REV14062

• Permit No: C1981010
• IBM Index Class Name: Revision
• Doc Date: 12/5/1997
• Doc Name: MEMO TRAPPER MINE PN C-81-010 VEGETATION COVER STANDARDS
• From: DMG
• To: DAVID BERRY LARRY ROUTTEN
• Type & Sequence: SL1
• Media Type: D
• Archive: No

STATE OF COI.III IIIIIIIIIIIIIIII

DIVISION OF MINERALS AND GEOLOGY
Dep.vlmenl n( N,°ural Resources
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To: David Beny
Larry Routten
From: Tom Kaldenbach
Date: December 5, 1997
Re: Trapper Mine, Permit C-81-010
Vegetation Cover Standards
MEMO
II~~
DEPARTMEM OF
NATURAL
RESOURCES
Roy Romer
Governor
lames 5. Lnchhead
E eecmrve Duecror
Michael B. Long
Dwnian D~reclor
I have finished researching the origin of the cover standards on the Trapper Mine. My seazch
included the permit (including the old Utah International permit), all files in our office, and all
archived files.
The most substantial pre-mining cover data that I found is in Table 4.4.4 on page 4-102h of the
existing permit. That table presents 7 years of premine herbaceous cover percentages for
Trapper's three reference areas, range sites A, B, and C (see enclosed copy).
Trapper's letter dated 5/28/87 said the year-to-year variability of data in Table 4.4.4 was too
great to use for establishing a cover standard and therefore proposed aback-calculation of the
USLE as an indication of the minimum cover necessary for adequate erosion control (see
chronology below). The Division approved the back-calculated cover percentage in PR-02 on
6/9/87 .
It is possible that the data in Table 4.4.4, in reality, were not too variable to use for establishing a
cover standard. In that table, Trapper calculated that at the 90% confidence level the actual
annual mean cover could range as low as 14.85% on range site A, and 7.62% on range site B.
These percentages appear suspiciously low because they are far beyond the numerical range of
the 7 yeazs of actual data. Although the statistical calculation itself may not be in error, the use
of this particular calculation in these circumstances may need review.
Confidence levels of 90% are required when comparing large data sets from undisturbed and
revegetated sites, but it seems a 90% confidence level may be inappropriate for a data set as
small as 7 elements because in such a small data set one extreme value can have an inordinate
effect on the calculated statistics. Page l2 of the Division's guidelines for compliance with
vegetation requirements (published October, 1988) calls for using a simple arithmetic mean
when a 7-year historic record is being used to establish a cover standard.