Ecological - Second Order
Forested watersheds supply drinking water for millions of people in the United States. The increased frequency and severity of wildfires during recent decades have elevated public concern regarding source water protection. Large, high-severity wildfires alter the physical and biological conditions that determine how watersheds retain and release nutrients and regulate stream water quality. The short-term water quality impacts of severe wildfire are often dramatic, but little is known about whether significant post-fire changes persist and what such changes mean for post-wildfire watershed resilience, sustained delivery of clean water from headwater catchments and challenges for water treatment. Our overall objective was to assess the effects of the largest fire in recorded Colorado history, the 2002 Hayman Fire, on stream water quality. Specifically, we set out to 1) examine stream nitrogen (N) and carbon (C) concentrations and export, 2) quantify the abundance and composition of precursors of potentially hazardous byproducts of treatment water chlorination (e.g., disinfection-by-products, DBPs) including trihalomethanes (THMs), haloacetonitrile (HANs), chloral hydrate (CHD) and haloketone (HKT) and 3) evaluate the chemical composition of C contained in soil organic matter. We sampled stream water and soils in burned and unburned catchments during 2015 and 2016, nearly 15 years after the wildfire. For stream nutrients, sediment and stream temperature we were able to compare current conditions with those found 1-5 years after the fire, and with pre-fire conditions. We analyzed soil C using pyrolysis Gas Chromatography / Mass Spectrometry (PyGCMS) and developed an automated data management and analysis pipeline to process and interpret large volume of chemical analytical information. The headwaters of the Upper South Platte River have yet to recover from the 2002 Hayman Fire. We found that stream nitrate, total dissolved N (DTN) and C (DOC) all remained elevated in burned catchments. The extent of a catchment that burned or that burned at high severity had a strong effect both on stream N and C. Nitrate-N was more than an order of magnitude higher in streams draining extensively-burned catchments (>60% of their area) compared to unburned catchments. In contrast to unburned catchments that retain more than 95% of atmospheric N inputs, extensively-burned catchments release more than half of N inputs. Conversely, DOC was higher in streams with moderate wildfire extent. Accordingly, DBP formation was generally greater in these catchments compared to unburned ones. Importantly, precursors of US EPA-regulated THMs (a group of potential carcinogens) were almost double in moderately burned compared to unburned catchments. We found that soil organic matter in burned landscapes had higher percentages of stable aromatic hydrocarbon and nitrogen-containing compounds but lower percentages of lignin compounds and phenol compounds than unburned soils. The slow return of forest vegetation is likely to contribute to the lasting stream and soil biogeochemical responses to the Hayman Fire. Annual, remotely-sensed estimates of vegetation cover and field trials both suggest that is will take several more decades before forest canopy cover, and expected nutrient demand, return to pre-fire levels. High stream N coupled with low dissolved organic C in burned catchments suggests that C may have replaced N as the primary limitation to in-stream production. Though the persistent stream N increases we report are below human health, drinking water thresholds, they exceed ecoregional reference stream concentrations and demonstrate that extensively-burned headwater catchments no longer function as strong sinks for atmospheric N in the Upper South Platte drainage.