Implications of land disturbance on drinking water treatability in a changing climate: Demonstrating the need for source water supply and protection strategies.

Synopsis: Large scale natural disturbances from wildfire and severe insect infestations can significantly impact water quality downstream of forested headwaters. Forest fires impact water quality due to sediment input of the ash and soot and by chemical changes to surface and groundwater. This has recently been extensively studied in southwestern Alberta, after the “Lost Creek” wildfire of 2003. Four years of comprehensive hydrology and water quality data from seven watersheds were evaluated and synthesized to assess the implications of wildfire and post-fire intervention (salvage logging) on downstream drinking water treatment. Burned and unburned reference watersheds were examined by the authors.

Far more contaminants of concern were exported downstream from burned watersheds than from unburned watersheds, while burned watersheds subsequently salvage logged had the highest rates of contaminant export. Some highlights included:

 Turbidity (NTU) measured in streams (95th percentile):

•   5.1 NTU for unburned watersheds

•   15.3 NTU in burned watersheds (3x higher)

•   18.8 NTU in burned watershed subsequently salvage logged (>3.5 x higher)

•   Over 6.5x as much nitrate (NO3-) exported downstream from burned watersheds in the first year

•   Over 4.1x as much dissolved organic nitrogen (DON) exported downstream in the first year

•   Observed increases in contaminants such as nutrients and heavy metals (mercury)

•   Increases were particularly elevated during snowmelt freshet as well as during storm flows

•   A rapid decline in mean concentrations of many nutrients three years after the fire, although high contaminant loads were still exported during the snowmelt freshet and following precipitation events

•   Much higher peak concentrations for many parameters during extreme events were observed in burned and burned + salvage logged watersheds (Figure2, Figure 4)

  There were persistent increases in sediment and other contaminants such as nutrients, heavy metals, and chlorophyll-a in discharge from burned and salvage-logged watersheds present important economic and operational challenges for water treatment processes. This is particularly the case for any “extreme” values which often dictate drinking water infrastructure and chemical consumption requirements. Many traditional source water protection strategies would fail to adequately identify and evaluate many of the significant wildfire- and post-fire management-associated implications to drinking water "treatability"; accordingly, it is proposed that "source water supply and protection strategies" should be developed to consider a suppliers' ability to provide adequate quantities of potable water to meet demand by addressing all aspects of drinking water "supply" (i.e., quantity, timing of availability, and quality) and their relationship to "treatability" in response to land disturbance. In addition, large scale wildfire and insect infestations have become more likely due to climate change.