Seasonal Changes in Biogeochemistry of a Natural Wetland Receiving Drainage from an Abandoned Mine Diane McKnight and Eric August – University of Colorado at Boulder, Institute of Arctic and Alpine Research

Acid Mine Drainage Formation FeS 2 (s) + 7/2O 2 + H 2 O  Fe SO H + Fe /4O 2 + H +  Fe /2H 2 O FeS 2 (s) + 14Fe 3+ + H 2 O  12Fe SO H + Fe 2+  Fe 3+ + e - or Fe 2+ Fe 3+ FeS 2 (s) Iron, sulfate and acid released into water Additional metals dissolve that are associated with pyrite or in surrounding rock: Zn, Pb, Mn, Cu, Ag, Al

Conventional Treatment of AMD Difficulties and Challenges * Mining in Colorado essentially stopped after the 1950’s * AMD Referred to as the greatest water quality problem in Western U.S. (Mineral Policy Center, 1997) * $$$ needed for chemical usage and maintenance * Requires accessibility and infrastructure in relatively isolated areas

Wetlands as a treatment to AMD Advantages * low cost – construction costs can be recovered within one year of operation, through savings in chemical usage (Kleinmann, 1989) * no continuous maintenance Disadvantages * long-term potential unknown * limited knowledge concerning AMD wetlands effect on watershed scale

Biogeochemical Processes in AMD wetlands 1. Ion exchange and adsorption 2. Complexation with organic matter Sedimentation 4. Plant uptake 5. Oxidation and precipitation of metal oxides 6. Sulfate reduction and precipitation of metal sulfides

Dinero Mine operated part of Sugar Loaf Mining District Dinero Tunnel and all Lake Fork Creek sites sampled 30 times: Sept (avg. every 12 days) Samples collected 3 times on each sampling date (8 am, noon, 5 pm) Sites within wetland sampled during summer

Groundwater comprises majority of flow from Dinero Tunnel (Westside Eng., 1983)

Fe: 90% Fe 2+ 85% dissolved Mn:100% dissolved Zn:100% dissolved Al:15 ug/L Cu:ND Cd:12 ug/L Pb:31 ug/L DOC:0.7 mg/L pH:6.4

Research Site §Lake Fork Creek and El Rojo Wetland located 5 miles west of Leadville, CO 50 m Site Fe (mg/L) pH inflow U U M L Fe OH -  Fe(OH) 3

Research Site §Lake Fork Creek and El Rojo Wetland located 5 miles west of Leadville, CO 50 m Site Zn (mg/L) pH inflow U1, ,3.9 M1, M L L

Research Site §Lake Fork Creek and El Rojo Wetland located 5 miles west of Leadville, CO 4 m 2 vegetation plots Divided into 4 sections and sampled for metals (Fe, Zn, Mn) and biomass – sampled every 30 days Concentration & biomass data combined to estimate mass of metal bound in vegetation

Total Mass: 1.1 kg 2.2 kg 2.4 kg 3.0 kg 3.1 kg

High zinc and sulfur sediment concentrations indicate precipitation of ZnS – mostly in mid and lower reaches of wetland ~ 46,000 kg of zinc in sediment (peat) of wetland

Total mass of zinc stored in wetland on par with amount retained during

Wetland became source of Zn in winter and snowmelt through tailings became source in May and June

High mass loading of zinc calculated during snowmelt period (early April – mid June)

Loading estimated from tailings only accounts for 1/3 of difference between ÄLFC and Dinero Tunnel mass flows Tailings source underestimated All inflows not measured Flushing in wetland by snowmelt

Research Site §Lake Fork Creek and El Rojo Wetland located 5 miles west of Leadville, CO 50 m Site Mn (mg/L) pH inflow U1, ,3.9 M1, M L L

Total Mass: 11 kg 23 kg 24 kg 31 kg 33 kg

High manganese and sulfur sediment concentrations indicate precipitation of MnS possible – with no clear spatial variation observed ~ 58,000 kg of manganese in sediment (peat) of wetland

Total mass of manganese stored in wetland on par with amount retained during

Stoichiometry indicates S and Mn + Zn would fall on line of slope = 1 Precipitation of MnS and ZnS likely Form of Mn and Zn in sediment not fully tested Other possibilities: metal-organic complexes, metals sorbed to inorganic species, metals sorbed to other metal oxides

Fe concentration relatively constant through seasons 20 times less than chronic toxicity level Mass flow controlled by LFC discharge

Conclusions Wetland retained over 50% of all metals flowing from Dinero Tunnel during summer months. Aerobic precipitation of iron oxides and anaerobic precipitation of manganese and zinc sulfides were the dominant removal mechanisms. High loading of metals from the tailings during snowmelt was a critical time of year – intensified by unnatural flow conditions downstream of Sugar Loaf Dam.

General Conclusions Seasonal trends (mine, wetland, stream) are important in understanding ecosystem impacts, making water management decisions, and planning remediation. 2. Wetland efficiency in removing metals may decrease with age. 3. In remediation of AMD sites, the role of mine tailings should be considered in design.