废弃矸石监测

Armed with CSI equipment, Montana’sSchafer &  Associate stakes on one of mining’s biggestenvironmental challenges
Evaluating the effectiveness of acid rock drainage controls

Acid Rock Drainage (ARD) problems associated with hard rockmetal mining are one of the largest environmental challenges formany mining operations.  The formation of ARD requires threecomponents: oxygen, water, and reactive rock (commonly found withina mine’s waste rock dumps).  Although ARD preventionstrategies are numerous, they typically focus on minimizing bothwater and oxygen flux into and through the reactive waste rock toreduce sulfide oxidation. A common and successful technique is tocover the reactive waste materials with a low permeability cap andvegetative cover, which isolates the reactive rock from oxygen andwater.  The effective-ness of such caps can be evaluated insitu by monitoring the water movement, oxygen content, andtemperatures within the waste rock.

Wireless telemetry, in this case cell phones,simplifies data retrieval from remote areas.

Schafer & Associates of Bozeman, Montana, has beenmonitoring waste rock cap performance and associated ARDdevelopment for over ten years. An integral part of thesemonitoring efforts has included CSI equipment to furthercharacterize water content and temperature changes in the reclaimedmaterials.  Typically, the monitoring sensors are installedalong a vertical profile within the cap system and under lyingwaste rock.  Each sensor is wired to a surface-located CSIdatalogger for continual data collection.  To maximize theusefulness of the data, a surface weather station is normallyinstalled to monitor precipitation events and changes in ambientconditions which can directly impact the cap performance. Figure 1 shows atypical instrumentation monitoring nest for a minewaste rock facility.

Subsurface water content and movement can be monitored with anumber of techniques and equipment designs, including neutronlogging, time domain reflectometry, gravity lysimeters, and heatdissipation units.  To enhance water flux interpretationsthrough various scapped systems, combinations of the abovetechniques can be employed. For example, Schafer & Associatesis effectively using CSI’s  229-L heat dissipation matrixpotential sensor to monitor water changes in a multi-layer capsystem consisting of a low permeability barrier layer beneath avegetation cover.  By installing heat dissipation sensorsimmediately above and below the low permeability layer, thebarrier’s effectiveness at limiting infiltration can beevaluated.  Also, these sensors demonstrate the wetness of thebarrier layer and indicate possible trends which can adverselyaffect cap performance.

Quarterly site visits are conducted to collectneutron-probe and oxygen-sensor data and to perform routinemaintenance.

Another effective technique for monitoring water content changesin waste rock materials has been the CS615 water contentreflectometer probe.  Installed horizontally within the uppersurface of an undisturbed waste rock pile, the CS615providesbaseline information on water flux into the pile.  Followingreclamation with a vegetated cap, the CS615 can be used to monitorthe net surface water flux into the waste rock while vegetationestablishes.  The collected data not only allows assessment ofthe vegetation cap performance, but also enhances input parametersfor water balance models.

During the formation of ARD, the oxidation of the pyrite in thewaste rock materials is an exothermic reaction.  As the pyriteoxidizes to ferrous sulfate andacid,1440 kJ mol-1of pyrite isgenerated(Harries and Ritchie 1981).  Schafer &Associatesis using CSI’s 107 temperature probes to monitor temperaturechanges across the vertical profile of capped waste rockpiles.  Using trend analysis techniques along with monitoringdata, both ARD development and constructed cap performance can beevaluated.

Monitoring reclaimed waste rock piles is an important tool inevaluating the performance of constructed caps.  Datagenerated from continuous monitoring techniques allow improvedinterpretations of water movement and potential reactions occurringwithin the cap and waste rock materials.  CSI equipment hasand will continue to play an integral role in our monitoring andevaluation of engineered controls to limit ARD.