DETERMINING THE ORIGIN OF PORTAL DISCHARGE BY UNDERGROUND SAMPLING, THE STANDARD MINE, CRESTED BUTTE, COLORADO

In many hard-rock-mining districts water flowing from abandoned mine portals is a primary source of metals and acidity to receiving streams. Portal discharge often is a complex mixture of groundwater from various sections of the underground workings and from the surrounding groundwater-flow system. Elevated metals in portal discharge from the Standard Mine near Crested Butte Colorado, led to its designation as an EPA Superfund site. The Standard Mine is an abandoned, underground, hard-rock mine where Ag, Pb, Zn, and Cu were mined intermittently from 1880 through the 1960s from a polymetallic vein system associated with the Standard fault. The mine workings consist of 4 levels, connected by stopes and raises, that follow a near vertical vein system. Water discharges from the portal at the lowest level (Level 1) which is collapsed. Underground sampling of the upper levels, seasonal sampling of the portal, and sampling streams and springs in the surrounding basin were undertaken to determine the sources of metals in the portal discharge. Samples were analyzed for major and trace elements, isotopes of O, H, S, Sr, and Pb, and tritium and dissolved noble gases. The pH of Level 1 discharge ranged from 5.5-6.6 except during late spring when it dropped to 3.4. Level 1 had elevated dissolved concentrations of SO₄ (up to 242 mg/L), Zn (up to 28 mg/L), Pb (up to 1.4 mg/L), Cu (up to 0.87 mg/L), and Cd (up to 0.16 mg/L). Constituent concentrations in samples from Level 3 and above were lower than concentrations of Level 1 discharge. For example, SO₄ typically was less than 111 mg/L and Zn typically was less than 9.5 mg/L. Concentrations of SO₄, Mn, Zn, and Cd in Level 2 were substantially greater compared to Level 3 water samples. These constituents approach or exceed concentrations observed in Level 1 portal discharge suggesting that increased water-rock interaction (oxygenated groundwater and sulfide minerals) between Levels 2 and 3 leads to elevated concentrations at the portal. Age dating of the portal discharge shows that it is dominated by young groundwater, and seasonal variations in the O and H isotopes are consistent with short residence times. Understanding the generation of mine-portal discharge is an important step in developing remediation plans.