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Hohner, Amanda K ¹ ; Cawley, Kaelin ² ; Rosario-Ortiz, Fernando L ³

¹ University of Colorado at Boulder
² University of Colorado at Boulder
³ University of Colorado at Boulder

The frequency, intensity, and duration of wildfires in the western United States has been increasing, and in recent years many forested watersheds have been impacted by such events. Therefore the threat of wildfires presents a growing concern to watershed management practices and drinking water utilities. The 2012 High Park Wildfire burned the Cache la Poudre (CLP) watershed, which serves as the drinking water source for several communities in northern Colorado. In order to evaluate the impacts of the wildfire on source water quality samples were collected from April-October, 2013 from three sampling locations, including a reference location upstream and outside of the burn area. Additionally, storm samples were collected during intense rainfall events. All samples were analyzed for nutrients, dissolved organic matter (DOM) properties, and disinfection byproduct (DBP) reactivity.

Results from 2013 CLP monitoring samples showed minimal differences between sampling locations for DOC and SUVA, suggesting that the system was dominated by baseline DOC and the impact of the fire was not significant in terms of altering the DOC. However, impacts of the fire were observed for nutrients (TDP, TDN) which showed greater variability between fire-impacted and control sites, suggesting increased mobilization of nutrients following the fire. Similarly to DOC, DBP yields for impacted and control sites were not substantially different, and fire impacted organic matter did not appear to have increased reactivity. The exception being several impacted and storm event samples which showed elevated HAN yields.

To supplement the watershed monitoring study, a laboratory based sediment leaching experiment was conducted in order to simulate fire-impacted water. Sediment samples mobilized from the fire-impacted region were collected at three locations (lower, middle, and upper) along the bank and five sites along the CLP River. In addition, sediments were collected from a reference location outside of the burned area. Sediment samples from all locations were then leached into reference water from the CLP River for 6 and 24 hour periods. Preliminary results showed differences in DOC concentrations between the reference location and samples leached with fire impacted soils, ranging from 4.0 – 17.5 mgC/L. This indicates differences between the yields of carbon per gram of sediment among the sites. Aromaticity also varied with SUVA values ranging from 3.0 – 4.9 L/mg-m. The fluorescence index (FI) was higher for impacted sites (max = 1.58) compared to the control site (min = 1.33), consistent with findings from CLP monitoring samples. DBP results showed high HAA5 and HAN yields, whereas TTHM yields were more typical of previous work. The control site had the lowest HAN yields, suggesting that fire impacted water may contain considerable HAN precursor material.