The transport and accumulation of pyrogenic black carbon in recently burned watersheds and implications for water quality

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The intensity and frequency of forest fires have increased since the 1950s and are predicted to continue to increase as a result of past fire management strategies, climate change, and shifts in land use. Black carbon (BC), in the form of soot and char, is generated by incomplete combustion of organic matter and is transported and re-deposited throughout downstream valleys and waterways by post-wildfire rainfall and flood events. Black carbon from wildfires has the potential for both negative and positive impacts on water quality. In the short term, for example, it may foul drinking water supplies and generate of anoxic conditions that are harmful to aquatic life. But the BC deposits produced on the landscape may act as carbon sinks and may sequester significant quantities of contaminants and heavy metals through sorption. This project will study the impacts of two recent wildfires in northern New Mexico with the goal of understanding the transport, accumulation, and residence times of BC in post-wildfire watersheds, as well as the environmental significance of these BC deposits. Black carbon will be measured in soils within burned areas, downstream of burns, and in areas unaffected by wildfires. Samples will be taken before and after two summer monsoon seasons, when mobilization is expected to occur, in order to understand transport and accumulation rates. Laboratory analysis will measure the amount of several heavy metals and burn-byproduct contaminants contained within the soil samples, and what percentage is held within the BC component of the soils. Further analysis will quantify the maximum capacity of the BC in the soils to sorb out and sequester these contaminants. Black carbon is important to consider for at least three reasons: global carbon cycling, contaminant sequestration, and soil fertility. BC has traditionally been viewed as a chemically stable form of carbon, with suggestions that the addition of BC to soils as an amendment could serve as a carbon sink. Recent work, however, has found that BC is removed from the soil more rapidly than previously thought. Researchers have found significant quantities of BC dissolved in river water, which may mean that water percolating through the soil dissolves out some BC. Erosion and transport of BC-containing sediment is an additional way by which BC may be removed from soils, and this mechanism has received much less attention. This work will contribute to quantification of this poorly understood component of the carbon cycle. The findings are anticipated to be particularly relevant in areas that utilize fire-prone forest watersheds as a source of drinking water, which is the case for most of the southwestern US. The presence of black carbon in the soils of watersheds that are municipal water sources may provide an additional buffer to contamination risks. The results will also assist managers in developing post-fire recovery plans and may also be useful for ecological risk assessment following chemical spills. This work will support the training of two hydrologists at the MS level and provide a research experience for one undergraduate from New Mexico Institute of Mining and Technology, a Hispanic serving institution.