Heap Leach Pad Monitoring To Manage
Risks In Gold And Copper Mining
G2 Imaging installed an electronic leak detection system
(ELDST) under a heap leach pad at a gold mine located in Northern Nevada. The system
consisted of 147 stainless steel electrodes placed in a rectangular grid within the
subgrade of the pad. Two PVC pipes were also installed in the subgrade to introduce
measured amounts of salt solution for system calibration.
The ELDST array contained a total of 215 data points
providing coverage for approximately 85% of the leach pad area. Monitoring was conducted
biweekly using an Iris Instruments Syscal Junior resistivity
meter. ELDST software downloaded data from the receiver, and translated the values to
produce contour and three-dimensional surface plots of the data.
The plots represented the difference between the current data
set and baseline values. Baseline values were collected from the grid prior to the
introduction of leachate solution, and during system calibration. The baseline data is
adjusted for periodic, seasonal, and yearly fluctuations in resistivity values normally
experienced by the system.
Data sets collected during routine monitoring are added to
the baseline data after calculation of the current offset. If the offset file shows a
large magnitude change from baseline conditions, it will appear as a spike which alerts
the operator to the presence and location of the leak.
At the example heap leach pad, a geosynthetic liner was found
to be leaking shortly after startup. Up to 5 gallons per minute of leachate solution were
produced at a physical leak observation port shortly after the introduction of solution
started. ELDST was used to determine the position of the leak, and verify the
effectiveness of the remediation activities performed on the liner.
After ore placement and leaching began, the ELDST revealed
four separate, significant leaks, each of which alone would have been large enough to
initiate facility shutdown. These leaks located at a downstream pipe penetration, a
perimeter solution conveyance ditch, and two interior liner rips. The first data sets
collected by ELDST located the suspected leak, and the area was excavated by hand. Liner
tears and pinholes where located and repaired, and the pad was re-buried.
When the pad was wetted again, solution shows were once again
present in the physical leak detection ports. The ELDST also indicated that leakage was
still present, adjacent to the previous leaks. The volumes of leakage produced at the leak
observation ports were somewhat less than prior to the repairs, but still represented a
total volume which was in excess of permit limits.
Once again the pad was excavated, and additional leaks were
found and repaired. Response of the ELDST to the gradual reduction of solution in the
subgrade was predictable, with the magnitude of the conductivity anomaly decreasing
through time. The physical leak detection system showed gradually decreasing flows as the
subgrade de-watered. The ELDST continued to be viable for monitoring of the pad over the
active life and during the closure period.
With a history of nine years successful service in the heap
leach sector of the mining industry, ELDST has proven to be reliable, cost effective, and
accurate for the detection and location of leaks within the sensor network. Over six
million square feet of liner are currently monitored by ELDST. It is a tool for monitoring
the entire area under a liner, not a point specific method like lysimeters or monitoring