Identifying Post-fire Effects on Downstream Water Supplies

Wildfires and their associated consequences pose serious environmental and safety hazards. Over the last decade an average of 3.7 million acres have burned each year in the U.S. Southern California’s Mediterranean climate in particular, along with dry, extended Santa Ana wind patterns, renders the region vulnerable to periodic and intense wildfires, many of which occur at the urban fringe. Of note are the 2003 wildfires that burned over 700,000 acres across Southern California, primarily in San Bernardino and San Diego counties. These devastating fires directly killed 20 people, and a resulting post-fire debris flow killed an additional 16 people in the Waterman Canyon area of the San Bernardino Mountains. Fires struck again in 2005 in western Los Angeles County, burning the upper reaches of Malibu Canyon near the cities of Agora Hills and Calabasas. Most recently, in the fall of 2006, the fifth largest fire in California’s recorded history consumed parts of northern Los Angeles and Ventura Counties. Known as the Day Fire, it burned over 160,000 acres (approximately 250 square miles) throughout September.

Although fire is an essential and natural occurrence for most ecosystems, decades of fire suppression have resulted in unnaturally large fuel loads in much of the western U.S. In the absence of necessary, periodic fires, vegetation grows quickly and densely, often leading to excessive available fuel and, depending on climate conditions, intense wildfires. Moreover, the interface between undeveloped natural ecosystems and sprawling urban lands is highly susceptible to fire. In recent decades billions of taxpayer dollars have been spent fighting fires at the fringes of our expanding population centers. Post-fire effects can be especially devastating; history shows that flash flooding and debris flows compromise the lives of downstream populations and destroy property.

The effects of wildfires reach beyond immediate safety and extend to important environmental and potential long-term health concerns as well. For example, fires also severely impact water quality in stream systems. Increases in nutrient concentrations, especially nitrate, commonly are observed in post-fire stream systems. The mobility and availability of trace elements, including iron, aluminum and other trace metals, also are altered during fires. These enhanced concentrations in stream water adversely affect the short and long-term quality of downstream water supplies and the rate of watershed recovery (ecosystem and vegetation re-growth). More alarmingly, limited studies (mostly in Canada) suggest that wildfires may contribute significantly to mobilization (movement) of mercury in affected ecosystems. Mercury is a toxic element that is emitted from both natural and anthropogenic sources. Due to atmospheric transport, mercury is found in all ecosystems, even pristine environments far from human influence. However, erosion from altered soil properties in burned watersheds may expedite the transport of inordinate amounts of mercury out of the affected system via surface waters.

After arriving at UCLA in 2003, Professor Terri Hogue and collaborators from the University of Arizona (UA) and University of the Pacific undertook field and laboratory investigations to study the physio-chemical response of watersheds in parts of the San Bernardino Mountains burned by the 2003 fires. The National Science Foundation’s Science and Technology Center program on Sustainability of Semi-arid Hydrology and Riparian Areas at UA funded the project. This research identified significant post-fire changes in stream chemistry, including decreased infiltration of water into the soil zone and increased overland flow, related directly to altered watershed flow paths. The changes are attributed to nearly complete vegetation loss in the burned watersheds, reduced permeability of the soils and the formation of a hydrophobic (water-repellent) layer from the burning of surface litter and vegetation. Flooding and debris flows also are linked directly to these altered soil profiles and vegetation loss. Ongoing research on the San Bernardino watersheds will help improve operational hydrologic models used to predict post-fire flooding across western U.S. basins.

In a continuation of this research agenda, Prof. Hogue recently teamed with C&EE Professor Jenny Jay to launch a unique interdisciplinary program to investigate mercury cycling in post-fire watershed systems. Hogue and Jay’s goal is to better understand the mechanisms and linkages between the physical response of burned watersheds and the resulting mercury cycling within downstream reservoir-stream systems. The in-house collaboration presents an opportunity to join the complementary skills and interests of a physical hydrologist (Hogue) with those of an environmental contaminant specialist (Jay). The expectation is that the cumulative benefit derived from pooling expertise from distinct disciplines – hydrology and water resources engineering and environmental engineering – will be greater than the benefit generated by a traditional, single-discipline approach to the study of natural processes impacting human health and ecosystem sustainability.

Initial sampling of soils and runoff events in the Los Angeles area after the 2005 fires (Upper Malibu and Arroyo Seco watersheds) revealed – in contrast to normal conditions – a loss of total mercury at the soil surface in the burn area. Hogue and Jay hypothesize that this is attributed to both vaporization of surficial mercury into the atmosphere and, more significantly, erosion and transport during early post-fire rain events. In support of this, stream samples collected from Upper Malibu Creek during the first post-fire runoff event revealed dramatically elevated mercury levels. Total mercury concentrations in the burned system were 30 times higher than samples taken from a neighboring control stream unaffected by the fires, reflecting extensive mercury mobilization during early post-fire runoff events. These elevated levels raise serious concerns for downstream populations and ecosystems exposed to this environmental contaminant.

Based on these and related preliminary findings, Hogue and Jay recently received an NSF Small Grant for Exploratory Research (SGER) to investigate the impacts of the September 2006 Day Fire. Although this fire did not threaten urban areas, several major drinking water reservoirs serving Los Angeles are located downstream from the burn area, clearly presenting an immediate concern for water resource managers and consumers. One of these reservoirs, Pyramid Lake, is located on the eastern fringe of the burn area and stores water from the California Aqueduct as well as natural inflows from the Piru Creek watershed. The Hogue-Jay team is investigating post-fire conditions that may contribute to the cycling or transformation of elemental mercury into methylmercury. Methylmercury is known to be a poison to the human nervous system and has a strong tendency to increase in concentration to toxic levels as it moves through food webs. Studies suggest that small increases in exposure may affect the heart and circulatory system, and exposure during pregnancy may affect fetus development.

In addition to Hogue and Jay, the interdisciplinary research group includes PhD student Megan Burke, undergraduates Sonya Lopez and Carolina Mendez, collaborators from University of the Pacific (led by Professor Laura Rademacher) and numerous volunteers from both the Hogue and Jay research labs. To meet its research goal, the group is conducting an extensive field campaign within the Piru Creek-Pyramid Reservoir system. The field work includes sampling in burned and unburned soils, sediment and core sampling throughout the Piru Creek stream system, bi-weekly aqueous grab sampling, high resolution stream sampling during storm events, and collecting macro-invertebrates (fish) to determine bioaccumulation of mercury within various aquatic species. In the lab, researchers fractionate and analyze soils for organic content and total mercury, while aqueous samples are analyzed for various geochemical parameters, nutrients, basic chemistries as well as total mercury and methylmercury.

Preliminary results from the project were presented at an American Geophysical Union conference in December 2006 and the Society of Environmental Toxicology and Chemistry annual meeting in April 2007. The results support findings of mercury loss at the top of the soil column in burned surfaces. Increases in mercury content at greater depths are more pronounced in burned systems compared with control samples. Also, fine grain aggregates consistently register higher mercury content in both burned and control systems throughout the entire depth profile. The implication for erosion and transport from burned surfaces to downstream water bodies is clear: highly erodible soil surfaces in burned systems contribute to transport of these fine particles during runoff events and significantly impact the water quality of downstream water systems. The group is working to further elucidate the conditions conducive to the mobilization and potential methylation of mercury and evaluate the impact on aquatic biomes. Ultimately, collected data and findings will be used to develop new mechanistic and watershed scale models of nutrient, metal and sediment transport in post-fire watershed systems.

If mercury levels become problematic within this recreational stream-reservoir system, relevant state and municipal agencies must consider consumption warnings. Along these lines, the Hogue-Jay group’s work already is having a direct and practical public benefit. The team is collaborating with the National Weather Service (NWS) and the United States Geological Survey’s Debris Flow Task Force. They are working in southern California to improve post-fire debris flow warnings issued to the public through the NWS offices in Oxnard and San Diego. The prediction and corresponding mitigation of post-fire runoff is an essential and vital concern to operational forecasters, flood control districts and emergency managers who must deal with post-fire consequences. In addition, understanding mercury cycling and possible methylation in post-fire systems is critical when alerting downstream users and managers of potential bioaccumulation of neurotoxins in aquatic systems and related degradation in water quality.