Effects of sedimentation and sediment pore water chemistry on water quality in a beaver pond, Deer Lodge County, Montana

Historic smelting activities by the Anaconda Copper Mining Company in Anaconda, MT emitted large quantities of metals and metalloids into the air that fell to the ground downwind of the smelter. The Mill Creek drainage is located southeast of Anaconda and was often downwind of the smelter, so it was drastically impacted by the fallout of metals and metalloids. The chemical concentrations in Mill Creek and its tributaries are often in violation of Montana DEQ-7 acute and chronic toxicity surface water quality standards for the metals and metalloids emitted by the smelter. Stormwater runoff carries these contaminants of concern (COCs) into the streams where some of the contaminants are in a particulate form that settle to the bottom in depositional areas and contaminate the stream sediments.;A beaver pond on Cabbage Gulch, a tributary to Mill Creek, was investigated for its effect on metal and metalloid concentrations, specifically arsenic (As), cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn). Surface water samples were taken above and below the pond during stormwater runoff events in the summer of 2013. The samples were analyzed for total recoverable metals, dissolved metals and total suspended solids (TSS). Flow rates were measured at both these monitoring stations as well. The flow rates and metal concentrations were used to calculate metal loadings into and out of the pond. The metal loadings were used to calculate the fraction of the influent load removed by the pond.;Total recoverable metals and TSS removals, on a mass basis, were 25% for As, 51% for Cu, 45% for Pb, 54% for Zn and 30% for TSS. Decreases in concentrations were -17% for As, 10% for Cu, 7% for Pb, 27% for Zn and 2% for TSS. Arsenic was not removed because all As was in the dissolved phase. Flow rates were higher at the upper site than at the lower site during the sampling periods; this difference in flow rates had a large impact on contaminant removals on a mass basis. Cadmium concentrations were almost all below the minimum reportable limit.;Sediment pore water samplers (peepers) were installed in the pond. The peepers collected samples that were analyzed for the five contaminants of concern. This data showed that As and Cu diffuse from the pond water into the sediment, indicating that pond sediment does not contaminate the surface water and the sediment does not have to be removed frequently.;Sediment core samples, analyzed using x-ray fluorescence spectroscopy, showed that the COCs with the highest concentrations in the sediment solids were Cu and Zn. Temperature profiles showed that the pond was stratified in one part but not in another part, where the influent mixed the water column.;The COC removal efficiencies can be used to estimate the size beaver ponds must be in order to remove sufficient amounts of Cu, Pb, and Zn so that surface water quality standards are attained for those COCs. Arsenic is problematic in that dissolved ions will not settle in a pond.