Wildfire effects on water quality in forest catchments: A review with implications for water supply

Increases for sulfate and chloride were relatively short-lived and did not exceed World Health Organization water quality aesthetic use guidelines for sulfate.

“Increases for both were relatively short-lived and did not exceed World Health Organization water quality aesthetic use guidelines for sulfate (Beche et al., 2005; Smith et al., 2011).”

PAH increases occurred in boreal, temperate, and Mediterranean biomes.

“PAH increases occurred in boreal, temperate, and Mediterranean biomes (Campo et al., 2017; Smith et al., 2011; Stein et al., 2012; Wisløff, 2018).”

Lakes also experience elevated inorganic chemical concentrations, including metals, following fires, but there was comparatively less information on elevated metals in lakes than in streams.

“Lakes also experience elevated inorganic chemical concentrations, including metals, following fires (McCullough et al., 2019; Smith et al., 2011), but there was comparatively less information on elevated metals in lakes than in streams.”

Absolute PAH concentrations post-fire rarely exceeded the World Health Organization guideline for benzo(a)pyrene (0.0007 mg/L).

“Still, absolute PAH concentrations post-fire rarely exceeded the World Health Organization guideline for benzo(a)pyrene (0.0007 mg/L, Smith et al., 2011).”

Compared to lower severity fires, higher severity fires consume a greater proportion of aboveground vegetation, forest litter, and other organic matter within the soil; these impacts expose more of the soil to precipitation and, in some cases, increase erosion by several orders of magnitude.

“Compared to lower severity fires, higher severity fires consume a greater proportion of aboveground vegetation, forest litter, and other organic matter (e.g., roots) within the soil these impacts expose more of the soil to precipitation and, in some cases, increase erosion by several orders of magnitude.5,31”

Streams that drain burned forested landscapes often have increased levels of phosphorus, nitrogen, dissolved organic carbon (DOC), sulfate, chloride, calcium, magnesium, sodium, and potassium.

“Streams that drain burned forested landscapes often have increased levels of phosphorus, nitrogen, dissolved organic carbon (DOC), sulfate, chloride, calcium, magnesium, sodium, and potassium.5,13”

Most previous reviews of wildfire impacts focused on water quality in general or primarily on drinking water.

“While wildfire impacts have been reviewed before, most reviews have focused on water quality in general or primarily on drinking water (e.g., Smith et al., 2011).”

Fire-mobilized pollutants include sediment, dissolved organic compounds, and metals that threaten drinking water sources.

“These pollutants include sediment (Dahm et al., 2015; Smith et al., 2011; Verkaik et al., 2013), dissolved organic compounds (Hohner et al., 2016, 2019), and metals (Bladon et al., 2014; Smith et al., 2011) that threaten drinking water sources.”

Cations like calcium, potassium, and magnesium generally follow nutrient trends, increasing after post-fire precipitation events as a function of burned area and burn severity.

“Cations like calcium, potassium, and magnesium generally follow nutrient trends, increasing after post-fire precipitation events as a function of burned area and burn severity (Bitner et al., 2001; Rust et al., 2018; Smith et al., 2011) as they are primarily generated from ash (Ranalli, 2004).”

Proposed mechanisms explaining increased responses of nutrients post-fire include volatilization and deposition, reduced plant and microbial uptake, and increased leaching and runoff into waterbodies.

“Proposed mechanisms explaining increased responses of nutrients post-fire include volatilization and deposition, reduced plant and microbial uptake, and increased leaching and runoff into waterbodies (Emelko et al., 2016; Rust et al., 2018; Smith et al., 2011).”

Wildfires can mobilize metals and other inorganic chemicals which are then transported by precipitation events into downstream waters.

“Wildfires can mobilize metals and other inorganic chemicals which are then transported by precipitation events into downstream waters (e.g., see Abraham et al., 2017b; Rust et al., 2018; Smith et al., 2011).”

Smoke and ash from fires can transport PCBs over longer distances and subsequently deposit them.

“Smoke and ash from fires can transport these over longer distances and subsequently deposit them (Smith et al., 2011), yet we found no data reported for these chemicals in the research papers reviewed.”

Previous reviews have emphasized the problems to water quality posed by sediments, nutrients, and metals, among other chemicals.

“Previous reviews have emphasized the problems to water quality posed by sediments, nutrients, and metals, among other chemicals (e.g., Bladon et al., 2014; Rust et al., 2018; Smith et al., 2011).”

Highly variable impacts of ash contamination on freshwater biota have been reported between different ecosystems, types of ash, fires and species.

“Highly variable impacts of ash contamination on freshwater biota have been reported between different ecosystems, types of ash, fires and species (Smith et al. 2011;Silva et al. 2015;Oliveira-Filho et al. 2018).”

Release of soluble elements and particulate matter from eroded ash and underlying soil into aquatic systems following fires can cause increases in water turbidity, pH, organic matter, suspended sediment and conductivity and depletion of dissolved oxygen.

“The release of soluble elements and particulate matter from eroded ash and underlying soil into aquatic systems following fires can cause increases in water turbidity, pH, organic matter, suspended sediment and conductivity and depletion of dissolved oxygen, among other effects (Smith et al. 2011;Tsai et al. 2017).”

PAHs and heavy metals present major biological concern owing to their carcinogenic potential, persistence within ecosystems and tendency to bio-accumulate.

“Both present major biological concern owing to their carcinogenic potential, persistence within ecosystems and tendency to bio-accumulate (Smith et al. 2011;Chen et al. 2018).”

High Na+ levels in freshwater systems can present an issue for water purification processes as they cannot be removed using conventional methods.

“High Na þ levels in freshwater systems can present an issue for water purification processes as they cannot be removed using conventional methods (Smith et al. 2011).”

The limited number of studies focusing on Na+, Cl-, SO4 2- and NO3- exports after a fire have found maximum levels sampled in ash fall well below recommended safe limits.

“The limited number of studies focusing on Na þ , Cl À , SO 4 2À and NO 3 À exports after a fire have found maximum levels sampled in ash fall well below recommended safe limits (Smith et al. 2011).”

The influence of key nutrients on Daphnia magna immobilisation is perhaps less well established.

“The influence of key nutrients on D. magna immobilisation is perhaps less well established (Smith et al. 2011)”