Methodological considerations for anticipating wildfire ash-associated organic carbon changes in water supplies

Wildfire can adversely impact the quality and quantity of water in forested regions by delivering excessive loads of sediment and burned materials into receiving waters via runoff. To evaluate implications to water treatability, bench- and pilot-scale investigations often involve wildfire ash addition to source water to reflect post-fire source water quality change. As methods are not standardized, a range of experimental conditions, such as different ash/water mixtures and mixing methods have been used—this can lead to contradictory results. Here, two key factors in source water preparation for investigating wildfire impacts on water treatability (i.e., mixing time and ash mass concentration) were investigated and their impacts on leached water extractable organic matter (WEOM) from wildfire ash were characterized. Specifically, a series of controlled bench-scale experiments were conducted to monitor and evaluate water quality changes in wildfire ash‒ impacted water (WAIW) under different mixing scenarios using both natural river water and ultrapure water. Water quality parameters including pH, alkalinity, conductivity, dissolved organic carbon (DOC), and specific ultraviolet absorbance (SUVA) were measured in WAIW samples. Further characterization of organic matter involved liquid chromatography-organic carbon detection (LC-OCD) analysis. The concentration and character of organic carbon changed considerably during the first 24 hours of mixing. Water quality and ash mass concentration limit the extraction of organic matter into water. These results emphasize the importance of (i) specifying experimental conditions and providing rationale for the approach utilized (e.g., demonstrating a worst-case scenario of maximal leaching of WEOM from wildfire ash, reflecting watershed conditions), (ii) providing a range of results obtained at different leaching conditions, or alternatively (iii) clarifying that results may be exploratory or comparative, but not necessarily quantitatively meaningful or relevant for decision-making.