Exploring wildfire impact on post-fire runoff water quality: field and laboratory investigation

Abstract:

Wildfire can have complex effects on physical and chemical properties of soil and post-fire runoff. Water quality issues in the post-fire runoff may have caused catastrophic events in aquatic ecosystem in the Virgin River in Southwestern U.S. To examined the mechanisms of the impact of wildfire on post-fire runoff water quality, field sampling and experiments were conducted on surfaces of various fuel types at a burned site in the Virgin River Watershed. Rainfall simulation tests were performed to generate runoff for water quality test including in-situ DO and pH measurement and laboratory tests on a number of water quality constituents. Soil/ash samples collected from burned surfaces were applied in a laboratory test to produce solutions of different concentrations and DO changes over a 24-hour period were measured. Results confirmed that, for runoff carrying large amounts of sediment or debris, DO values can be substantially reduced to a level close to or lower than 5 mg/L. Fire effects may enhance this trend, but is not necessarily a critical reason for the reduction of DO levels.

Laboratory runoff and soil sample analysis show that the post-fire runoff in this watershed may contain a large amount of ammonia (NH₃ and NH₄⁺). The concentration of ammonia can be higher than the lethal level to many (if not all) fish species. Fire effects appear to have a significant impact on the ammonia level, which lead to an increase of several times to one order of magnitude in the ammonia concentration in the runoff or soil solution under burned conditions. These results provide information to better understand post-fire water quality in this and similar watersheds.