Seasonal variation in metal concentrations in a stream affected by acid mine drainage, St. Kevin Gulch, Colorado

Mining of mineral deposits in the Rocky Mountains has left a legacy of acidic inflows to otherwise pristine upland watersheds. Since 1986, the U.S. Geological Survey has studied physical, chemical, and biological processes that affect the transport and transformation of metals in St. Kevin Gulch, an acidic, metal-rich stream near Leadville, Colorado. Well-known chemical processes have been quantified in the context of on-going physical transport by defining the hydrology with instream tracer-dilution experiments. These processes affect the partitioning of metals between dissolved and colloidal transport phases. In this acidic stream, pH increases during snowmelt runoff. At the most acidic stream site, pH varies from 3.15 to 4.00 during seasonal changes. Conservative effects of dilution are quantified using manganese as a natural, conservative tracer. Aluminum, copper, and zinc also are relatively conservative throughout the seasonal changes. Sulfate and iron, on the other hand, are removed with respect to manganese. The loss of iron through precipitation of hydrous Fe oxide is consistent with thermodynamic calculations. The loss of sulfate, however, cannot be fully explained.