Dames & Moore, 1999 - USDA Forest Service

The chemistry of these monitoring wells is compatible with a groundwater flow pattern that includes
natural groundwater fiom upslope and affected groundwater originating fiom the east waste rock pile
andlor the mill buildings area. Further, these groundwater concentrations may be influenced by a,
groundwater flowing within the former creek channel which underlies the tailings. This relatively
permeable unit may control and channel the flow of affected groundwater fram the weathered source
areas, including the portal drainage, lagoon, waste rock piles, and mill area towards the SP-2 seep.
Tailinas Pile 2 and 3
Groundwaters sampled from the underlying surficial material in the central areas of tailings piles 2 and 3
(e.g., wells PZ-lA, TP2-SA, TP3-6A) generally had low metal concentrations (e.g., Cu: Q to 67 p&; Zn:
32 to 400 pg/L; Cd: 0.2 to 22 p&), although an occasional elevated iron level (5,710 pg/L iron) was
observed in PZ-IA. This elevated level is probably the result of groundwaters leaching natural materials or
minor incursions of tailings waters infiltrating the substrate.
The metals load discharged by the seeps (SP-3, SP-4, SP-5 and SP-18) appears to be the result of oxidation
of tailings, possibly due to the interaction with groundwaters derived from Railroad Creek. Groundwater
wells in the substrate near the tailings pile edges (e.g., TP2-4A, PZ-6A, TP3-8) had higher concentrations of
iron (7,040 to 58,300 pL), sulfate (340 to 1,500 mg/L) and total dissolved solids (500 to 2,400 m a).
Limited data on groundwaters in saturated tailings (e.g., PZ-IB) report elevd iron (69,000pgL) and
elevated sulfate concentrations (850 mgL). These values are similar in magnitude to the concentrations
recorded for the seeps (Fe: 23,900 to 154,000 pgL; SO,: 530 to 880 ma).
6.5.2.2 Metal Removal
Metal removal mechanisms are described Wow and are summarized in Figures 6.5-1 7 to 6.5-19.
Tailings Pile 1 Area
Seepage emerges from the base of tailings pile 1 at SP-I and SP-2 and enters Railroad Creek. The seeps
are strongly acidic @H4) and contain elevated concentrations of sulfate (>2000 mg/L) and iron (-1000
ma). As the acidic seepage mixes with Railroad Creek water, it goes through several rapid chemical
changes including:
Conversion of unoxidized ferrous iron to ferric iron by atmospheric and dissolved oxygen
Increiising pH due to dilution and stream water alkalinity allowing additional ferric iron
to precipitate as flocculent (in surface water) or goethite (in groundwater)
Co-precipitationJsorption of copper with flocculent or ferricrete
Further pH increase due to dilution and stream water alkalinity resulting in precipitation
of aluminum as flocculent or aluminum hydroxide precipitate
Formation df copper carbonate flocculent
Attenuation occurs by precipitation of femcrete, aluminum hydroxide, copper carbonate and co-
precipitation of copper with ferricrete.
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17693M1J-01 Wuly 27. 1999;4:11 PMDRAFT FINAL RI REPORT