydrologic system, and increases the rate and volume of stormwater runoff that reaches the receiving watercourses. Due to the sensitivity of hillslope environments, and because upland activities may have damaging consequences downstream, development on these hillslopes presents many unique challenges for stormwater management. This research project uses a watershed approach to examine the impacts of land use (agriculture and urban development) on hydrological processes and surface water quality in a mixed land use setting. In Chilliwack, forest land on the hillslope is being converted into urban developments, and plans are under way to house up to 50,000 people on the hillslopes in Chilliwack over the next 25 years. The impact of this conversion on hydrology and water quality was examined in streams draining recently completed urban development (up to 2000 houses) by comparing the results with streams originating from undisturbed forested land. Using samples collected at twenty stations, a baseline was established for water quality and trace metals in sediments for various sub-watersheds in the study area. These results indicate that the lowland agricultural activities are the major source of NPS pollution in the watershed. Nutrient levels are elevated during the wet season, and many of the agricultural tributaries show evidence of eutrophication in the summer season. Trace metals associated with agricultural operations (Cu, Fe, Mn, Cd and Zn) were also elevated in the sediment of agricultural streams. Spatially, ammonia, orthophosphate and trace metals increased in the downstream direction along Interception Ditch (a large agricultural drainage ditch) indicating the effects may be cumulative. Results from the hillslope urban sites indicated that the hydrologic impacts of the development are the most important at this stage. Peak runoff was shown to be up to 1416% higher and lag times were up to 30 hours shorter in the suburban hillslope catchment (26% TIA) than for the forested catchment (4% TIA). While the impact on water and sediment quality was minimal, concentrations of orthophosphate, dissolved magnesium and potassium did show significantly elevated concentrations compared to the forested tributaries. Currently, the City of Chilliwack is experimenting with a number of innovative stormwater management designs (e.g. on-site detention ponds, infiltration galleries) in attempts to infiltrate much of the stormwater into the soil in these hillslope developments, before it reaches the streams. It is suggested that incorporating these low impact designs and source control methods may be more effective at mitigating the impacts of development than conventional stormwater management systems.

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