By Catherine Ortega Klett, NM WRRI Program Manager
The 2011 Las Conchas fire in the Jemez Mountains of New Mexico has had lingering effects on nearby surface water and waterways, primarily due to the leaching of burnt soils. Such effects of wildfires are typically exacerbated by the fact that the monsoon season follows after the wildfire season. Amanda Otieno, an NM WRRI 2017 student grant recipient who is completing her master’s degree studies in Water Resource Management at UNM, has just published her final report on a field and laboratory study of the chemistry of soils exposed in varying degree to the wildfire. Her faculty advisor for this research is Professor Rebecca Bixby, of the UNM Biology Department. Additional assistance has come from UNM Professor Ali of the Earth and Planetary Science Department and Professor Cerrato of the Civil Engineering Department.
For this study, thirty-two soil samples were collected from the Sierra de los Valles dome within the Valles Caldera National Preserve (VCNP) in Jemez, New Mexico. Eight samples were collected from each of four different areas classified in terms of burn severity, labeled respectively as high, moderate, low, and unburned areas. The top three inches of soil were collected, and the soil texture type was found to be mostly loamy, containing roughly equal proportions of sand, clay, and silt, although six sandy loam samples, containing less clay, were collected from the unburned area. Equal numbers of samples were taken in winter and spring to capture some seasonal influences.
Soil samples were analyzed chemically by first partially dissolving them in strong acids to produce positively charged molecular fragments in solution (metal cations), and negatively charged counterparts (anions). Dissolved metal concentrations were then determined by inductively coupled plasma optical emission spectrometry. In this technique, small samples are sprayed into an extremely hot plasma flame, which produces highly energized metal ions which, in turn, emit light with a spectrum of wavelengths that is different for each element and can, therefore, serve as a means of identification. Since anions are often small molecular groups that would be destroyed by intense heat, a different approach, called ion chromatography, was used to determine their concentration. Chromatography involves the migration of charged molecules in a controlled chemical environment.
The research shows that the Las Conchas fire has had long lasting impacts on the chemical composition of soil in the VCNP. Twenty-two metals were analyzed and sixteen were found to be significantly different among the burn severity categories. The majority of differences were between the high/moderate and low/unburned groups for elements: Al, As, Ba, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Si, Sr, and V. Cobalt and vanadium were the only two metals showing a significant mean difference among sites for both December and April samples. Lead was one of the few elements to show differences between the high and moderate burns. Concentrations of magnesium and nickel in the low burn severity sites differed from those in the remaining three categories. The presence of heavy metals in particular is of concern for their influence on plant and animal life in these environments. Wildfires simultaneously mobilize heavy metals in the soil that are typically inert and restrained, and introduce metals from the overlying vegetation as ash deposits. Burn severity influenced the mean concentrations of metals but there was no trend indicating higher or lower mean concentrations by burn severity, whereas mean anion concentrations appeared to vary collectively with burn severity. The overall distribution of anions was higher in high/moderate burn areas; however, statistical tests showed that this outcome may have been due to chance. The only anions having a significant difference among the four burn categories were bromine in December and nitrite in April.
Soil is a medium for metals and anions to collect, but may also be a transporter of these substances when a disturbance is introduced into a system. A wildfire will disrupt the aggregate capacity of soils, making them more susceptible to erosion during precipitation events. Thus, burnt soils have a high potential for entering and affecting waterways in areas like the VCNP. Continuous inputs of these constituents may possibly affect the aquatic and/or terrestrial environment. It is therefore important to understand the distribution of metals and anions in surface soils of different burn severities, as well as their interaction with nearby surface water. This study helps to describe some of the long-term changes expected for soils exposed to wildfire, which may therefore provide additional insight into achieving better management practices. The New Mexico Environment Department, the Valles Caldera National Preserve, and the Forest Service are examples of agencies that may benefit from such analyses.
The final report for the project entitled: Characterization of Metals in Soil Contributions to Runoff Events Following Wildfires, contains many tables and graphical displays of metal and anion concentrations for the different circumstances considered in the study. These results can be viewed online by clicking here.