DISTINCTION OF DISSOLVED AND COLLOIDAL METAL CONCENTRATIONS DURING SNOWMELT RUNOFF, LITTLE COTTONWOOD CREEK, UTAH

Colloidal iron is commonly found in drainage from mines and from hydrothermally altered bedrock, and its presence complicates the regulation and monitoring of streams for dissolved metals. Iron colloids range in size from nanometers to micrometers, and some can pass through typical 0. 45-micrometer (ìm) filters, the legal size for defining dissolved concentrations. The presence of iron-rich colloids in “filtered” samples may result in an operational bias once those samples are acidified for analysis by creating “dissolved” metal concentrations that are higher than actual ambient stream dissolved concentrations. Samples of snowmelt runoff from Little Cottonwood Creek, Utah were collected downstream from the Alta mining district in 2006. Total-recoverable concentrations were consistently greater than filtered concentrations for iron, copper, lead, manganese and zinc, consistent with the observed behavior of these elements to form or sorb to colloids in mine drainage. Ultrafiltrate (10,000 Dalton) concentrations were consistently lower than those from the 0.45-ìm filtrate for iron, copper, and zinc, but for manganese and zinc the two filtrate concentrations were not substantially different. The difference between filtrates increased during the period of greatest snowmelt runoff. This was the period when the total-recoverable concentration also increased, and the difference among filtrates could result from more of the iron colloid, and sorbed metals, passing through the 0.45-ìm filters. These results suggest that the use of 0.45-ìm filtration to operationally define dissolved metal concentrations may lead to overestimates of dissolved concentration, affecting the interpretation of these concentrations relative to water-quality standards used for regulatory purposes.