Wildfire Impacts on Soil
Authors: Veneris Max A. 
Published: February 2025

The increasing prevalence of wildfires in today’s day and age has amplified the
need for post-fire remediation and environmental restoration, especially in the
wildland-urban interface (WUI). One of these impacts is soil hydrophobicity, or
soil water repellency, which is a well-documented phenomenon after wildfires.
Hydrophobicity can significantly impacting revegetation, slope stability,
runoff, and erosion potential. To better understand the soil fire-induced
hydrophobicity and its impact on other soil properties, variable levels of
hydrophobicity need to be repeatably induced in several types of soils under
different controlled testing conditions to measure other soil properties. To
reach this goal two ingredients are essential: (i) a method capable of inducing
controlled levels of hydrophobicity, and (ii) more robust and accurate
hydrophobicity measurement procedures. Currently, there is no robust, repeatable
method to create controlled hydrophobic soil samples. The most popular
hydrophobicity measurement method, Water Droplet Penetration Test (WDPT), which
consists of counting the seconds for a droplet to infiltrate into the soil under
testing.

Attempts were made to induce various levels of hydrophobicity onto sands in a
laboratory setting using two methods: (i) condensates of volatilized hydrophobic
organic compounds and (ii) using hydrophobicity-inducing surrogates. This was
done to achieve and measure various degrees of water repellency. The results
were highly dependent on the establishment of the correct particle coating
produced from volatilization of the fuel, and the measurement method. Low burn
severity in the field does not necessarily correlate to low burn severity in the
laboratory. Hence, the use of a hydrophobicity-inducing surrogate at various
dilution levels was selected and a method was developed to induce various
controlled levels of hydrophobicity.

On the other hand, despite its popularity WDPT is very subjective and inaccurate
when utilized outside of its one-dimensional intent. The subjectivity of WDPT
stems from the determination of full penetration and lack of precision for
accurate classification of the degree of hydrophobicity due to the lack of
calibration or correction for significant properties of the soil, including
grain size, moisture content, and particle lift. When hydrophobicity is induced,
the WDPT was shown to be unsatisfactory when used solely to determine a
consistent classification of soil water repellency. This was discovered when
consistent degrees of surrogate was added to consistent test specimens yielding
differing results. Infiltration data outlined both rapid and delayed
infiltration rates when replicated, which can be attributed to the instability
caused by particle lift, a function of degrees of repellency and grain size.
This finding shows the inability of the test method to analyze particularities
in water repellent soil as well as any insight into the impact of particle lift
on various grain sizes and nature of the degrees of water repellency.
Understanding this phenomenon can pave the way for further calibration or finer
measurement methods and scales.