16-23 Wildfire-Induced Elemental Alterations of Soils: Salem, Massachusetts

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

Wildfires alter soil chemistry and mobility dynamics, however their effects remain understudied in the Northeastern US even though wildfires are projected to double in occurrence by 2100 (Gao et al 2021). Here we study a small-scale fire that occurred in the Forest River Conservation Area, Salem, Massachusetts (August 2025). Based on our initial field observations of vegetation - no overstory trees were visibly affected - we identified this fire as low-severity. 

Soil samples representing both the burned and adjacent unburned areas (n=5 each, 10 total) were collected using methods outlined by the USGS. Homogenous samples were analyzed via X-ray fluorescence (XRF) and elemental analyzer (EA) to determine if the maximum temperature of the fire could be estimated via changes in elemental concentrations and those elements’ known volatilities.  “Volatile” elements (C, N, S, Pb, Cd) all show a decrease in concentration from unburned to burned, although none were significant. “Stable” elements (Ti, Mn, Ca, Mg, Fe, K) all show an increase, though of these, Ti and Mg are the only significant increases (p≤0.02) from 5581±794 to 6901±700ppm and 294±103 to 2198±834ppm, respectively. Some metals (As, Cu, Ni, Zn) show little difference in concentration. Data normalized against Ti showed significant decreases in C (40±19 to 18±5, p=0.04), N (2.1±0.8 to 1.2±0.2, p = 0.04), and S (0.3±0.1 to 0.2±0.002, p=0.02), in addition to the normalized Mg concentration still showing an increase (0.04±0.03 to 0.3±0.1, p=0.001).

Fire temperature can be constrained using these data.  Since organic C and N can oxidize at temperatures as low as 100°C, and since normalized concentrations are significantly lower in the burned samples, we can estimate a minimum temperature of 100°C.  Organic S typically volatizes between 300-600°C, indicating maximum fire temperature.  We can constrain this further with As data.  Organic As can volatize at 275°C, while inorganic As requires temperature of 600°C.  Since As is not significantly different in the samples, this suggests that the maximum temperature was closer to 275-300°C than 600°C and/or most As is inorganic.  Pb volatizes at 280°C, and since its concentrations are not significantly different between the sample groups, these data further support the interpretation that the maximum fire temperature was between 275-300°C.  Ongoing polycyclic aromatic hydrocarbon (PAH) analyses will help constrain our interpreted wildfire temperatures. 

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