Dames & Moore, 1999 - USDA Forest Service
Dames & Moore, 1999 - USDA Forest Service Dames & Moore, 1999 - USDA Forest Service
snowmelt on the adjacent valley slopes, so that primary groundwater flow directions are perpendicular to the trend of Railroad Creek. Groundwater discharge to Railroad Creek decreases over the summer. Through the summer months as groundwater levels decrease, groundwater beneath the Site begins to flow downstream to the east rather than directly toward Railroad Creek. The flow loss occurs because water levels in Railroad Creek are above water levels in the alluvial aquifer near the eastern portion of the site. The area immediately east of tailings pile 3 replenishes groundwater storage and is assumed to discharge back to Railroad Creek along the reach in and near SP-21, immediately east of RC-2. This assumption is based on the observed exposure of bedrock on the south bank of Railroad Creek, immediately downstream of SP-21. The presence of bedrock, and absence of alluvial material, indicates that the groundwater likely becomes surface water (Railroad Creek) at this location. Seep discharge is largest in the spring and essentially stops by late summer, indicating snowmelt as the primary source of seep flow. 6.1.3.2 West Side of Site The west side of the Site includes the Honeymoon Heights drainage area, mine area, underground mine . workings, the east and west 'waste rock piles, mill building area, and the maintenance yard (Figures 6.1- 1 and 6.1-la). Some of the upslope run-on probably enters nk-surface discontinuities in bedrock south and upslope of the Site and flows downward through bedrock fractures, through abandoned stopes and mineralized (unrnined) portions of the underground mine and contacts residual mineralization on rock faces of the stopes and tunnels as shown on Figures 6.1-2 and 6.1 -2a. The water emerges as either surface water overland flow (i.e., 1500-level main portal drainage) and can reinfiltrate as seeps that emerge as overland flow andlor as diffuse groundwater discharge to Railroad Creek. Some of the overland flow from upslope run-on also moves across the Honeymoon Heights waste rock piles (800 and 11 00 level) or the mill area, and the maintenance yard and then travels downslope to other drainage features such as the lagoon and other miscellaneous drainage channels (Figures 6.1 - 1 a and 6.1 - 2b). Not all groundwater comes into contact with the underground mine workings. Some portion of groundwater flow from the west side of the site is assumed to be diverted into the abandoned Railroad Creek channel and also flows beneath the tailings piles. 6.133 East Side of Site The east side of the Site includes tailings piles 1, 2, and 3 (Figure 6.1-la). Upslope surface water overland flow from direct precipitation and snow melt is transported to Copper Creek and also infiltrates across and through tailings piles 1,2, and 3. Surface water is further transported to other drainage features including ditches that divert water to Copper Creek, an abandoned decant tower near the southern margin of tailings pile 1, the Copper Creek diversion, and the sauna dipping pool. Groundwater recharge from upslope run-on and infiltration occurs through the fractures within the bedrock found along the valley sidewalls and in the alluvium/reworked till, where present (Figures 6.1-3 and 6.1-3a). Infiltration occurs through the tailings piles from a combination of sources including upslope run-on as well as snow-melt and direct precipitation on the tailings piles. Infiltration through these features contributes recharge to groundwater in the alluvium/reworked till beneath the tailings piles which eventually discharges as seeps and groundwater baseflow to Railroad Creek. The discharge rate decreases after the spring snow melt period. Some portion of groundwater flow from the west portion of the site is assumed to be diverted into the abandoned Railroad Creek channel and also flows beneath the tailings piles. \U)M-S~I\VOLI\COMMOMWR~W)S~\~DIQ~-~\~~,~~~ 6-6 1769300U)l Wuly 27.1999,4:11 PMDRAFT FINAL RI REPORT
Evidence of significant surface and channel erosion in Site drainages was not observed and therefore, there does not appear to be a direct water borne pathway for significant quantities of metal-containing sediments to enter Railroad Creek fiom the Site. Under extreme flow events, bank erosion in Railroad Creek and channel shifting with subsequent erosion of tailings in Copper Creek can potentially transport tailings into Railroad Creek. Channel scour and re-suspension of flocculent and fine particulates originating as iron oxide (and other) metal precipitates, and which settle into the Railroad Creek streambed, is also a transport mechanism which can carry metals downstream. Although large channel scour events in Railroad Creek which transport bed material long distances downstream are not common (due to the size of the dominant bed material and bed armoring), transport of iron-oxides as fine suspended material was observed several times during high flow events in 1997. 63 GEOCHEMISTRY INTERPRETATION TOOLS Interaction of minerals with the atmosphere and the formation of new minerals control the chemistry of waters originating from the Site. A critical component of the geochemical interpretation is therefore the determination of which minerals in the rocks are controlling the original source of the water, and which new minerals are forming and removing metals from solution. These two groups of minerals are referred to as primary, indicating that they are present in the rocks, and secondary, indicating that they are formed when metals released from the primary minerals react with other dissolved or solid constituents. The first group does not include all minerals in the rocks because some minerals are inherently unreactive. The following subsections describe the interpretative tools that were used to evaluate chemical processes occurring at the Site. 6.2.1 Contribution of Primary Minerals Subsection 6.1.1 described the types of primary minerals reported to be present in the rocks within the Holden Mine. The processes that contribute the components of primary minerals to mine water chemistry are introduced below and described in more detail in'subsection 6.3. The controlling process on water chemistry at the Site is the oxidation of iron sulfide (pyrite), summarized as: The acid (IT) produced by this reaction, can then dissolve other minerals, for example, calcite: or, other more abundant minerals (for example, potassium feldspar): It can be seen from reaction (6-1) that each mole of pyrite produces two moles of sulfate ion and four moles of hydrogen ions (representing acidity). Reactions (6-2) and (6-3) indicate that these four hydrogen ions then react with calcite and produce two ions of calcium or four ions of potassium. Therefore, if 176
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Evidence of significant surface and channel erosion in Site drainages was not observed and therefore,<br />
there does not appear to be a direct water borne pathway for significant quantities of metal-containing<br />
sediments to enter Railroad Creek fiom the Site. Under extreme flow events, bank erosion in Railroad<br />
Creek and channel shifting with subsequent erosion of tailings in Copper Creek can potentially transport<br />
tailings into Railroad Creek. Channel scour and re-suspension of flocculent and fine particulates<br />
originating as iron oxide (and other) metal precipitates, and which settle into the Railroad Creek<br />
streambed, is also a transport mechanism which can carry metals downstream. Although large channel<br />
scour events in Railroad Creek which transport bed material long distances downstream are not common<br />
(due to the size of the dominant bed material and bed armoring), transport of iron-oxides as fine<br />
suspended material was observed several times during high flow events in 1997.<br />
63 GEOCHEMISTRY INTERPRETATION TOOLS<br />
Interaction of minerals with the atmosphere and the formation of new minerals control the chemistry of<br />
waters originating from the Site. A critical component of the geochemical interpretation is therefore the<br />
determination of which minerals in the rocks are controlling the original source of the water, and which<br />
new minerals are forming and removing metals from solution. These two groups of minerals are referred<br />
to as primary, indicating that they are present in the rocks, and secondary, indicating that they are formed<br />
when metals released from the primary minerals react with other dissolved or solid constituents. The first<br />
group does not include all minerals in the rocks because some minerals are inherently unreactive.<br />
The following subsections describe the interpretative tools that were used to evaluate chemical processes<br />
occurring at the Site.<br />
6.2.1 Contribution of Primary Minerals<br />
Subsection 6.1.1 described the types of primary minerals reported to be present in the rocks within the<br />
Holden Mine. The processes that contribute the components of primary minerals to mine water chemistry<br />
are introduced below and described in more detail in'subsection 6.3.<br />
The controlling process on water chemistry at the Site is the oxidation of iron sulfide (pyrite),<br />
summarized as:<br />
The acid (IT) produced by this reaction, can then dissolve other minerals, for example, calcite:<br />
or, other more abundant minerals (for example, potassium feldspar):<br />
It can be seen from reaction (6-1) that each mole of pyrite produces two moles of sulfate ion and four<br />
moles of hydrogen ions (representing acidity). Reactions (6-2) and (6-3) indicate that these four hydrogen<br />
ions then react with calcite and produce two ions of calcium or four ions of potassium. Therefore, if<br />
176
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