Evaluation of Water Vapor Sorption Isotherms to Quantify Wildfire Ash in Soil

Movement of wildfire ash into soil can alter soil hydrologic behavior and consequently influences the stability of burned hillslopes. There are no standard methods to quantify the concentration of ash in soil. This paper evaluates water vapor sorption as an index test for ash quantification. Soil and ash samples collected after the 2021 Green Ridge Fire, WA, were dry-mixed in mass-controlled ratios and the water vapor isotherms were measured. Ash gave smaller specific surface area than soil, but the maximum adsorbed water content of ash was greater than that of soil because of relative amounts of adsorbed and capillary water. The best-fit of various isotherm models was evaluated to represent the measured data. The results suggested that the Guggenheim-Anderson-de Boer (GAB) model well represents (R-2 similar to 0.99) the isotherms of ash-soil mixtures and the GAB parameter (k(b)) can be used to quantify ash in soil. However, the nonlinear increase in k(b) with ash content suggested a stepwise calibration that changes at 25% ash content.