Arsenic loads and source areas in and along Bear Creek, Park County, Montana, September 2000

Abstract

This report quantifies arsenic loading to Bear Creek and identifies the principal source areas using the results of a tracer-injection and synoptic-sampling study conducted in Bear Creek during September 2000. Arsenic loads were calculated for mainstem sites along the approximately 3-mile long study reach by combining streamflow determined by tracer-injection methods with concentration data determined in synoptic waterquality samples.Arsenic was the only trace element in Bear Creek that had concentrations greater than the minimum reporting level. Dissolved arsenic concentrations increased from 0.5 to about 11 micrograms per liter (jiig/L) downstream through the study reach. Although cadmium, copper, lead, and zinc were not detected in any water samples from Bear Creek, these metals were detected in both right-and left-bank inflows downstream from Old Tailings North. Arsenic concentrations were elevated in many right-and left-bank inflows, and ranged from 0.5 to 228 |0,g/L.Dissolved arsenic loads in Bear Creek increased by 5,817 micrograms per second (jiig/s) through the study reach. North Fork Bear Creek contributed only about 1.5 percent (90 (ig/s) of the arsenic load to Bear Creek. Subsurface inflow along a 550-foot subreach near the Former Arsenic Mill accounted for about 36 percent (2,087 |U,g/s) of the arsenic load. Inflow from Outfall 001 accounted for about 8 percent (455 (ig/s) of the arsenic load./The remaining 54.5 percent of the arsenic load entering thejstudy reach was attributed to diffuse surface and subsurface inflows.Based on quantification of arsenic loads in Bear Creek, diffuse surface and subsurface inflows that probably derive arsenic from natural weathering of the host rock collectively constitute a large source of arsenic. Reductions in arsenic loads to Bear Creek could be maximized by remediation efforts that focus on more concentrated source areas, such as the subsurface inflow along the subreach between sites 1,496 and 2,046 (near the Former Arsenic Mill) and on the surface inflow from Outfall 001 (site 3,278).