Samples were collected at or near baseflow conditions. Water pH and specific conductance were measured in the field, with specific conductance also measured in the laboratory and calculated based on the ionic strength of samples based on laboratory analyses. Water samples were collected for laboratory analyses using a peristaltic pump with silicon tubing. Samples were filtered through a reusable plexiglass filter holder with nylon screws using 0.45m polyethersulfone membranes that were rinsed with approximately 20mL of sample water prior to collection. Splits were also collected unfiltered to determine trace metal concentrations associated with suspended sediments. The filtration apparatus was rinsed with deionized water between samples. Separate filtration apparatuses were used for mining-impacted sites and background sites to minimize sample cross-contamination. Field equipment blanks were collected at the end of a field day using the full sampling apparatus used for mining-impacted sites to determine carryover of constituents between samples. Samples were chilled to approximately 4C and transported by overnight shipping to USGS analytical laboratories. Mercury and methylmercury were analyzed in the USGS Mercury Laboratory in Middleton, WI. Note that mercury concentrations in the field blanks exceeded concentrations in sites considered to be background. Sources of sample contamination may be the filter, which was not pre-cleaned, and the filtration apparatus that was previously used in stream reaches directly impacted by mercury mining. The detection limit for mercury was <0.04 ng/L. Field blanks had 1.96 and 2.45 ng/L Hg.
Streambed sediment samples were collected as grab samples and stored in pre-cleaned glass jars, frozen on dry ice to minimize microbial activity associated with the formation and degradation of methylmercury. Sediment samples and shipped to USGS laboratories in Denver, CO for total elemental analysis, including mercury, and to Middleton, WI for methylmercury analysis. Sediment was dominantly fine to coarse sand trapped between cobbles in a high-energy stream environment. Samples were sieved to less than 2mm and hand-crushed to homogenize prior to trace metal analysis in Denver.