Assessing Spatial and Temporal Variation in Source Water Quality and Drinking Water Treatability Across a Gradient of Forest Harvest on Vancouver Island, BC

  • Author / Creator
    Bourgeois, Alyssa K
  • On Canada’s Pacific Coast, forestry is integral to society. Although economically important, harvesting practices may alter source waters that originate in forested watersheds through changes in suspended solids (SS) and dissolved organic matter (DOM). Each of these metrics has the potential to degrade source water quality, which poses challenges for drinking water treatment. While harvest impacts on source waters are generally well understood, effects differ regionally due to variations in climate, topography, and soil characteristics. Research is therefore needed across different ecozones, and until now, few studies have focused on the Pacific Maritime. Within this ecozone, one question that remains is how the magnitude of change caused by recent forest harvest compares to the background spatiotemporal variation in source water quality, and how this impacts drinking water treatability. To assess these effects, I examined variation in stream water SS and DOM across different flow conditions in forested subwatersheds with contrasting forest harvest histories and diverse characteristics (e.g., topography, soil depth and clay content). This work was done on the east coast of Vancouver Island, British Columbia, in the Comox Lake watershed. Subwatersheds ranged greatly in size (0.21–214.08 km2) and mean elevation (371–1366 m above sea level). Total annual precipitation in the watershed averaged 2193 mm yr-1 between 1981 and 2019. Streams were sampled from April 2019 to March 2020 across synoptic (thirty sites sampled four times), seasonal (ten sites sampled ten times), and storm (four sites; two storm events captured) sampling campaigns. Synoptic and seasonal sampling were conducted during stable baseflow conditions, while storm sampling targeted peak flows. Samples were analyzed for turbidity, total SS, DOM carbon concentration (DOM-[C]), and DOM composition to evaluate source water quality, as well as disinfection by-product formation potentials (DBP-FPs) to infer drinking water treatability. Targeted water quality monitoring revealed that recent forest harvest did not overwhelm the background spatiotemporal variation of SS and DOM in a recovering landscape. Instead, storm event and seasonal changes in hydrology were the primary control on water quality (i.e., SS and DOM) and resultant DBP-FPs: each exhibited a greater change with seasonal and storm flows than across spatially distinct subwatersheds. This highlights that temporal (i.e., hydrologic) factors, rather than spatial ones (i.e., forest harvest and catchment characteristics), were key in driving variation in water quality and treatability. Temporally-driven changes in SS, DOM, and DBP-FPs were not amplified by forest harvest, which suggests weak anthropogenic influences on source waters in the region. Here, a muted response to recent harvesting may be attributed to forestry practices (i.e., slash removal) and the prevalence of second growth forests (i.e., historical harvest in this region). Although responses were subdued, water quality and treatability metrics did vary according to harvested area in the Comox Lake watershed; this highlights the necessity of evaluating the relative importance of forest harvest and flow conditions when assessing source waters. Overall, this study contributes to our understanding of spatiotemporal effects on SS, DOM, and DBP-FPs in forested Pacific Coast subwatersheds.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.