Mercury controls on arsenic speciation at the Black Butte abandoned Mercury mine, Cottage Grove, Oregon

The Black Butte mercury mine near Cottage Grove, Oregon has recently been added to the Federal Clean Up List (2010). Water samples were collected from streams within the site and analysed for trace metals. Water samples were separated in the field into As(III) and As(IV) species for subsequent measurement using ICP-MS. Tailing piles on the mine site originating from the retort furnace contain cinnabar (HgS) up to 20mg kg-1, as measured using XRF, which has previously been established as the source of mercury polluting the adjacent Coast Fork Willamette River and Cottage Grove reservoir. Mercury methylation is largely confined to lake sediments with fish consumption advisories in effect.
The andesitic lavas hosting the mercury ores at Black Butte are naturally elevated in arsenic, with groundwater from wells in the region reported above the EPA MCL level of 10μg L-1.Arsenic levels in streams within the study area ranged from 0.3 – 9.6μg L-1. The highest levels were found in water flowing from the main mine adit. Over 80% of soluble arsenic is in the form of As(III), rather than As(V); this reflects the greater mobility of As(III) species. Dissolved mercury concentrations from site run-off are below detection limits; most of the mercury transportation is as sediment. Relationships between arsenic speciation and the mercury content of sediments are discussed.