generation should be reached. If the infiltration control system, particularly
the geomembrane,
works as designed, no leachate would be generated after the cells had been
capped and the tailings
had dewatered. Figure 3-1 shows the predicted maximum and minimum seepage
from an individual
tailings cell. The potential long-term effects on groundwater also were evaluated
assuming a
partially deteriorated geomembrane (some infiltration through the cap). The
long-term seepage
under this worst case scenario predicted a rate of 0.93 gallons per minute
(0.067 in/yr over 270 acres).

MWDF Leachate Characteristics - Table 3-1 provides the predicted tailings
ponds leachate quality
prior to and after attenuation by the soil liner and underlying soils. The
major leachate components
are sulfate, calcium, magnesium, and iron. Attenuation processes within the
liner and underlying
glacial soils would reduce the concentration of many toxic metals in the
leachate.

                                             Table 3-1
                               Predicted MWDF Leachate Chemistry
                                                    Initial Leachate
            Parameter                              Characteristics      
   After Attenuation
            pH                                             7            
         7-8
            Total dissolved solids (TDS)               3,000            
        3,000
            Sulfate                                    2,000            
        2,000
            Arsenic                                        0.50         
           0.03
            Barium                                         0.03         
           0.1
            Cadmium                                        0.50         
          <0.001
            Chromium                                       0.06         
           0.001
            Copper                                         0.10         
          <0.01
            Iron                                          30.0          
            0.02
            Lead                                           0.04         
           0.01
            Manganese                                     20.0          
         <20.0
            Mercury                                        0.01         
          <0.001
            Selenium                                       0.10         
           <0.001
            Silver                                         0.03         
           <0.001
            Zinc                                          10.0          
            0.2
            Calcium                                      500.0          
         500
            Magnesium                                    250.0          
         250
            Hardness                                   2,000            
        2,000

1 pH in pH units; all others in mg/I (ppm)
Source: In part from Table A-4, Appendix 4.1A, EIR

The concentration of dissolved solids and metals in the leachate would change
over time as the
tailings slurry water was flushed from the tailings. If the final cover remained
intact, any percolat-
ing water would probably not contain sufficient dissolved oxygen to decompose
the pyrite in the
tailings. Without oxygen, pyrite decomposes slowly, if at all, and thus is
unlikely to contribute dis-
solved constituents. Water transport without corresponding oxygen transport
would only displace
slurry water trapped in the pores of the tailings. Under the worst case infiltration
scenario (0.067
in/yr), about 3600 years would be required before the tailings pore water
would be replaced by clean
percolating water.

Other wastes disposed with the tailings would contribute relatively small
amounts of dissolved
constituents to the pore water, even if pyrite decomposition did not occur.
Wastewater sludge from
the treatment plant and reclaim ponds would contribute leachate constituents
very similar to those of
the tailings themselves. The sludges would provide a small amount of neutralizing
capacity to the
waste mass, but it would be of minor importance. The physical and mineralogical
properties of the
tailings would be a more important source of pH control.

Storage of salt cake (sodium sulfate, a by-product of the reverse osmosis
water treatment process) in
the MWDF is proposed until a determination of commercial marketability would
be made. If stored
within a bermed portion of tailings cell T-1, sodium sulfate would absorb
water (hydrate) from excess
available water if left uncovered. Following hydration it would begin to
dissolve and contribute



sodium and sulfate ions to the leachate. Approximately 2000 cubic yards of
sodium sulfate would be
produced annually.



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