Drinking Water Source Quality and Treatability Impacts of Severe Wildfire at the Large Basin Scale: The Legacy of the 2016 Horse River Wildfire in Fort McMurray, Canada

Abstract:

Wildfires can have severe and lasting impacts on water quality in aquatic ecosystems and thus drinking water treatability. Our understanding of these impacts is largely founded on data from small watersheds with high hydrologic connectivity between hillslopes and receiving waters. Despite the predominance of large, low-relief rivers across the fire-prone Canadian Boreal forest, the extent to which drinking water source quality and treatment can be impacted by severe wildfire in these systems, which are generally understood to buffer upstream disturbance signals, has not be described. In addition to a basin-scale water quality monitoring program initiated by the provincial government, a focused long-term investigation of reservoir water and sediment quality and drinking water treatment performance was conducted—the first four post-fire years of this investigation are reported herein and contrasted to several years of historical records. This study shows that the wildfire had an appreciable impact on water quality in the Athabasca River watershed, although only <4% of the watershed was burned and thus, substantial dilution of any wildfire impacts on water quality might have been expected. Brief pulses of suspended solids that were exceptional relative to historic records were observed in the study rivers following the wildfire, likely due to uneven mixing of heavily-impacted tributaries with high-order rivers including the Athabasca River. Notably, as a result, the wildfire also had a significant and continued impact on drinking water treatment in Fort McMurray. Specifically, delivery of sediment and associated contaminants (including bioavailable phosphorus and dissolved organic carbon) from the wildfire impacted tributaries disproportionally affected reservoir water quality and subsequent treatment leading to over CDN $9 million in additional treatment costs to date. Amongst the impacts, and consistent with previously-reported headwaters investigations, increased variability and maxima in coagulant demand and disinfection by-product formation potentials were observed. Linked to carbon and phosphorus fluxes across a large spatial and temporal scale, such extreme, long-lasting shifts in drinking water treatability and treatment as a result of wildfire have not been previously reported.