HYDRO29057

• Permit No: M1988112
• IBM Index Class Name: Hydrology
• Doc Name: TABLE OF CONTENTS
• Media Type: D
• Archive: No

Appendix D, Comment Letters arrd Pnbtic Meeting: EPA Comments (Cont.): Page 7
Table Value Standard (7'VS) calculations require the input ojhardness data for the receiving water in the vicinity of the point of
compliance. Discharge standards were evaluated jor a point of corrrpliance at RS-2 under a condition of zero flow (worst case
scenario) under pre-mining conditions. Hardness data collected jrorn /987 tlrroug/r /989, w/ten the mine opened, were used in
the calculations to refiec[ pre-mine conditions in the Rito Seco downgradient ojthe mine deposit. The average hardness value
calculated x~as 63.2 nrg/L xath a standard deviation oj9.5 mg/L. This results in a 95'"percent confidence inren•al range oj57to
69.4 mg/L. Hardness data were also evaluated at RS-3A, where both low flow and analytical data were available. The
hardness calculated using the water quality data collected at RS-3A was 64 mg/L, which jails within dre confidence intenal
range for the more statistically defensible set ojdata available from surface rater station RS-2. 77ris value x•as then selected as
the hardness valrte jor use in the NS-based calculations. Therefore, on this basis, the Division believes that the hardness value
of 64 mg/L as CaCO~ is an appropriate slue that reasonably represents the natural influences on water quality along the reach
oJthe Rito Seco potentially affected by rheJlows associated with water treatment activities in the West Pit.
Comment:
Ill. Add iron and Manganese Limits far Outfa[1002.•
Response.•
The implementation of the TR-26 water nranagernent plan has resulted in the reversal of the hydraulic gradient in the West Pit
and groundwater flow from the West Pit into the adjacent alluvial aquifer no longer occurs. The reversal of the hydraulic
gradient between the West Pit and the adjacent Rito Seco alluvial aquifer is the fundamental best management practice (BMP)
upon which all other considerations associated x~ith the West Pit are based. The condition is based on physical parameters,
which provides for a readily measured compliance parameter, that is groundwater level. Through the implementation of the
groundwater level permit [imitation and this BMP, the hydraulic gradient has been reversed, resulting in no source of
groundwater flow from tJre West Pit and no discharge from orttfa[l 002. Thus, no rater quality standard based limits need to be
established jor outfal[ 002. BMRI xd!! be required to maintain the groundwater level in the West Pit sufficiently [ow to ensure
that the current no-outflow condition pres•ails tliraughout the period of the CDPS permit. During the period ojreclamation and
rernediatian in the vicinity ojthe West Pit, any proposed change in groundwater levels to be maintained in the West Pit would
require review and approval from the Division and a permit amendment may be required.
Comment:
/V. Other Potential Water OualiN Based E,~luent G'rnits for Outfall 002.•
Response:
As noted previously, the primary limitation jor outfal[ 002 is the groundwater level requirement jor we/l BF-4 and the associated
BMP in the permit (which is also specified in the TR-026 water nranagernen[ plan) is the reversal ojthe hydraulic gradient to
eliminate the Jlow ojgroundwater from the West Pit into the adjacent Rito Seco alluvia! aquifer. The rater level in the West Pit
has been lowered sufficiently to reverse the hydraulic gradient in the vicinity of the West Pit. Thus, [Ire West Pit will not serve
as a source of discharge jrorn Outfa[l 002 during t/re CDPS pernrit period. T/rerejore, the Division believes that the appropriate
compliance condition jor Outflow 002 during the period of active reclamation and remediation in the West Pit would be to
maintain the groundwater level sufficiently low to maintain the hydraulic gradient reversal.
Associated with the BMP jor the CDPS pennir is a monitoring program that xil/provide the information necessary to verify that
the groundwater system /ws responded as projected. BMR/ has implemented an approved performance monitoring program (as
is required under their DMG permit) in conjunction xdth the TR-026 water management progrmn. The performance monitoring
program consists oja cmnbination of groundwater level monitoring with rater quality monitoring. The water quality monitoring
is conducted at both surface water and groundwater locations. This monitoring plan data xRll also be submitted as a CDPS
permit requirement.
In response to [he comments relating to TR-26 and TR-28, reference to these two documents is no longer included within the
permit.
Comment:
V. Add Aluminum Limitations:
Response:
The Division has determined that limitations and monitoring xdll be included in the permit jor potentially dissolved aluminum for
outfalls OOIa, 0016, and OOIc. Since concentration of dissolved alumimun have been greater in the pit backf~ll monitoring well
data (0.208 mg/l per Appendx A) than the applicable 30-dav average limitation (0.087 rng/l), there is a reasomble potential to
suppor adding permit limits jor [his parameter. Also, since there has been no monitoring data for aluminum from the treatment
facility, alumirturn will need to be evaluated as pan of this permit.