Assessing Mercury Loads in Cottage Grove Reservoir

Science Center Objects

Cottage Grove Reservoir is an impoundment of the Coast Fork of the Willamette River and is managed by the U.S. Army Corps of Engineers (USACE) as part of the Willamette Valley Flood Control Plan.  The reservoir is currently under a fish consumption advisory because of elevated levels of mercury in fish tissue observed in an Oregon Department of Environmental Quality sampling survey in 2003 (Oregon Department of Human Services 2004).  A probable source of mercury to the reservoir is mine tailings from the Black Butte Mine superfund site upstream of Cottage Grove Reservoir. Assessing the dominant source of mercury into the reservoir, the controls on mercury transport and bioavailability, and the relation of mercury concentrations to streamflow is important information for the USACE and its management of the reservoir.


  1. Continue water sampling efforts in FY 2013 to provide data to refine mercury load estimates over a range of streamflows and seasons and to investigate interannual variability of mercury loadings into and out of Cottage Grove Reservoir.
  2. Estimate (a) the flux of dissolved and particulate fractions of total mercury (THg), methylmercury (MeHg), dissolved organic carbon (DOC), and suspended sediment concentration (SSC) into and out of the reservoir using continuous streamflow data along with concentrations of these constituents from 2012 and 2013 sampling events, and (b) use these data to identify dominant transport mechanisms of mercury. 


  1. Objective 1 will be achieved by continuing the sampling efforts on the inflow and outflow sites in FY 2013 at the same frequency as the FY 2012 sample events (10 samples, approximately every 6 weeks to further cover seasons and the range of the hydrograph).
  2. Upon completion of the sampling events for FY 2012 and 2013 and receipt of laboratory analyses, mercury flux into and out of the reservoir will be estimated by developing regression models for estimation of constituent loads. Explanatory variables within the regression model include various functions of streamflow, seasonal cycles, decimal time, and additional user-specified data.  Correlations between dissolved and particulate mercury species and DOC and SSC will give insight into potential dominant mercury transport mechanisms.