oa A Comparison Of Natural Acid Rock Drainage Occurrences In Colorado

A difficult question that arises in matters related to metal mining concerns the<br>background concentration of elements in natural waters prior to mining. The answer to this<br>question speaks to both regulatory limits for operating facilities as well as cleanup and<br>remediation practices for abandoned mines. This study characterizes the aqueous geochemistry<br>of naturally-occurring acid rock drainage (ARD) for three historic mining districts of Colorado<br>and considers the magnitude of the initial impact as well as natural amelioration processes.<br>Water quality data are reported for seeps and streams in drainage basins with weathering<br>sulfides hosted in metasediments, metamorphic terrain, and highly altered volcanics. Water<br>samples and field parameters were collected from both natural ARD seeps, and from within<br>natural ARD streams. Water samples were analyzed at the Colorado Department of Health for<br>dissolved Al, Ca, Cu, Fe, Mg, Mn, K, Na, SO4, and Zn. Additionally, select samples were<br>analyzed for dissolved As, Cd, Cr, Co, Pb, Hg, MO, Ni and Se. Results indicate that two types of<br>water contribute to natural ARD: 1) dilute ARD, influenced by storm events; and 2) concentrated<br>seeps. The naturally-occurring ARD from concentrated seeps showed a range in pH from 2.39 to<br>4.49 and TDS from 1890 mg/L to 2 18 mg/L. The naturally-occurring ARD from stream samples<br>showed a similar range in pH, but with TDS measurements as low as 18.3 mg/L.<br>Naturally-occurring ARD is observed to be attenuated by precipitation of secondary<br>solid phases and simple dilution. The precipitation of ferrihydrite and other minerals, as<br>determined by extraction analysis and equilibrium modeling, accumulate to create ferricrete<br>mounds. This process removes iron and trace metals from ARD solutions. The concentrated<br>ARD seeps negatively impact the water quality in receiving streams, but are quickly diluted by<br>the primary stream flow and/or storm events. This is evident by low flow volumes having high<br>concentrations, and high flow volumes having low concentrations. The combination of<br>precipitation and dilution is observed to restore the water quality of one stream within 12<br>kilometers.