Effects of Wildfire on River Water Quality and Riverbed Sediment Phosphorus

Abstract

Before and after an intense wildfire in a forested area of Colorado in June 2012, river water and sediment samples were collected to study temporal and spatial trends related to the event. Water quality and soil properties were disturbed by the fire, but the magnitude was relatively small without precipitation. After precipitation, in-stream total nitrogen and total phosphorus (TP) concentrations significantly increased in the upstream section located within 10 km of the burned area. Large amounts of particulate P associated with highly correlated total suspended solids were introduced to the upstream section. Along with significantly increased in-stream concentrations of soluble reactive P (SRP) and dissolved organic P (DOP) after rain events, SRP dominated dissolved P in the river replacing DOP that was the main dissolved species before the fire event. In the riverbank, TP load increased significantly after the fire, and silt-clay and organic matter mass concentrations increased after precipitation. Riverbed TP mass concentrations decreased due to a reduced sorption capacity leading to a considerable P release from the sediments. The results indicate that fire-released P species will impact the downstream area of the watershed for a considerable time period as the bank erosion-sorption-desorption cycles in the watershed adjust to the fire-related loading.