THE IMPORTANCE OF IRON COLLOIDS IN ACID MINE DRAINAGE AND IMPLICATIONS FOR REMEDIATION

The geochemistry of Mineral Creek, a metal-rich stream in Southwestern Colorado, is quantified using data from three synoptic studies. These studies provide a detailed 'snapshot' of the watershed that includes stream chemistry and discharge. Data from the upper study reach are used in a reactive transport model to quantify the role of sorption and precipitation reactions in determining the observed spatial profiles of pH and metal concentration. Additional modeling is conducted to estimate the stream water quality that could result from two hypothetical remediation plans. Both remediation plans involve the addition of base to raise the pH of an acidic inflow to ~7.0. This pH increase results in a reduced metal load that is routed downstream by the reactive transport model, providing an estimate of post-remediation water quality. The first remediation plan assumes a closed system wherein inflow Fe(II) is not oxidized by the treatment system; inflow Fe(II) is therefore unaffected by treatment and allowed to oxidize after discharge. Under the second remediation plan, an open system is assumed, and Fe(II) is oxidized within the treatment system such that the effluent has a low Fe concentration. Both plans increase instream pH and substantially reduce concentrations of Al, As, Cu, and Fe at the terminus of the upper study reach. Under the first remediation plan, dissolved Pb concentrations are reduced by ~18%. This reduction is due to sorption of Pb onto Fe colloids within the treatment system and the instream sorption of Pb entering farther downstream. Instream sorption of Pb is made possible by the oxidation and subsequent precipitation of Fe discharged from the treatment system. In contrast, the second remediation plan results in a ~150% increase in dissolved Pb concentrations over existing conditions. As with the first remediation plan, Pb is removed from the treated inflow via sorption. Instream sorption is limited, however, because of the low Fe concentration of the treatment effluent; i.e. oxidation and removal of Fe(II) within the treatment system limits the instream formation of Fe colloids that can attenuate downstream sources of Pb. This difference in remediation plans with respect to Pb is less important in the middle and lower study reaches as iron-rich inflows provide additional sources of Fe colloids.