REV07329

• Permit No: M1988112
• IBM Index Class Name: Revision
• Doc Date: 3/25/1999
• Doc Name: SECTION 4
• Type & Sequence: TR26
• Media Type: D
• Archive: No

•

The gradient reversal will prevent groundwater flow from the West Pit to the seep neaz the Rito
Seco. Details of the water management plan aze discussed in the following sub-sections.
6.1 Initial Phase Water Management Plan
Initially, BMRI will implement pumping from the backfill to create a reversal of the hydraulic
gradient between the West Pit and the Rito Seco. The pumped groundwater will be evaporated
using enhanced evaporation methods during the time period from May until approximately
October. During the winter, the pumped water would be treated using nanofiltration (see Section
4). After the hydraulic gradient reversal has been verified, the extraction rate will be reduced to
the minimum flow rate required to maintain the reversal.
6.1.1 Reversal of the Hydraulic Gradient from the West Pit
This section describes the general principles of groundwater flow involved in reversing the
hydraulic gradient between the West Pit and the Into Seco. The water management plan for
reversing the hydraulic gradient between the West Pit and the Rito Seco is based on the observed
hydraulic properties and flow conditions in the backfill and Rito Seco alluvium. The
permeability of the backfill material is sufficiently high that dewatering can be achieved without
inducing significant water level variations in the backfill.
The goal of modifying the hydraulic gradient between the West Pit and the Rito Seco alluvial
aquifer is based on the fundamental hydrologic principle that water flows from high to low
hydraulic head. Currently, the hydraulic head in the backfill is approximately 12 ft higher than
the water level elevation of the Rito Seco in the region affected by West Pit outflow. Dewatering
the backfill to a water level elevation that is slightly lower than the elevation of the 12ito Seco
would effectively reverse the existing hydraulic gradient and would prevent the flow of
groundwater from the West Pit to the Rito Seco. Modifying the current hydrologic system in the
vicinity of the West Pit by pumping from the backfill has the following advantages over other
forms of hydraulic control (e.g., installing a well field in the alluvium):
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