Wildfires Alter Forest Watersheds and Threaten Drinking Water Quality

Abstract

CONSPECTUS: Wildfires are a natural part of most forest ecosystems, but due to changing climatic and environmental conditions, they have become larger, more severe, and potentially more damaging. Forested watersheds vulnerable to wildfire serve as drinking water supplies for many urban and rural communities. The highly variable nature of wildfire behavior combined with spatially complex patterns in vegetation, landscape, and hydrologic factors create uncertainty surrounding the postfire effects on water supplies. Wildfires often cause dramatic changes in forest vegetation structure and soil conditions, and alter the watershed processes that control streamflow, soil erosion, nutrient export, and downstream water chemistry. The authors' work centers on field and laboratory studies to advance knowledge of postfire changes in soil and water chemical composition that influence drinking water treatment. High intensity postfire rainstorms typically increase runoff that erodes ash and soil from burned landscapes and dramatically elevates turbidity, nutrient, and dissolved organic carbon (DOC) levels in surface waters, which can cause short-term challenges for water providers. There is also growing evidence that water quality impacts can persist after high severity fires due to slow vegetative recovery, and nitrogen and DOC have remained elevated for 15 years following high severity fire. Low-moderate temperatures during wildfire may also influence water quality. Research by the authors showed that the solubility of organic matter, and C and N released from soils increased following soil heating at temperatures ≤ 350 °C. Further, the water extracted organic matter from soils heated at 225−350 °C included higher proportions of condensed aromatic structures, such as black carbon and black nitrogen. Short-term postfire water quality degradation following high intensity rainstorms can force water treatment plants to shut down or can significantly challenge treatment process performance. Extreme turbidity and high DOC in poststorm water, coupled with compositional organic matter changes, reduced the coagulation efficiency of postfire water supplies. Field and lab-based studies documented the formation of small, aromatic soluble compounds during wildfire that contribute to inefficient DOC removal from postfire stormwater. Due to increased postfire DOC concentrations, and poor treatability of poststorm runoff, toxic disinfection continued...

Watershed‐scale vegetation, water quantity, and water quality responses to wildfire in the southern Appalachian mountain region, United States

“…Concentration and export of DOC and particulate organic matter (as TVS) were affected by the fires as reported in previous work in the western United States (see reviews by Hohner et al, 2019; Rust et al, 2018; Smith et al, 2011). Similar to Rhoades et al (2019), watersheds with intermediate levels of high severity burn extent (i.e., CA, 20% high severity) had greater DOC concentrations and export than watersheds burned at greater or lesser high severity extent.…”

“…For example, in a 5.6 km 2 forested watershed in the southeastern United States burned by a low severity wildfire, Jensen et al (2017) found elevated levels of particulate mercury per unit TSS for the first eight months post fire. Taken together, these results suggest that there is elevated risk of localized erosion and sedimentation of streams draining severely‐burned southern Appalachian watersheds that could impact aquatic ecosystems and water supply (Bladon et al, 2014; Hohner, Rhoades, Wilkerson, & Rosario‐Ortiz, 2019; Smith et al, 2011).…”

“…Jensen et al (2017) also found elevated DOC concentrations (accompanied by elevated mercury concentrations) across the flow regime in a forested watershed burned at low severity (8.5% high severity extent) in the southeastern United States. This non‐linear relationship between high burn severity extent and DOC concentration and export has been attributed to unburned and partially charred organic material that remains on the forest floor following less severe wildfires that becomes sustained carbon inputs to streams compared to catchments burned at greater high severity extent (Hohner et al, 2019). Concentration and export of particulate organic matter (as TVS) in burned watersheds at baseflow were generally less than corresponding unburned watersheds, but peak TVS concentration during storms was greater in the most severely burned CA‐TO watershed than the corresponding unburned watershed AR‐UP.…”

Laboratory simulation of postfire effects on conventional drinking water treatment and disinfection byproduct formation

“…The increase in particulates can pose challenges for water conveyance infrastructure (Sham, Tuccillo, & Rooke, ) and treatment processes (Emelko, Silins, Bladon, & Stone, ; Hohner, Cawley, Oropeza, Summers, & Rosario‐Ortiz, ). Research also suggests that wildfires may alter the quantity and quality of dissolved organic matter (DOM) (Hohner et al, ; Hohner, Rhoades, Wilkerson, & Rosario‐Ortiz, ; Wang, Dahlgren, Erşan, Karanfil, & Chow, ). DOM is an important parameter for drinking water utilities as it influences the formation of disinfection byproducts (DBPs) (U.S. Environmental Protection Agency (USEPA), ) and may dictate the type of water treatment required.…”

Do Fungal Symbionts of Salt Marsh Plants Affect Interspecies Competition?

“…The cumulative effects of forest fire, such as percolation, and the capacities of soil absorption, can change a healthy productive watershed to poor condition. With watershed conditions altered after a forest fire, post-fire hydrologic conditions on the surrounding landscape can pose increases in the amount of overland flow, erosion, and soil loss (Neary, 2003;Hohner, 2019). These fire-related effects are the beginning stages of altering the hydrologic responses to precipitation and snow melt within a watershed.…”