retardation factor (Rd). Rd is the ratio of the rate of groundwater movement
compared to the rate
of movement of a contaminant in the groundwater. Table 3-2 shows the Rd'
s for several of the
anticipated leachate constituents. For example, copper with an Rd of approximately
fourteen would
move about fourteen times slower than the groundwater. In contrast, sulfate
has an Rd=1 and would
move at the same rate as groundwater.

                                            Table 3-2
                             Representative Retardation Factors (Rd)
                                       for Glacial Materials

                                                 Outwash                
       Till
                   Parameter                     Rdl                    
       Rd'
               Filterable residue (TDS)           1                     
      1
               Sulf ate                           1                     
     1
               Arsenic                            BD.2                  
         BD
               Barium                             BD                    
         BD
               Cadmium                            >13                
            >14
               Chromium                           BD                    
         BD
               Copper                             >13                
            14
               Iron                               BD                    
         BD
               Lead                               BD                    
         BD
               Manganese                          < 7                
            ( 7
               Mercury                            >13                
            >14
               Selenium                           >13                
            >14
               Silver                             >7                 
            >14
               Zinc                               BD                    
         BD

i From projected tailing seepage pH of 9.
2   Rd Values reported as "BD" represent soluble metal concentrations
below detection limits.
>   Means greater than.
<   Means less than.

Impacts to the Groundwater from the Mine Refuse Disposal Facility (MRDF)
Groundwater Impacts - MRDF leachate would contribute a number of common ions
to the
groundwater. The MRDF would contribute more chloride, magnesium, BOD (biological
oxygen
demand), COD (chemical oxygen demand), and nitrate than the MWDF per unit
of leachate, but much
less sulfate. Attenuation mechanisms would reduce leachate concentration
as it flowed through
underlying soils. Toxic metals would be retained in the wastes, liner, or
unsaturated subsoils. Only
traces of the organics would remain in the leachate by the time it reached
groundwater. Leachate
reaching the saturated sediments would have a minor impact on groundwater
quality. Compared to
MWDF seepage of thousands of years, the MRDF seepage duration of 40-60 years
would be very
short. Subsequent dilution by natural recharge and mixing within the aquifer
would reduce the overall
impacts, and no groundwater standards are predicted to be violated at the
compliance boundary (150
feet from the facility).

Leachate Generation - The proposed MRDF design (large area of side slope,
high permeability drain
layer, low permeability liner, and short flow distance to the leachate collection
system) would result
in high leachate collection efficiency. The design, however, would concentrate
the leachate on the
base grades (2% slopes), and thus may result in higher seepage if aggressive
leachate pumping were
not pursued.

Seepage through the base of the MRDF should be less than 10% of the precipitation
falling on an
operating cell. Since annual precipitation is about 30 inches, the resulting
seepage would be about
three inches per year (500 gallons per day). In comparison, natural infiltration
is 6.5 inches per year
or more, thus there would be substantial dilution of leachate by the surrounding
recharge. Actual
leachate production would depend on the permeability of the liner and the
drain layers, precipitation
gains, and evaporation losses. Thus, the volume of leachate could be several
times larger or smaller



than was estimated. Following placement of the final cover, the refuse should
dewater quickly due to
its high permeability and low moisture holding capacity. The volume of leachate
collected should
decrease rapidly after closure.



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