Case Study: South Wall of Pit Expansion Penetrates Groundwater System Connected to Local Potable Water Supply

Dewatering design proven effective and mine is operating efficiently with sufficient water supply

Challenge: The carbonate groundwater system has the potential to produce considerable groundwater flows to the pit and is connected to the local potable water supply.

Solution: Schlumberger Water Services designed and implemented a drilling and aquifer testing program, developed a groundwater model to estimate the dewatering rate and to assess impacts from pumping, and designed and implemented a full scale dewatering and discharge system.

Result: The open pit expansion has been successfully executed over a six-year period through an effective dewatering design and an effective recharge scheme that mitigates impacts to the local potable water supplies.

Considerable groundwater flow expected for south wall of pit expansion

The Robinson Mine consists of three main open pits that produce copper, gold, and molybdenum. The mine has been in and out of operation since the late 1800’s depending on precious metal demand. Due to increasing demand, the eastern pit (known as the Ruth Pit) will be widened and deepened over a six-year period from 2009-2015.

The primary groundwater system connected with the Ruth Pit is composed of highly permeable carbonate bedrock. This system has a large volume of water in storage, receives substantial recharge from the Egan Range, and serves as a source of local potable water.

Schlumberger Water Services (SWS) was selected to conduct a hydrogeologic characterization of the groundwater system, design and implement an open-pit dewatering system, design the dewatering system in a manner which meets the mine’s water-supply requirements, and design and implement a system to manage excess dewatering discharge not consumed by the mine.

Hydrogeologic Characterization Solution

The hydrogeologic characterization was initiated in 2006 to confirm the dewatering design for the new Ruth Pit expansion plan and to assess potential impacts from the dewatering operation. SWS designed and executed the study to achieve the following:

• estimate the required pumping rate for dewatering the Ruth Pit expansion
• evaluate the potential impacts to surrounding groundwater systems and local potable water supplies  increase overall understanding of the groundwater system and the water budget
• simulate return of excess dewatering discharge (not used as mine water supply) to the hydrographic basin under various artificial recharge scenarios
• estimate a range of pit filling rates during closure
• predict a range of post-closure equilibrium pit lake elevations
• evaluate the potential for postclosure pit-lake flow through to surrounding groundwater blocks.

The program included the construction of additional piezometer wells, management of a large-scale pumping test, and development of a numerical groundwater model.

Data from more than 350 wells were used to develop the conceptual and numerical groundwater models including ~150 wells in the USGS database, ~130 wells in the Nevada Department of Water Resources database, and ~80 wells from the Robinson Nevada Mining Company database. These included domestic and municipal water supply wells, monitoring wells, and piezometers covering three hydrographic basins. The finite difference code MODFLOW-SURFACT was selected for the groundwater model.

Dewatering System Design Solution

The dewatering approach for the Ruth Pit expansion was to maintain groundwater levels below the pit floor in advance of mining and throughout the life of the mine. To achieve this, SWS developed primary and secondary dewatering strategies.

The primary dewatering strategy focused on reducing groundwater levels with high-capacity vertical pumping wells in permeable zones around the perimeter of the pit to dissipate pore pressures in the pit walls. The secondary dewatering strategy focused on lower permeability zones along the pit perimeter and inside the pit, using a number of small-diameter dewatering wells. The potential use of passive dewatering measures inside the pit were evaluated. The dewatering measures implemented to date have allowed the pit walls to be developed in a safe manner and per the desired mine plan.

Dewatering Discharge Solution

The dewatering system was designed to meet the mine’s water supply requirements by linking the dewatering discharge system to the existing water supply infrastructure. The new system replaces several historic water supply wells, which relied on a deeper groundwater source.

To manage the excess dewatering production, SWS evaluated the injection of water into a number of historic mine water supply wells and artificial recharge basins. This assessment involved the characterization of the groundwater systems, evaluation of the performance of the alternative strategies, and the design of the injection well system.

Results

As predicted by empirical-based estimates and the numerical groundwater model, groundwater inflows to the pit are primarily from the carbonate groundwater system (98%). Based on this evaluation, a dewatering rate of 16,000 gpm is being used for long-term mine planning and to maintain a safe working environment. The impacts on local water supplies were accurately predicted and mitigation measures have been successfully implemented in accordance with regulatory agencies. Upon cessation of dewatering, the groundwater conditions are predicted to recover to pre-mining conditions.

Full-scale production well drilling in the carbonate groundwater system began in 2008. The dewatering design has been proven effective and the mine is operating efficiently with sufficient water supply and water management. The recharge scheme for managing the excess dewatering discharge is effectively mitigating impacts to the local potable water supplies.

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