2008-06-06_REVISION - C1981019

• Permit No: C1981019
• IBM Index Class Name: REVISION
• Doc Date: 6/6/2008
• Doc Name: Adequacy Review ‎(Memo)‎
• From: Kent Gorham
• To: Jim Stark
• Type & Sequence: TR73
• Media Type: D
• Archive: No

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STATE OF COLORADO
DIVISION OF RECLAMATION, MINING AND SAFETY
Department of Natural Resources
1313 Sherman St., Room 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone] Interoffice Memo
To: Jim Stark
From: Kent Gorham
CC: Sandy Brown
Date: June 6, 2008
RE: Adequacy comments, TR-73, Colowyo Mine
COLORADO
D IV IS I ON OF
RECLAMATION
MINING
SAFETY
Bill Ritter, Ir.
Governor
Harris D. Sherman
Executive Director
Ronald W. Cattany
Division Director
Natural Resource Trustee
Jim, as per your request dated May 15, 2008 1 have completed my preliminary adequacy
review for Technical Revision No. 73 (TR-73) for the Colowyo Mine, #C-81-019.
General Comments
As I understand this revision, Colowyo is proposing to modify the methodologies and
assumptions section of the permit text in Exhibit 7 as it relates to all sediment ponds except
those designed and previously permitted via PR-02, South Taylor. Volume 13 contains
information specific to South Taylor sediment pond design, as stated in the applicants cover
letter that accompanied the revised materials submitted. Therefore, this proposed change
would only affect any future storm runoff modeling for existing ponds and associated ditches
and permanent drainages.
I recall conversations briefly discussing the possibility of having an approved range of curve
numbers for particular stages of land condition, rather than single, specific values for each
land condition. However, I don't understand the rationale for changing design information
specific only to sediment ponds that are already constructed and also, in some cases, for
watersheds that are entirely reclaimed. How will the new design information be used in the
future? Application materials provided no explanation.
Furthermore, no information is apparently offered or provided for support of the new ranges
of curve numbers for each land condition. Specifically, for "disturbed land, no topsoil" a range
from 85-91 is proposed, from a currently approved value of 91. For "topsoiled and seeded" a
range is proposed of 79-86, from a currently approved value of 86. For 1-2 year
reclamation" and range is proposed from 61-71, up from a currently approved range of 58-61.
For "3+years growth" a range of 56-67 is proposed, up from a currently approved range of 48-
56.
If approved, these changes allow more flexibility regarding design but of major concern is the
fact that using these ranges could result in even more liberal SEDCAD modeling resulting in
estimates of less storm runoff and sediment generation. Give our past and ongoing concerns
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