Response No.                                     Response

        seeping from the MWDF is assumed to flow directly into Hemlock Creek
is calculated by
        allowing 400 acres of lined area to release sulfate for one year.
(The MWDF design as
        modified by Exxon consists of a 270 acre lined area.)

        a)   Total mass of sulfate released per year =

              mg S04 = (400 acres)  43,560 ft2  0.67 inches  1 ft  7.48 gal
 3.8 liters
              year                     acre         year       12 in.  1
ft       1 gal.

              2000 mg S04 = 5.53 x 109 mg S04/year
              liter

        b)   The Q7,z flow for Hemlock Creek at SG-6 is about 2.0 cfs, therefore
the yearly flow =

              liters =  2.00 ft3    7.48 gal  3.8 liters  3600 sec  24 hr
   365 days
              year        sec          ft3        gal          1 hr     
day       year

              = 1.8 x 109 liters/year

        c)   Therefore, the maximum increase in sulfate concentration in
Hemlock Creek at SG-6
              would be:

              mg SO4 =    5.53 x 109 Mg S04           =  3.1 mg/liter
              liter       1.8 x 10 liters/year

              Downstream on Swamp Creek at SGA3 where the Q7,2 is 6.7 cfs,
the increased sulfate
              concentration would be 0.9 mg/liter.

              Even without the effects of dispersion which would retard the
arrival of the sulfate
              at Hemlock Creek for several thousands of years, the steady
state worst case
              calculation as made above shows insignificant environmental
surface water impacts
              can be expected from MWDF seepage at the projected, conservative
rate.

  301   Exxon may not cause an unreasonable detriment to public rights in
surface waters. After
        closure, it must continue to mitigate mine-induced surface water
impacts as long as
        necessary.

  302   The longevity of geomembranes is addressed in response #264. In addition,
the long-term
        success of the waste containment facility depends on geomorphic and
geological stability
        more than on membrane permeability. It is unlikely that the small
amount of water which
        would leak through the final reclamation cap would contain sufficient
amounts of oxygen to
        cause significant pyrite oxidation.

  303   Exxon' s proposal did not include a provision for clean out of underdrain
pipes. Refer to
        response #265 on underdrain clogging and response #283 on geomembrane
longevity. Also
        refer to the discussion on DEIS page 214 and Fig. 4-6 on underdrain
clean out alternatives.



304   Stability of the geomembrane liners is addressed in response A
      the bentonite-modified soil liner is not expected to occur. TI
      liner would be covered by the geomembrane, a drain layer, an
      The topsoil would support a plant community. The bentonite-
      remain at a relatively constant moisture content following inc

      Construction of the final reclamation cap, as explained on DE
      the tailings had consolidated. The grading layer would have t
      suitable base for the liners and limit air and water movement



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