Dead rainbow trout in the Big Tujunga Watershed during the 2009 Station Fire, California. (Credit: U.S. Geological Survey)
Many studies to assess the impacts of wildfires on the water quality of streams have only looked at how they affect sediment loads or nutrient levels, but relatively few investigations have considered what effects there are on trace elements.
In a project to measure the impacts on levels of trace elements to Southern California streams after the 2009 Station Fire, researchers with the U.S. Geological Survey (USGS) have sampled a number of waterways in the San Gabriel Mountains north of Los Angeles. Levels of some elements, including iron, lead, nickel and zinc, were elevated after the fire to concentrations higher than standards set by the U.S. Environmental Protection Agency for protecting aquatic life.
During and after the fire, scientists gathered discrete water samples from 13 sites. Some of the sites were located within the fire perimeter while others were located outside of it.
Water samples were collected from all 13 sites after the start of the fire but prior to the first storm of the winter season (pre-storm) to represent water quality in the unburned watershed under base-flow conditions. The samples were divided into three groups based on site location and type of sample: pre-storm samples, stormflow samples located at sites outside of the burn area, and stormflow samples located at sites inside the burn area.
The samples were assessed using standard USGS procedures with help from the agency’s National Water Quality Laboratory in Denver, Colorado and its Sediment Laboratory located in Santa Cruz, California.
Filtered concentrations of iron, manganese, mercury and total concentrations of most trace elements in storm samples were elevated as a result of the fire. In contrast, scientists found that concentrations of copper, lead, nickel and selenium and total concentrations of copper were elevated primarily due to storms and not the Station Fire. Total concentrations of selenium and zinc were elevated as a result of both storms and the Station Fire.
So what do those results mean for aquatic life? USGS researchers say that some at-risk frog populations might be affected by water chemistry changes due to fire in their habitats. But overall, the findings can help to inform resource managers about the potential risks wildfires pose to fish and frogs that live in these watersheds, a few of which are critically endangered and close to extinction.
The scientists also recorded extremely high pH levels, up to 12, in water leachates of ash samples collected from burned landscape areas prior to storm events. These levels can adversely affect terrestrial and aquatic life. Such high pH values were not found in the storm water samples, possibly due to dilution and chemical reactions with rainfall.
In addition, many of the creeks studied feed into public drinking water reservoirs. While water from these reservoirs is tested and treated before supplies are delivered to customers, results will help water treatment managers prepare for future water quality issues arising after fires and storms.
The Station Fire in the Angeles National Forest was the largest of 20 wildfires in Southern California in 2009, burning from Aug. 26 to Oct. 16. It is the largest fire ever recorded in Los Angeles County.
The USGS study area included portions of eight watersheds, of which approximately 680 square kilometers were burned by the fire. Full results of the investigation, including more on methods involved, are published in the open-access journal PLOS ONE.
Top image: Dead rainbow trout in the Big Tujunga Watershed during the 2009 Station Fire, California. (Credit: U.S. Geological Survey)