_REVISION - M1980244 ‎(31)‎

• Permit No: M1980244
• IBM Index Class Name: Revision
• Doc Name: Adequacy Response
• From: Cripple Creek & Victor Gold
• To: DMG
• Type & Sequence: AM8
• Media Type: D
• Archive: No

21
6. Page 38, 2"d par.
It appears from discussions in the amendment and site knowledge that overburden from the Easr
dCresson and North Cresson pits, which were determined to have net negative neutralization
;potential, are deposited in pan in the 5GOSA. Acid generated by this rock may be neutralized
;,by reaction with carbonate in the lower part of the diatreme. Absent precipirarton or
adsorpttan, zinc that maybe dissolved jrom warts rock in the mining zone and carried into the
>groundwacer system could Increase the overall concentration of zinc to the diatreme and !n the
,,Carlton discharge.
Response: A critical conditional statement in the comment shove is the phrase "absent precipitation or
adsorption." W e believe that the modeling performed shows solid solution of zinc in calcite is a61e to
account for the geochemical behavior of tine in the Carlton Tunnel, as well as the physical presence of
secondary calcite containing zinc confirms that there is a mechanism for explaining zinc removal under
currant•conditions (see Attachment 11 to this response document). Additionally, we believe that this
geochemical system will remove zinc (and acidity), even if their load contributions marginally increase
under Amendment No. 8 activities.
The text indicates that carbonate which precipitates in the diatreme derives from "increased
interaction of the ground water with carbonate minerals" in the diatreme. It is not evident how
=a solution that becomes carbonate-saturated due to water•rock interactions along a flow path
can then precipitate Carbonate without a change in some other estrinsrc property such as a
change in temperature, pH, salinity. Ca+~ or C03' concentration, or pCO~ decrease. Because
the only documented control over removal of dissolved Zn is Zn adsorption via carbonate
precipitation, it seems important that the mechanism of carbonate precipitation be understood.
Please explain in more specific geochemical terms how carbonate currently forms in the
~diat~eme.
Please explain how the diatreme solutions can simultaneously dissolve and precipitate
• ~ carbonate to sorb 7n.
Respottse:`Ttia tnade- that was presented called upon degassing of cazbon dioxide to cause precipitation of
calcite from downward percolating water. This is a trasoaabie ptt>cess baausc acidic water that moves
downward in fractures essentially enters a "closed" system where gaseous exchange with the
atmosphere is prohibited. Reaction with carbonates causes an increase in the pressure of cazbon dioxide
gas is the water in the closed system of the fractures. This is also facilitated by the slow ldnetics of
degassing of carbon dioxide. Upon entering the "open" drainage system of the Carlton Tunnel, the
carbon dioxide exsolves and causes precipitation of calcite. However, because of concern by the
OMGR',-shout this process, the geocbemical ittodeling, including the carbonate disaolutioNprecipitation
processes, has bees extended. The modeling results support that dtece processes will account for the
eleva[ed: carbon dioxide levels and the precipitation of secondary calcite at the mouth of the Carlton
Tunnel;'. A detailed discussion of the mndaling r~aulte is provided scpacatcly (sec Attachment 11 to this
respotue document).
M. GENERAL CGMMENTS
'The geology section indicates that most 4f the water derived from surface i-tlllnation moves
'' 'downward through the diatreme via discrete fracture flaw rather than by more homogeneous
'' :;porous media f Iow. The model for acid neutralization by carbonates in the deeper part of the
:;: