The Fires forestry can trigger the transformation of minerals present in the soil, converting harmless elements into dangerous pollutants that endanger groundwater quality for years to come.
This was suggested by a study by the University of Oregonpublished in Environmental Science and Technology. The work shows that the Chrome, an essential micronutrient in its natural form, could become one toxic heavy metal after exposure to extreme temperatures caused by fire long-term environmental risks in fire-prone regions.
The study led by Chelsea ObeidySoil scientist at California Polytechnic State University, Humboldt, and carried out in the professor’s laboratory during his doctoral thesis Matthew Polizzotto at the University of Oregon focused on analyzing how chromium 3, the predominant and harmless form of this element in the environment, is converted into chromium 6, a Class A carcinogen linked to lung, sinus and nasal cancers.
Obeidy and his team collected soil samples in Eight Dollar Mountaina hill of Rogue River-Siskiyou National Forest in southwestern Oregon, characterized by chromium-3-rich serpentinite deposits and increasing wildfire risk.
The research simulated wildfires in the laboratory by exposing soil samples to temperatures between 200 and 760 °C (392 and 1,400 °F) for two hours. The results showed that fires with temperatures between 400 and 590 °C (750 and 1,100 °F) produced the most chromium-6, although the location of the soil on the slope influenced the optimal temperature for this conversion.
On the peaks and surrounding areas, where erosion is greater and the rock breaks down and releases more chromium 3, the highest production of chromium 6 was observed at around 400 °C (750 °F). Instead, chromium 6 occurred at the base of the slope primarily at temperatures around 590 °C (1,100 °F).
To assess the possibility of groundwater contamination, the team simulated rain leaching: They filled plastic columns with the burned earth and allowed rainwater to flow through them for a week, simulating about half of the area’s annual rainfall.
By analyzing the drained water, it was possible to identify the locations on the hill where there is the greatest risk of chromium 6 being released into groundwater. According to the results, depending on the location on the slope, chromium 6 could keep water pollution above the limits US Environmental Protection Agency (EPA) for periods ranging from six months to almost two and a half years.
Obeidy emphasized the importance of these findings for environmental management in fire-affected regions: “This could have lasting effects on a burned landscape. Maybe we should study burned environments in these specific rock types.”
In addition, he noted that lower temperatures, such as those achieved in prescribed or cultural combustion, do not appear to produce significant amounts of chromium-6, although he cautioned that this aspect requires additional study.
The teacher Matthew Polizzotto highlighted the complexity of soils and the need for a detailed approach to risk assessment: “Soils tend to be very variable. They change on very small spatial scales. If we want to assess risks, we need to know the extent to which things can vary from place to place.”
Obeidy noted that there are already efforts to analyze other heavy metals in post-fire scenarios, such as manganese, lead and nickel, which can also be incorporated into the soil and leached into water sources after fires.
“Conducting various metal tests after wildfires could provide valuable information,” the researcher said, emphasizing that the type and extent of pollutants vary significantly, especially in landscapes with human presence.
The research highlights the need to expand post-fire environmental monitoring protocols to include specialized testing for chromium 6 and other heavy metals in soils rich in minerals susceptible to oxidation. The team warns that the conversion of chromium 3 to chromium 6 depends not only on the temperature reached during the fire, but also on factors such as erosion, mineralogical composition and the topographical location of the soil.
Obeidy explained that the motivation for the study came from the increase in the number and severity of fires in the Pacific Northwest: “We wanted to see if there was a connection to the pollutants there and whether the fires might be mobilizing pollutants of interest.”
The researcher emphasized that while chromium 6 is usually associated with industrial processes, wildfires can produce this pollutant from natural sources, expanding the spectrum of environmental risks in fire-prone areas.
The study also warns that the spatial variability of soils can make it difficult to predict critical contamination points, reinforcing the need for detailed sampling adapted to the geological and topographic features of each region. Polizzotto concluded: “We are really in the early stages of determining all the things we need to know.”
The investigation of the University of Oregon provides evidence of conversion of soil minerals into pollutants following wildfires and highlights the urgency of reviewing and expanding environmental monitoring protocols to protect water quality and public health in vulnerable areas.
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