By Catherine Ortega Klett, NM WRRI Program Manager
Amanda Otieno received a master’s degree from the University of New Mexico’s Water Resources Program in December 2017. In 2017, she was awarded an NM WRRI Student Water Research Grant for a study on the effects of the 2011 Las Conchas fire on soil chemistry (see July 2017 eNews), under the advisement of Dr. Rebecca Bixby of UNM’s Department of Biology. Amanda is now working for a precious metals company in Rio Rancho as a chemist.
Amanda is a co-author on a recently published paper entitled, Metal Reactivity in Laboratory Burned Wood from a Watershed Affected by Wildfires. The paper was published in 2018 in Environmental Science and Technology (Environ. Sci. Technol. 2018, 52, 8115-8123). Authors include Asifur Rahman, Eliane El Hayek, Johanna M. Blake, Rebecca J. Bixby, Abdul-Mehdi Ali, Michael Spilde, Amanda A. Otieno, Keely Miltenberger, Cyrena Ridgeway, Kateryna Artyushkova, Viorel Atudorei, and José M. Cerrato. Both Asifur Rahman and José Cerrato also have received funding through the NM WRRI.
The paper describes how wildfires, such as the Las Conchas fire, have many lingering effects on local water quality, primarily due to the leaching of metals from burned soils and wood ash. The main objective of the reported study was to investigate the interfacial processes affecting wood ash reactions with water by integrating laboratory experiments, spectroscopy, microscopy, and aqueous chemistry methods. The release of metals and dissolved organic carbon (DOC) was assessed in batch experiments reacting laboratory burned wood ash with water. Additional experiments were conducted to investigate sorption processes that affect ash−metal interactions.
Results from the soil analysis showed that soils from different burn severities have different concentrations of major elements including iron, copper, chromium, zinc, and manganese. For example, iron was substantially higher in the high severity burned soils, compared to the unburned soils. This soil chemistry information was essential in understanding the composition of ash from different tree species. Burned soil and ash in contact will naturally exchange metallic ions, so that the determination of the concentrations of trace metals in ash as a function of temperature, which was the primary focus of this report, was supported by knowledge gained through the concurrent analysis of burned soils.
The investigation of soil chemistry changes from different burn severities help to establish the physical factors that control the metal and DOC content associated with the water leaching of ash produced by different trees, such as Pine, Spruce, and Aspen, and at different burn temperatures. An interesting qualitative result is that many metals are found to leach from the ash into water initially, but then to subsequently reassociate with the ash surface over time. For example, up to 94% of copper ions were removed from water solution over a three-hour period, by subsequent readsorption to Pine ash created at a burn temperature of 350⁰C. This ash also produced higher DOC concentration in water compared to ash burned at higher temperatures.
Results, such as these, may provide additional insight when considering watershed management after a wildfire, specifically one with different burn severities. The detrimental effect of the transport of wood ash into nearby streams can be more properly evaluated if the effect of burn intensity of the mineralogy and reactivity of metals associated with wildfire ash is better understood. Post-fire recovery strategies might be improved by taking into account the affected types of vegetation and the burn intensities experienced. The New Mexico Environment Department and the U.S. Forest Service are examples of agencies that may benefit from such analyses.