Wildfire Effect on Mercury Levels in Putah and Cache Creek Watersheds

Science Center Objects

A serious consequence of wildfires is the erosion which occurs during storm events in areas where vegetation has been burned away. In cases where such land is near a creek or stream, mercury (Hg) and other contaminants from the eroding soil can make their way into the waterways, impacting the health of fish and wildlife, and the quality of the water supply for local communities (see: USGS Fact Sheet: Mercury Contamination of Aquatic Ecosystems).

One such scenario played out in the summer of 2015, on the eastern side of Northern California’s Coast Range, in Colusa, Lake, Napa, and Yolo counties. That summer, four wildfires burned through the watershed areas of two creeks: Cache Creek, which flows eastward from Clear Lake, and upper Putah Creek, which flows into Lake Berryessa (see upper map).

Concentrations of methylmercury (MeHg), a highly toxic form of mercury that becomes magnified as it works its way up the food chain, have been observed to increase after wildfires in sediment, plants, and animals. To understand this process more fully, better understanding is needed of the transport and methylation of mercury following wildfires.

Objective

The California Water Science Center team will assist the Bureau of Reclamation in developing models illustrating the effect of wildfire on mercury and methylmercury transport in streams. Data from Cache Creek, collected before and after the 2015 wildfires, will be used to calibrate the models.

Science Plan

The team will compare pre- and post-fire conditions as they relate to concentrations of mercury and methylmercury at various sampling sites in the Cache Creek watershed. The team will review known sources of mercury in the area. They will consider the effects that certain organic elements (such as organic carbon) have on mercury and how wildfires might alter chemical conditions in the soil. They will also study how mercury is transported through the watershed. Using sample data collected in previous studies, conceptual models will be constructed illustrating the effects of wildfire on mercury and methylmercury transport.

Water-quality data from taken sampling sites in Cache Creek before and after the 2015 fires will be used for model calibration. The USGS had previously modeled sediment transport in the Cache Creek watershed using HSPF (Stern and others, 2016) for the period 1955-2006 with an hourly time step as part of an analysis of the entire Sacramento River system. For the current project, the HSPF model will be re-calibrated for the reach of Cache Creek between Clear Lake and Rumsey (see lower map) using an hourly time step.

The initial focus of the HSPF modeling will be on Water Year 2015 (Oct. 2014 through Sept. 2015) and Water Year 2016 (Oct. 2015 through Sept. 2016). Additional water years (2000 and 2017) will be modeled to improve calibration over variety of flow conditions. Output from the HSPF model will be used to provide boundary conditions to the USBR for development of their new model describing transport of sediment, mercury, and methylmercury before and after the summer 2015 wildfires.