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1. Post-wildfire soil hydrophobicity and slope erosion remediation by applying environmentally friendly modifiers

   https://doi.org/10.1016/j.gete.2025.100740  

   Ma, Wenpei; McKlin, Henry; Chan, Russel; Kim, Caitlin; DePoint-Spang, Teagan; Tomac, Ingrid (September 2025, Geomechanics for energy and the environment)

   This paper investigates the use of environmentally friendly remediation materials and techniques for rain-induced post-wildfire soil erosion on burned slopes. During wildfires, vegetation and organic matter combust and release hydrophobic chemicals on soil grains. Hydrophobicity reduces the water infiltration rate, prolongs the wetting process, increases erosion, and causes severe debris flows over watersheds. This comparative study presents the most effective approaches for mitigating hydrophobicity effects through environmentally friendly biopolymers and surfactants. Experimental techniques evaluate the dynamics of water drop penetration into treated and untreated soil, downhill water drop mobility, and erosion. The waterdrop contact angle measurements indicate that biopolymer Xanthan Gum (XG) slightly reduces hydrophobicity, whereas surfactant Sodium Cocoyl Isethionate (SCI) reduces it by a factor of a thousand. In addition, SCI can decrease slope erosion at low-inclined and moderate-inclined slopes. Sands' infiltration rates (IR) are very fast due to high permeability in normal conditions; however, surface hydrophobicity significantly reduces IR. Results from artificially treated extremely water-repellent samples of mixed sand show a six orders of magnitude decrease in IR. Then, after treatments XG and SCI modifiers, the IR increased by an order of magnitude after the XG treatments, and by four orders of magnitude under SCI treatment. Although XG is wettable and attractive to water, the crust and webs it forms between sand particles prevent effective water infiltration. Mild slopes exhibit similar IR rates as horizontal surfaces for all the cases; however, steeper slopes reduce IR for treated hydrophobic soils because they allow for downhill motion of water that is faster relative to the infiltration speed. 

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2. Evaluating the Recovery of Hydraulic Conductivity in Fire-Affected Soils

   Gerdes, Nolan; Webb, Ryan; Liston, Glen; Mooney, Kori (September 2024, McNair Scholars research journal)

   In the natural environment, wildfires affect how water interacts with soil leading to potentially catastrophic phenomena such as flooding, debris flows, and decreased water quality. Wildfires can cause soil sealing from increased soil water repellency, which in turn reduces infiltration and increases flood risk during rainfall. A 2017 meta-analysis found two properties that were affected by soil burning processes: Sorptivity (the capacity of a soil to absorb or desorb liquid by capillarity, S) and hydraulic conductivity (the ability for soil to transmit water when saturated, Kfs). Changes in these properties act synergistically to reduce infiltration, which increases erosion by accelerating and amplifying surface runoff. Thus, this research seeks to understand how soils subjected to severe burning compare to unburned soils. Using a mini-disk infiltrometer, field tests measured hydraulic conductivity of soils burned under slash and burn piles during the winters of 2016-17, 2020-21, and 2023-23 to better understand changes that occur in soil-hydraulic properties over time. These slash and burn piles served as approximate impacts for wildfires. Slash and burn piles also allow for paired measurements of unburned soils immediately adjacent to the burned area. Hydraulic conductivity was not significantly different when comparing burned and unburned soils 1 year after being burned. However, there was a significant difference between the hydraulic conductivity of soils burned 3 years ago compared to both unburned soil and soils burned 1 year ago. This suggests an interim process between 1- and 3-years post-burn that reduces hydraulic conductivity of burned soils. 

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3. Assessment of the Effects of the 2021 Caldor Megafire on Soil Physical Properties, Eastern Sierra Nevada, USA

   https://doi.org/10.3390/fire6020066  

   Sion, Brad; Samburova, Vera; Berli, Markus; Baish, Christopher; Bustarde, Janelle; Houseman, Sally (February 2023, Fire)

   The Caldor fire burned ~222,000 acres of the Eastern Sierra Nevada during summer–fall 2021. We evaluated the effects of this “megafire” on the physical properties of a sandy soil developed from glacial tills to document fire-induced soil modifications in this region. We measured soil water retention and hydraulic conductivity functions as well as the thermal properties of five core samples from control (unburned) areas and eight core samples from burned soil of the same soil unit. Soil water repellency was measured in terms of water drop penetration time (WDPT) in the field and apparent contact angle in the laboratory on control and burned soil as well as ash samples. Soil organic matter (SOM) and particle and aggregate size distributions were determined on control and burned soil samples. Additionally, scanning electron microscopy (SEM) was used to image microaggregates of control and burned soil samples. We found a significant difference in SOM content and sand and silt aggregate size distribution between control and burned samples, which we associated with the disintegration of microaggregates due to the fire. We found no significant difference between soil water retention and hydraulic conductivity functions of control and burned soil but observed greater variation in saturated hydraulic conductivity and systematic shifts in thermal conductivity functions of burned compared to control samples. WDPT and apparent contact angle values were significantly higher for burned soils, indicating the occurrence of fire-induced soil hydrophobicity (FISH). Interestingly, the average apparent contact angle of the control soil was >90°, indicating that even the unburned soil was hydrophobic. However, the ash on top of the burned soil was found to be hydrophilic, having apparent contact angles <10°. Our results indicate that SOM and microaggregates were readily affected by the Caldor fire, even for sandy soil with a weakly developed structure. The fire seemed to have moderated thermal properties, significantly and soil wettability but had only minimal effects on water retention and hydraulic conductivity functions. Our findings demonstrate the complex nature of fire-soil interactions in a natural environment and highlight the need for additional investigation into the causes and processes associated with FISH and structure alterations due to fire to improve our ability to rapidly determine potential problem areas in terms of hazards commonly associated with fire-affected soils. 

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4. Post-fire erosion potential of clayey sand soils and slopes – A laboratory study

   Tabassum, Tanzila (October 2021, 10th International Conference on Soil Erosion and Scour)

   One of the principal side effects of wildland fire extreme events is mass soil erosion event of the soil and slopes denuded by the fire. These soil erosion events may be devastating and extreme in their own rights, damaging critical infrastructure downslope or downstream of the fire burn scars. While there are many variables influencing the severity of post fire erosion, the amounts of soil erosion are largely dependent on the water content of the soil at the time of the fire coupled with the fire intensity. Fires that are lower intensity or soils that are “wet” at time of burning have significantly less damage to root structures of grasses and other plants while showing lessened soil erosion potential. Fires that are higher intensity or on dry soils have higher damages to root structures and increased soil erosion potential. In this laboratory study, a single clayey sand soil material common to the ground surface across the Black Hills of South Dakota and Wyoming is studied for erosion potential after burning in a controlled container burn. This material is varied by initial water content and burned at a soil surface temperature of 800 Celsius for 75 minutes, a temperature-time continuum consistent with severe wildland fire. Burning is used rather than kilns to preserve the same atmospheric conditions as in the field fire event. A laboratory soil-erosion device is then used to measure soil erosion potential across a range of fluid velocities and soil slopes. The results of this study show that the initial water content of the soil at the time of fire is a key parameter in understanding soil erosion potential post-fire. While not a complete study on time-temperature-water content across many soil types, this pilot research shows promise for future models and mapping tools. These future tools will enable planners to target resources for post wildfire erosion mitigation based on surficial soil water content at the time of the fire. 

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5. Upland and Riparian Surface Soil Processes in an Urban Creek with Native and Non-Native Vegetation after Fire

   https://doi.org/10.3390/fire5020032  

   Kinoshita, Alicia M.; Becerra, Rey; Miletić, Marta; Mladenov, Natalie (April 2022, Fire)

   Wildfires can pose environmental challenges in urban watersheds by altering the physical and chemical properties of soil. Further, invasive plant species in urban riparian systems may exacerbate changes in geomorphological and soil processes after fires. This research focuses on the 2018 Del Cerro fire, which burned upland and riparian areas surrounding Alvarado Creek, a tributary to the San Diego River in California. The study site has dense and highly flammable non-native vegetation cover (primarily Arundo donax) localized in the stream banks and has primarily native vegetation on the hillslopes. We estimated the post-fire organic matter and particle distributions for six time points during water years 2019 and 2020 at two soil depths, 0–15 cm and 15–30 cm, in upland and riparian areas. We observed some of the largest decreases in organic matter and particle-size distribution after the first post-fire rainfall event and a general return to initial conditions over time. Seasonal soil patterns were related to rainfall and variability in vegetation distribution. The riparian soils had higher variability in organic matter content and particle-size distributions, which was attributed to the presence of Arundo donax. The particle-size distributions were different between upland and riparian soils, where the riparian soils were more poorly graded. Overall, the greatest change occurred in the medium sands, while the fine sands appeared to be impacted the longest, which is a result of decreased vegetation that stabilized the soils. This research provides a better understanding of upland and riparian soil processes in an urban and Mediterranean system that was disturbed by non-native vegetation and fire. 

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