GENERAL48922

• Permit No: C1980001
• IBM Index Class Name: General Documents
• Doc Date: 10/29/1981
• Doc Name: Proposed Decision and Findings of Compliance
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

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The portion of West Ridge that has been prcviousli~ Wined is now occupied by ungraded
spoils. This area exhibits poorly defined surface drainage and numerous closed
depressions. These features tend to promote deep Percolation at the expense of
surface runoff. McWhorter, et a1. (1979), estimate that approximately 4 percent of
the total runoff is surface flow from this area. Once the area has been remined,
graded and topsoiled, it is anticipated that the surface runoff will be increased to
approximately 7 percent of the total. This increase will tend to improve the quality
of the crombined rrrnoff because the increase will occur at the expense of deep perco-
lation which has a larger concentration of dissolved solids.
Post-hfi Wing [•la ter c,JUali ty
A model that can be utilized for the prediction of post-Wining water quality (p. 2.5-100)
is based upon a combined balance of water and T0.S and was colibrated previously for
the Edna mine (McWhorter, et a1., 1979).
The quality of water in Trout CrceF: will be measurably affected only by the mining
of areas previously undisturbed, and, to a lesser c:rtent, the re mining of a portion
of West Ridge. Considering West Ridge first, the pc•rccnt of the total f:'est Ridye
drainage area that is presently disturbed is 68°'. Monitoring at station C3 at
the toe of the slope and sampliny of springs indicates that the concentration of
dissolved solids in the subsurface runoff is 3030 my/1 (hlc[•Jhorter, et al., 1979).
Ground water runoff from the undisturbed portion of t-h•~ :.~atc•rshed i.~as estimated to
contain 460 mg/1 of dissolved solids and the TLS concentration in sufface runoff was
estimated at 150 mg/1. The pre-Honing grater balance data give surface runoff as
0.08 inches or 4 percent of the total on the mined land and 0.15 inches or 7 percent
of the total on undisturbed ground.
From these data, the pre-mining concentration o% dissolvcrl solids in drainage from
the West Ridge area is calculated to be 2122 my; 1.
The value measured at C3 orer a three year period averaoed 1840 mg/I (McWhorter, et a1.,
1979) which is in fairly good ayreement mi th tLo above nunibe r.
The computation for the post-mir,iny r.~ater quality nn Plest Ridye is similar, except
that almost the entire ldest P.idge ar_a [.•ill have been mined and the fraction of total
runoff that is surface flow increases to 0.07. Calcularions show (see p. 2.5-101)
that mining the remainder of file unr?isturbcq nma or, [~e;t Rig?qe is expected to
increase the mean annual TDS concentration from 2122 mg/1 to 2828 mg/1.
Similar computations are made for the Moffat area. Approximately 300 acres of a
total of 1,030 acres in the hloffat area are expected to he mined.
The ground water quality from the undisturbed a ma is estimated from previously
discussed data to be 570 mg; 1. Also, as indicated previously, the post-mining spoil
water is expected to contain about 3000 mg/1 of dissolved solir]s. Thus, (p. 2.5-1031,
it is predicted that the discharge weighted, mean annual TDS concentration in com-
bined surface and subsurface runoff from the hloffat area will be 2290 mg/1 in the
post-raining period. This includes the runoff from both the disturbed and undisturbed
portions. The corresponding TAS concentration from the same area at present is
estimated to be 541 mg/1.