Physicochemical Characterization and Treatment of Wildfire Ash Particles in Drinking Water

Mrittika Hasan Rodela

doi:10.7273/000006300

Wildfires activity is increasing in many regions globally due to climate change and accumulated fuel loads. Wildfires often accelerate post-fire erosion and increase turbidity resulting in shifts in surface water quality. Forested watersheds provide potable water supplies for billions of people around the world. Understanding the properties of wildfire ash that drive particle stability, downstream mobilization in aquatic systems, and the effects on drinking water treatment are crucial for effective post-fire management and treatment of water sources. Wildfire ash samples were collected from 2020 Oregon and California fires and compared to unburned soils. Ash was also produced under laboratory conditions. To understand the impacts of wildfire ash particles on source water quality and drinking water treatment, three studies were conducted: 1) characterization of physicochemical properties of different color wildfire ash, 2) comparison of wildfires ash and lab-produced ash water quality effects and particle stability in different water chemistries, 3) conventional coagulation treatability study of wildfire ash and lab ash particles. Particle size, specific surface area, pH, electrical conductivity, turbidity, alkalinity, zeta potential, dissolved organic carbon (DOC), nutrients and optical properties were analyzed. Analyses of solids showed lighter colored ash (white and gray), indicative of greater combustion temperatures, had higher pH, electrical conductivity, specific surface area, and zeta potential, and smaller particle size than darker ash (dark gray and black) and unburned soils. In different background water chemistries, white wildfire ash and 650 ℃ lab ash had the highest initial turbidity, but settled faster compared to dark wildfire ash and low temperature lab ash. Leached organic carbon and nitrogen concentrations were the greatest for dark gray ash and 250 ℃ ash, and lowest for white ash, 650 ℃ and unburned soils. Among all ash-water mixtures, white ash showed the highest zeta potential (-27.4 ± -7.80 mV) suggesting higher particle stability. All ash samples had higher zeta potential than unburned soils. Results indicate post-fire source water may contain more stable particles and increased organic matter. The treatability evaluation showed that 250 ℃ lab ash was the least amenable to coagulation at all alum doses (20, 40, 60, 80 mg/L) resulting in lowest turbidity and DOC removal while 650 ℃ lab ash showed the greatest removal among all ash samples. White wildfire ash also required a higher alum dose (60 mg/L) than unburned soils (40 mg/L) to reduce zeta potential to -5 mV. pH adjustment of ash-water mixtures resulted in significantly lower finished water turbidity and DOC at an alum dose of 20 mg/L implying post-fire source water may be treated with conventional processes if pH is adjusted. Water providers who experience wildfires and degraded source water quality from ash may need to increase coagulant doses and lower pH to reduce turbidity, zeta potential, and DOC effectively.

Physicochemical Characterization and Treatment of Wildfire Ash Particles in Drinking Water

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