Dames & Moore, 1999 - USDA Forest Service

control and buffering by alumino-silicates. The Baker Mine is believed to have a regional stable
geochemical conditions. Likewise, the Holden Mine is also believed to be stable.
The similarities with other mines indicates that the experience gained at other sites will be applicable to
planning remediation at the Holden Mine.
6.8 FLOCCULENT AND FERRICRETE
6.8.1 Flocculent
Iron flocculent is a colloidal material that is generated fiom baseflow groundwater contributed to Railroad
Creek from the tailings piles. As the pH of the groundwater increases upon contact and mixing with surface
water, iron oxyhydroxides become stable and precipitate. Some of the copper that is in solution and lesser
amounts of cadmium, zinc and other metals coprecipitate with the iron as the iron flocculent is formed.
During the fall and winter months when groundwater flow is low apd the flow in Railroad Creek is low, the
majority of the flocculent that is generated settles in the base of the creek bed with some limited transport
downstream.
In the spring, when both groundwater and surface water flows are high, flocculent continues to be generated.
Due to the high flow, spring flocculent and flocculent that has accumulated in the Creek bed h m
falVwinter are mobilized and are both transported downstream. Flocculent is transported during spring
runoff and precipitation events.
6.83 Ferricrete Formation
Femcrete is typically defined as a conglomerate consisting of sand and gravel cemented into a hard mass by
fenic oxides and sulfates derived fiom the oxidation of percolating iron-bearing solutions. The vertical and
lateral variation in thickness and character of femcrete in Railroad Creek suggests that percolating solutions
infiltrate whatever material is present. Fenicrete formation is dependent on the supply of salts and degree of
oxygenation of the solution. Increased infiltration of waters to the tailings dissolves the accumulated iron
and metal salts in the tailings, putting iron and metal cations, sulfate and acidity into solution. Ferrous iron
(Fe23 in groundwater, discharges as baseflow to Railroad Creek where oxidation and an increase in pH
occurs. Ferrous iron converts to femc iron (Fe33, complexes with hydroxides forming iron oxyhydroxides
and then precipitates. Other metals may also coprecipitate with the iron. Because of these factors, the rate
of formation of fenicrete is not constant and is dependent on the local conditions.
The formation of ferricrete is probably an important factor limiting the development of a hyporheic zone in
Railroad Creek. The hyporheic zone is defined as a zone of mixed groundwater and surface water that may
occur in the interstices of the bed sediment in direct contact with the water (for example, Bemer et al.
1995). If groundwater contains elevated concentrations of contaminants, it is conceivable that elevated
concentrations could be present in the hyporheic zone. The substantial ferricrete deposits may limit direct
mixing between groundwater originating from the tailings piles and Railroad Creek water by armoring the
stream bed. It is expected.that iron-bearing groundwater will be oxidized and neutralized within the
ferricrete, preventing the development of the hyporheic zone and encouraging precipitation of metals within
and beneath the fenicrete.
17693-00M19Uuly 27.1999,4:11 PM;DRAFT FINAL RI REPORT