A leakage collection and recovery system (LCRS) for process water ponds and some tailings impoundments typically incorporates a two-layered synthetic liner with drainage media between the layers to drain and dissipate any leakage through
primary liner, preventing leakage through the secondary liner. If a vibrating wire piezometer (VWP) at the base of the sump indicates a head higher than the elevation of the primary liner, the secondary liner could leak from the base of the sump.
SRK have designed options (alternative LCRS options) that include:
• pumping from the LCRS sump to measure the rate of leakage through the primary liner into the sump
• gravitating from the LCRS sump via an outlet through the secondary liner to an external sump at a low elevation
• constructing a low-permeability layer under the LCRS base that considers the full life of the facility, including closure.
Pumping from LCRS Sump
In Nevada, sump inflows of greater than 150 gpd are considered a violation. Typical sump construction includes drainage media that can limit the potential for such a high flow rate pump to dissipate the full volume of water stored in the sump. A high pump flow capacity and a low-permeability drainage media can cause underestimation of actual leakage rates through the secondary liner. Additionally, if the sump is operated without a VWP, it cannot be demonstrated that there is no hydrostatic head transfer from the primary to the secondary liner.
Gravitating from the LCRS Sump
This design includes a prefabricated LCRS drain outlet installed in a prepared foundation, a pipe-in-pipe connection to an external sump/pond and methods of
operation that will eliminate transference of hydrostatic head from the primary to the secondary liner.
The prepared foundation grading is steeper in the immediate vicinity of the LCRS drain outlet to facilitate gravity drainage toward the low-point inlet of the pipe-in-pipe system. The foundation consists of a prepared soil subgrade underlying a mass concrete foundation slab housing the prefabricated LCRS outlet, which has a smooth finish to allow wrinkle-free installation of the HDPE apron welded to the outlet.
Constructing a Low-Permeability Barrier Under the LCRS Base
The thickness of the foundation depends on how long the system will require LCRS operation and the compacted hydraulic conductivity of the foundation soil. For example, if the timeframe for facility operation and closure is 20 years, and the compacted hydraulic conductivity of the foundation soil is approximately one foot in 10 years, a two-foot-thick layer of foundation soils would provide adequate mitigation against leakage.
