In Southern California, United States (US), a post-fire monitoring study helps stormwater managers develop strategies to prevent downstream water contamination. Rebekah Guill of the Riverside County Flood Control and Water Conservation District and Garth Engelhorn of Alta Environmental, an NV5 Company, explain how post-fire stormwater runoff affects water quality, based on scientific data.
In August 2018, the “Holy Fire” caused devastating damage to approximately 23,000 acres of the Cleveland National Forest in Southern California. The high-to-moderate burn severity of the fire led to a loss of vegetation, created hydrophobic soils, changed the soil erosiveness of the steep forested lands, and created a risk of mass wasting (e.g., slope movement or mass movement of soil and rock).
The altered landscape stability and the potential for mud- and debris-flows created an immediate concern for not only the safety of the community downstream, but the potential for water waterbodies – including Lake Elsinore and Temescal Creek. Lake Elsinore is a natural freshwater lake that has an "impaired" designation under Section 303(d) of the Clean Water Act with an established Total Maximum Daily Load (TMDL) for nutrients. In Southern California watersheds, the contaminants elevated in post-fire runoff are often the same constituents already elevated in the receiving water. Research has identified that ammonium- or phosphorous-based compounds used to fight fires may contribute to elevated nutrient concentrations in stormwater. Coordinated post-fire monitoring is essential to provide information that can be used to manage, repair, and protect sensitive waterbodies from the effects of post-fire runoff.
The Riverside County Flood Control and Water Conservation District (District) played an important role in assisting the local authorities and agencies responding to this emergency within the affected community of Lake Elsinore. In addition to assisting with evacuation mapping, assessing erosion risks, and providing volunteers to communicate to the residences, the District anticipated severe debris flows to come from the large footprint of the burn area in the coming winter months.
Crews immediately began preparations to install new real- time surveillance and Automatic Local Evaluation in Real Time control measures, and debris- resistant retrofits to the District’s debris basins, and align resources in anticipation of flood emergency response that would be needed during the first intense storms of the winter season. As part of this proactive thinking, the District’s Watershed Protection Division also took a specific interest in the stormwater discharges that would likely be entering Lake Elsinore.
The District serves as the principal permittee in the Santa Ana River Region of Southern California as regulated by the municipal separate storm sewer system (MS4) National Pollutant Discharge Elimination System (NPDES) Permits, and undertook a post-fire monitoring study on behalf of the permittees within the Santa Ana River Region of Riverside County to assess the of the Holy Fire. In coordination with Alta Environmental, an NV5 Company, with feedback from the Regional Water Board staff as well as the Lake Elsinore and Canyon Lake TMDL Task Force, the District developed a monitoring plan. The plan was based on the guidance included in the Post- Fire Water Quality Monitoring Plan prepared by the Southern California Coastal Water Research Project and Southern California Stormwater Monitoring Coalition (SMC), titled “Effects of Post-fire Runoff on Surface Water Quality: Development of a Southern California Regional Monitoring Program with Management Questions and Implementation Recommendations.”
Water quality monitoring of post-fire runoff
The monitoring design focused on addressing one priority SMC Post-Fire Water Quality Monitoring Plan: How does post- fire runoff affect contaminant flux? The goal of the study was to assess contaminant concentration and flux by sampling stormwater runoff from the terminal end of burned catchments. The data were compared to reference sites to assess the effects of the Holy Fire on the hydrologic response, sediment loads, and contribution of pollutant loads (metals, nutrients, and organic contaminants) from post-fire runoff. Flux calculations were used to compare the relative mass contributions of contaminants from the burned catchments versus the unburned natural areas.
A key factor that influences post-fire erosion and increased runoff flow is fire-induced soil water repellency, which increases after the soil is burned and can reduce watershed infiltration rates after a wildfire, according to the US Forest Service. Consumption of the rainfall-intercepting canopy and of the soil-mantling litter and duff, intensive drying of the soil, combustion of soil-binding organic matter, and the enhancement of formation of water-repellant soils can result in decreased significantly increasing overland flow and runoff in channels. The above-average rainfall with numerous high-intensity rainfall events during the 2018-2019 winter storm season resulted in increased storm flows, channel erosion, and sediment runoff from the burned catchments throughout the wet-weather season.
Three primary sampling locations were identified in the study and included two sites at the terminal end of burned catchments and one reference site from an unburned catchment of similar size and land cover. The burned catchments were chosen to evaluate runoff discharging to downstream proximate receiving water bodies, including Lake Elsinore. During two storm samples and storm discharge volume estimates (flow) were collected from the three primary sampling locations, along with visual observations from seven additional locations.
Safety is a key consideration for monitoring at the terminal end of burned catchments. High- intensity, short-duration rainfall rates are the primary cause of debris flows and in order to capture study, field teams were mobilized within active evacuation zones during significant storm events. The unpredictable nature of the post-fire runoff and evolving site communication amongst project and real-time modifications to successfully capture post-fire runoff while ensuring the safety of the field staff. Fortunately, the monitoring sites for this study were located downstream of the District’s debris basins, which provided safe and accessible sampling locations during the debris flows.
As expected, the water quality results indicated that the pollutant concentrations from the burned catchments were generally higher than the reference site. However, the pollutant concentrations from the post-fire runoff were significantly higher than what is typically seen in urban stormwater discharges. The first storm event was the “first flush” from the burned catchments following the fire and significant post-fire sediment and debris flows were observed. The concentrations from the burned catchments were generally highest during the first storm event in November 2018 and lower during the second monitored storm event in January 2019. This was consistent with the expectation that the first flush of runoff following a fire usually contains the highest concentrations of contaminants.
How does post-fire runoff affect contaminant flux?
The contaminant flux was calculated as the ratio of the mass loading in kilograms (kg) and the contributing catchment 2) for each storm monitoring event. Mean total phosphorus and total nitrogen flux were estimated to be between 69- and 98-fold higher from burned catchments, and total copper, lead, and zinc flux were estimated to be between 659- and 11,169-fold higher compared to unburned natural areas. Mean total suspended solids flux were estimated to be 27,177-fold higher from burned catchments compared to unburned natural areas. Similar to the pollutant concentrations, the flux from the burned catchments were lower during the second storm event compared to the first flush storm event – indicating the attenuation of contaminant concentrations and loads decreased as the winter storm season continued to experience numerous high- intensity rainfall events.
In a similar study from 2012 titled, “Stormwater Contaminant Loading Following Wildfires,” post-fire stormwater runoff from wildfires in Southern California was assessed and mean copper, lead, and zinc flux were between 112- and 736-fold higher from burned catchments, and total phosphorus was up to 921-fold higher compared to unburned natural areas. Key findings of the 2012 study revealed elevated flux values appeared to be driven mainly by rainfall magnitude while contaminant loading from burned landscapes has the potential to substantially contribute to the total annual load to downstream areas in the first several years following fires.
The contaminant flux results characterized the potential water waterbodies of Lake Elsinore and Temescal Creek. This 2018 Post-Fire Stormwater Monitoring study provided stormwater managers and stakeholders with data to evaluate the post- fire contribution of nutrient loads in context with other sources within the watershed. Understanding the effects of the Holy Fire on contaminant flux provides information that can inform management actions, including strategies used to comply with nutrient TMDLs.
Post-fire pollution prevention effort
In anticipation of the increased storm flows and sediment runoff following the Holy Fire, the District also implemented post-fire debris flow clean-up operations to improve and maintain overall debris basin capacity, capture eroding sediment mobilized by major rain events, protect downstream communities, and prevent further impacts to downstream receiving waters. The District's response protected the communities affected downstream by capturing approximately 136,781 cubic meters (178,904 cubic yards) of post-fire mobilized sediment from District facilities and diverted the material to local landfills for disposal.
As part of this effort, the District conducted a Sediment Quality and Nutrient Load Reduction Study to evaluate the nutrient loads removed from District basins. From September 2018 through April 2019, the estimated amount of nutrients removed from two key District flood control facilities were found to have captured nearly 7,530 tons of total nitrogen and 120 tons total phosphorus. The District's effort prevented further by preventing these nutrient loads from entering Leach Canyon Channel, and ultimately into Lake Elsinore.
Tim Moore of Risk Sciences (based in Brentwood, Tennessee, US) serves as a TMDL compliance expert for the Lake Elsinore and Canyon Lake TMDL Task Force and recognizes the value of the District’s effort in capturing and diverting the Holy Fire nutrient load from entering Lake Elsinore. “The District’s actions prevented a mass extinction event at Lake Elsinore,” he said.
The District gained knowledge from the experience of the Holy Fire, which is now being applied for future emergency planning should the need again arise. Likewise, the scientific data gained from their research efforts is available to build upon and help in future management decisions.
Rebekah Guill is the monitoring programs manager at Riverside County Flood Control and Water Conservation District in Riverside, California. Garth Engelhorn is water resources senior project manager of Alta Environmental, an NV5 Company, headquartered in Long Beach, California.
This article is based on the conference paper “Post-Fire Debris Flows are Expected, Communities are Evacuating, and It's Time to Sample Because That's What We Do! Key Takeaways from Evaluating Contaminant Flux of the 2018 Holy Fire" presented at the 2019 California Stormwater Quality Association (CASQA) Conference. Contact the authors for a complete reference list at Garth.Engelhorn@altaenviron.com or rguill@RIVCO.ORG.
Click here to read this article on the World Water: Stormwater Management website.