USGS/DNR Model Results - Strong horizontal flow occurs in the highly permeable
outwash. Most of
the contaminant movement would occur in the outwash even though higher concentrations
could
develop in the less permeable till and bedrock units. Figure 3-9 shows the
direction of groundwater
flow as modeled. Discharge of groundwater from the MWDF area into Hemlock
Creek prevents
contaminants from moving beyond Hemlock Creek. Figure 3-12 shows steady-state
contaminant
concentrations for various dispersivity ratios assuming MWDF seepage was
held constant at 0.067
in/yr and the sulfate concentration of the leachate was set at 2,000 parts
per million. To provide a
check on model sensitivity the groundwater recharge rate was doubled, producing
contaminant
concentrations of 30 to 50 per cent that of the lower rates. The complete
destruction of the MWDF
membrane cap was simulated with a steady state seepage of 0.67 inches per
year. This produced
compliance boundary sulfate concentrations near that of the 250 mg/l drinking
water standard. The
complete loss of the cap membrane is considered such a remote possibility
that it is not considered as
part of a worst case scenario. Figure 3-11 has transient simulations showing
sulfate concentrations
after 50 years of MWDF operation. It should be realized that arrival of contaminants
at the water
table will be delayed by 20 to 120 years due their passage through the unsaturated
glacial soils
beneath the MWDF. Therefore, modeling the high rates of operational seepage
for 50 years is a
conservative case. A worst case short-term scenario (lack of a cap and slow
tailings dewatering
during the first 50 years) was simulated by using MWDF seepage rates of 0.6
to 2.4 inches/year
(2.5-10.2 gpm). An additional assumption was that the cap would not be fully
functional at this time,
so the groundwater recharge at the MWDF margin was set at normal levels.
Table 3-4 summarizes
the model results for sulfate concentrations expected at the 1,200 foot MWDF
compliance boundary.

                                            Table 3-4
                         Results of USGS-DNR Steady State and Transient
                         Groundwater Quality Modeling (Vertical Profile)



Steady State Simulations'
    Dispersivity Ratio (aL/aT)2
    Seepage 0.067 inches/year
         10/1
         60/1
         60/0.1
    Seepage 0.067 inches/year; recharge doubled
         10/1
         60/1
         60/.1
    Seepage 0.67 inches/year
         10/1
         60/1
         60/.1
Transient Simulations3
    Seepage 2.4 inches/year
         aL/aT = 60/1
    Seepage 2.4 inches/year
         aL/aT = 60/0.1
    Seepage 0.6 inches/year
         aL/aT = 60/1



  Maximum Sulfate Concentrations
  at Compliance Boundary (ppmj
Outwash         Till            Bedrock



20
15-20
25-40



10
<10
10



200
150
250



30-40

35-40

2-3



25-30
15-20
35-45



10
10
10



250
200
250



10-25

10-25

ca. 1



ca. 60
35-45
35-45

25
10
15

250
250
250

1-10

ca. 1

(1



    All have high groundwater recharge at the MWDF margins and a steady state
MWDF seepage
    rate as noted.
2
    aL = Longitudinal dispersity/aT = Transverse dispersity. Longitudinal
dispersivity considers
    dispersion of a contaminant along the direction of groundwater flow.
Transverse dispersivity
    evaluates contaminant dispersion at right angles (perpendicular) to the
groundwater flow. Where
    the rate of longitudinal dispersivity to transverse dispersivity is large,
contaminant dispersion
    throughout the aquifer would be minimal. If the ratio is high, contaminant
would be more
3   dispersed throughout the aquifer.
    No marginal groundwater recharge used. Sulfate concentrations are after
50 years of MWDF
    operation.



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