CHAPTER 12 WATER MANAGEMENT DURING MINE CLOSURE
| Jurisdiction | United States |
(Jan 2002)
WATER MANAGEMENT DURING MINE CLOSURE
Parsons Behle & Latimer
Salt Lake City, Utah
Jeffrey V. Parshley
SRK Consulting
Reno, Nevada
I. Introduction
The number of hard rock mines that are or will be undergoing closure in the foreseeable future has increased substantially over the last few years-. The increase in closure properties is due to a number of factors; for example, many mines are reaching long-planned end of mine life. Additionally, depressed metal prices have accelerated the closure time line for certain operations. Notably, the increase in the number of mines addressing closure-related issues has also been accompanied by a general increase in the sophistication of both the regulators and the regulated community with respect to reclamation techniques and long-term environmental planning. This atmosphere has resulted in the need to commit additional resources to closure and commence closure planning earlier.
The experiences gained over the last decade have demonstrated the common goals of both the regulators and mining community; the closure goals for a mine are typically aimed at achieving all closure criteria (and the associated bond release) as expeditiously as possible, returning the site to a viable post-mining land use, and avoiding any long-term management problems or obligations. While this approach may be generally characterized in reclamation plans throughout the life of mine, complicating issues typically arise when closure is imminent and the closure plan details are being developed. In that regard, the most significant legal, technical and economic challenges to accomplishing closure objectives are frequently related to the short- and long-term management of residual water at a site and the associated water quality concerns. Regulators, many of whom have worked on or studied historic mining-related watershed contamination, oftentimes view mine closure and the associated obligations from that perspective.1 The result is intense scrutiny and corresponding compliance requirements for owners and operators desiring to achieve final closure.
This paper focuses on the technical and legal issues associated with water management during and subsequent to mine closure. Specifically, the paper provides (1) a brief overview of technical considerations including means for minimizing the amount of water that requires management during and after closure; (2) certain examples of technical solutions to closure water management; (3) an outline of select federal environmental and state regulatory programs in so far as they may apply to residual water management; and (4) examples of legal solutions to closure-related water management. This paper does not
[Page 12-1]
purport to be an exhaustive overview of closure-related water management but is intended to illustrate certain situations, technical considerations, regulatory programs and obligations triggered by the selection of a closure strategy.
II. Types of Water Subject to Management During and Subsequent to Closure
Mine closure plans must evaluate the management of at least five types of water including (1) process solution inventory; (2) drain down; (3) remaining water; (4) infiltration; and (5) storm water (Figure 1). The following describes some of the characteristics and technical considerations associated with these water types.
Figure 1 - Water Types
A. Process Solution Inventory
The process solution inventory typically consists of water in both the ponds and the beneficiation and processing facilities. Depending on the water requirements of the mineral process and the operating practices employed at the mines, the amount of solution inventory requiring management at closure can vary greatly. If rinsing is used as part of the closure process, additional water may be added to the solution inventory during closure.
B. Drain Down
The amount of water contained within the voids of tailings and heap leach pads may not be visible but can represent a significant volume. The portion of the pore water that will drain from a heap leach pad or tailings impoundment once the application of process and/or rinse solutions ceases is commonly referred to as drain down water.
While the majority of these solutions will drain from the facilities over a period of time, the rate at which that drainage occurs will (as further detailed below) vary based on the physical characteristics of the material and the configuration of those facilities. In addition to the physical controls, the rate of drainage is directly proportional to the driving head (i.e., the height of the water column in the materials). As the material drains, the
[Page 12-3]
reduction in water column height results in a decreasing flow rate. The changing rate of drainage flow can be plotted as a drain down curve.
The typical method for determining the curve is to model the key hydraulic properties; those properties are considered in the preparation of flow estimation models, taking into account hydraulic head and permeability of the materials (Figure 2). Because drain down solutions are typically former process or rinse solutions, the solutions tend to exceed drinking water standards (i.e., Maximum Contaminant Levels ("MCLs")) for a number of constituents. As described later in this paper, the quality of the solutions is an important consideration when evaluating potential regulatory options.
Figure 2 - Drain Down Curve
C. Remaining Water
Figure 3 - Entrained Moisture
Remaining water can include water entrained in the heap leach pad or tailings materials or water associated with pit lakes. Entrained moisture is the water that is held in the pore spaces by matric suction created by surface tension between grains (Figure 3). The entrained moisture will remain in the heap or tailings indefinitely and will generally have a chemistry similar to that of the last solution or rinse applied to the material. Some of the entrained moisture may be displaced by infiltrating meteoric waters if the material becomes sufficiently saturated. An evaluation of closure typically incorporates some analysis of the fate of the water in the pore spaces.
Pit lakes often reach equilibrium after substantial periods of time subsequent to closure. The size of the pit lake will be determined by a number of parameters that define the water balance for the pit. These include groundwater inflows, precipitation, evaporation, surface water inflows and storm water inflows. The chemistry of any pit lake will be
[Page 12-4]
controlled by the chemistry of the inflows and chemical reactions with minerals in the pit walls. Typically, the secondary mineralogy is the primary control on the equilibrium chemistry of the pit lake.2
D. Infiltration
Infiltration of meteoric water through a facility can present one of the key long-term closure concerns. Proven engineering techniques are available to minimize the meteoric water infiltration through closed facilities and may include the installation of a designed closure cover to reduce the flow of such water. The closure engineering can also incorporate grading designs, i.e., plans to send meteoric water off a facility, to minimize infiltration.
E. Storm Water
Closure plans (similar to the water management approach typical of most operations) typically include measures to minimize run-on onto the mine site. The construction of engineered controls can result in reducing the amount of water to be managed by directing storm water around and off mine sites. The evaluation of storm water flow is essential to facilitating long-term water management at the site.
III. Select Sources of Remaining Water at Mine Sites
The expenditure and risk associated with managing the types of mine site water can often be mitigated through proper design and operation of the mine's individual components. Such management planning generally focuses on tailings impoundments, leach pads, waste rock dumps, and pit lakes. The following briefly describes some of the associated technical challenges.
A. Tailings Impoundments
Tailings impoundments are designed for the disposal of solids, not water. In contrast, tailings impoundments often become the primary fluid disposal site for most mining operations. Excess water from the process facilities, seeps and dewatering flow streams may end up in the tailings impoundment. This practice-while convenient in the short term-tends to exacerbate the substantial water-related closure issues. At the time of closure, tailings impoundments may contain a significant amount of water in the supernatant pond and water in the seepage collection system. All of these sources of water will require management during closure; minimization of water through prudent water management during operations can substantially reduce the cost and effort required to close a facility expeditiously and efficiently.
The most significant long-term risks for most tailings impoundments are related to the potential for (1) embankment instability and (2) poor quality seepage from the facility. Those risks can be mitigated through specific water management practices. Spillways, drainage control and other methods that eliminate the potential to impound water can enhance the post-closure condition of the impoundment by improving stability and reducing
[Page 12-5]
flow through the impoundment. For example, management of deposition toward the end of operations can be used to establish positive drainage.
B. Leach Pads
The variables involved in the closure of heap leach pads are numerous, but principal among them are the requirements for chemical stabilization, the chemistry and rate of draindown, disposal of existing solution inventory, and the characteristics of long-term seepage. For example, solution inventories at gold heap leach...
To continue reading
Request your trial