A 486 VOLUME 109 | NUMBER 10 | October 2001 • Environmental Health Perspectives

Innovations

Environmental Health Perspectives • VOLUME 109 | NUMBER 10 | October 2001 A 487

Innovations • Probing the Depths of a Solution for Acid Mine Drainage

Acid mine drainage (AMD), caused by the physical andchemical weathering of sulfide-containing minerals such as iron pyrite,

has been called one of the biggest threats to the environment in theUnited States. It is not confined to the United States, however; it is

a global issue, affecting not only any country where miningactivities have taken place but also neighboring countries,whose environments may be adversely affected by migrating

pollution. According to the U.S. Environmental ProtectionAgency, AMD has polluted ground and surface water, and hasrendered streams and even whole river systems unfit for humanconsumption and unable to sustain any form of wildlife. Effortsto treat the problem and control its spread cost the UnitedStates alone billions of dollars annually.

To fight AMD, scientists around the world must monitorand take samples from sites where it may potentially

occur to check for oxidation, the first step in theprocess that results in acid drainage. Ideally, scientistswould like to be able to sample in situ. Now, thanks

to a new probe developed by Australia’s Common-wealth Scientific and Industrial Research Organisation

(CSIRO), that goal may be within reach.

AMD: Soil and TroubleWhen minerals are ground up during the process ofextracting ore, the grain size of the mineral isreduced, and the surface area available for weather-

ing increases. In the eastern United States, the min-ing of high-sulfur bituminous coal is a prime source ofAMD, while in the West the hard rock mining ofcopper, gold, silver, lead, zinc, and associated sulfideminerals contributes a great deal. Acidic debris canalso come from road cuts and blasting through rock

strata for highways.Rainfall or stream runoff combines with oxygen from

the atmosphere and sulfide from exposed minerals to createsulfuric acid. Acidic hydrogen ions are released into the

waters that run from the mine tunnels andpiles of excavated matter. Hydrogen-pro-ducing iron(III) ions are formed throughthe oxidation of iron(II), a process that isaccelerated at low pH by the sulfide-oxi-dizing bacterium Thiobacillus ferrooxidans.Then the iron(III) ions hydrolyze in waterto form iron(III) hydroxide, which releasesstill more hydrogen ions into the waterand continues to lower the pH unless suf-ficient acid-neutralizing minerals are pre-sent in the rock to buffer any generatedacid. The iron(III) hydroxide precipitatesout of the water as a yellowish-orangeslime that coats river beds and kills off vir-tually all bottom-dwelling aquatic life.

Not only can sulfides of heavy metalsincluding cadmium, lead, and arsenicundergo the same process, this acidic solu-tion can also leach heavy metals out of theground, delivering them into waterways.

Lisa Kirk, a senior geochemist withMaxim Technologies of Bozeman, Mon-tana, and a past chair of the NorthwestMining Association’s EnvironmentalIssues Committee, points out that whenoxidation occurs, it doesn’t automaticallyfollow that acid is generated. “Thatdepends greatly on the availability of neu-tralizing material such as limestone withinthe waste,” she says. “The characteristicsof the site have to be taken into accountin all regards.”

Still, when AMD does occur, it canhave very undesireable effects. Results caninclude contaminated drinking water, dis-rupted growth and reproduction of aquatic

plants and animals, a decline in valuablerecreational fish species such as trout, andnegative impacts on outdoor recreationand tourism.

AMD in the United States has pollutedabout 75% of a total 180,000 acres ofreservoirs and 12,000-plus miles of streamsand rivers, according to the EnvironmentalMining Council of British Columbia, aVictoria-based environmental reformgroup. The estimated cleanup cost is some$32–72 billion, according to Burden ofGilt, a 1993 publication by the MineralPolicy Center, a Washington, D.C., non-profit organization.

The council estimates in its onlinereport Acid Mine Drainage: Mining &Water Pollution Issues in BC that inCanada as of 1998 mining operations hadyielded 351 million metric tons of wasterock, 510 million metric tons of sulfide-containing tailings (ground-up minedrock), and more than 55 million metrictons of other potential mining sources ofAMD such as displaced surface matter.The council further states that cleanup atthe more concentrated acid-generatingmines in Canada will cost some Can$2–$5billion, and in British Columbia alone, thestockpile of acidic and heavy metal–contaminated tailings and waste rock frommining is growing by 25 million metrictons each year.

One common method for preventingthe formation of AMD is by keepingacidic waste rock deposits either continual-ly submerged in water and isolated from

air or as dry as possible, buried under abarrier such as clay, to eliminate one or theother of the reaction components. But aswith any system where nature and humansinteract, there is no perfect barrier.Leakage takes place, and the key is to beable to detect that leakage and any sulfideoxidation that results, and to deal with itbefore it contaminates the environment.

To date, the only way to check forsuch infiltration of air and/or water isthrough taking an actual physical sampleof subsurface gas. Brought back to the sur-face, a gaseous sample is run through a gaschromatograph, which can detect the pres-ence and level of oxygen. If oxygen isfound in the sample, it very likely indicatesthat the containment system is allowing airinto the waste.

But it’s possible to accidentally contami-nate a sample with oxygen once it is broughtto the surface. This is where the need for anin situ sampling method comes in.

Down and Dirty Means CleanerSamplesBradley Patterson, an environmentalchemist with CSIRO Land and Water, ispart of a team that has developed a smallelectrochemical probe that can be buriedin a mine waste site. The probe sends a sig-nal up a thin wire to the surface, where arecording station receives the signal foranalysis, either on location or via cellphone connection for remote locations.

“Our group has been active in researchregarding remediation of contaminatedsites,” says Patterson, “especially those con-taminated by organics like gasoline. Formicrobial cleanup of an organic com-pound like that, you need the presence ofoxygen, so we originally developed thisprobe to determine the amount of oxygenbeing delivered to subsurface sites. It was-n’t a great leap from that point to use theprobe to detect oxygen in sealed minewaste sites.”

According to Patterson, the main bene-fit of this technology is that it allows insitu monitoring, thus avoiding the risk ofsample contamination. “With this probe,”he says, “you don’t have to collect a physi-cal sample and bring it to the surface. [Theprobes are] designed to be lower in cost,they last for years, and they yield datathat’s relatively easy to interpret.”

Patterson says the probes are made ofridged polyvinyl chloride, an inert sub-stance resistant to the acidic environmentthat occurs in these sites, and they candetect oxygen in air and water at concen-trations as low as 0.2%. Approximately 20cm long and 4 cm in diameter, the 200 gprobes are typically buried 50–100 m

A 488 VOLUME 109 | NUMBER 10 | October 2001 • Environmental Health Perspectives

Innovations • Probing the Depths of a Solution for Acid Mine Drainage

A drain on the environment. Acid mine drainage of heavy metals including cadmium, lead, andarsenic threatens waterways with toxic contamination. Jo

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apart in drier soils, more closely in water-saturated soils.

According to Patterson, the probes aredesigned to be used in conjunction withthe types of covers industry typically usesfor these wastes. “You prevent the forma-tion of an acidic solution by preventing theingress of oxygen, and the probe will tellyou if you’re doing a good job or not,” hesays. “If it picks up oxygen, you knowyou’ve got a problem, because as the pyriteoxidation in the AMD process consumesoxygen, if oxygen was present this wouldindicate possible significant oxygen ingresscompared to oxygen consumption, andyou can address it before the situation getsout of control.”

The probe’s design relies on diffusionthrough a slotted screen into a chamberhousing a semipermeable membrane, anddiffusion through that membrane into a1.2 mL–volume space that houses an elec-trochemical sensor. The oxygen sensorconsists of a lead oxygen cell with a leadanode and gold cathode. Oxygen enteringthe sensor is converted to water at the goldelectrode, producing a current flow that isproportional to the oxygen concentration.That current is measured as a voltage andconverted to an oxygen concentration,using exposure to the atmosphere prior toinstallation as a reference.

In the late 1990s, CSIRO ran an exper-iment where several probes were buriedwithin a sulfide-rich tailings repository, inboth clean sand above and tailings below asynthetic liner that had been installed toprevent oxygen from reaching the mineraland triggering the formation of sulfuricacid. Measurement of oxygen above andbelow the liner allowed researchers to com-pare oxygen ingress from the atmospherewith oxygen usage at the particular depthsof the probes.

Monitoring for six months, the teamdiscovered that while oxygen concentrationsin the clean sand were near atmospheric lev-els, there were considerable fluctuations inreadings below the synthetic liner over time,which indicated the liner was poorly sealed.Water was pumped in to further seal thebentonite clay used to secure the syntheticlayer, and further monitoring showed a ces-sation of the fluctuations and a decrease inoxygen concentration to less than 0.5%,suggesting the leak had been sealed (normalatmospheric oxygen concentration is about21%, by contrast).

Toward a Rock-Solid TechnologyExceptionally robust, the probes aredesigned to last from two to five years(some test site probes have been buriedthat long and are still functioning), but

Patterson admits the wires are a potentialweak link in the system. “There is certainlythe potential for damage, especially in anindustrial or mining situation,” he says.“When you’re dealing with larger earth-moving equipment, the probes would haveto be well-buried and clearly marked.”

Has the CSIRO team considered thepossibility of using a wireless data trans-mission method? “That’s not somethingwe’ve looked at,” Patterson admits,“although it is certainly possible. The

problem with that would be twofold—itwould require greater power than we haveavailable in the probe, and it would raisethe cost substantially by having to build inmore technology. This is designed as alower-cost solution to the problem.”

Patterson says he and his colleagues arecurrently in negotiations regarding com-mercialization of the product, and declinesto speculate on exact costs of the probe andmonitoring equipment.

To date, the probes have been installedonly in sandy or clay environments, notrock dumps. “I would assume that in arock dump [which shifts over time], you

may need to install the probes in some sortof bedding sand to reduce the [risk] ofcrushing the probes, and encase the wiresin some type of conduit,” says Patterson.

“It would be an interesting develop-ment,” says Lisa Sumi, research director ofthe Environmental Mining Council ofBritish Columbia, “of course, only as longas you could be assured it was a reliablemonitoring method. And the other prob-lem is that this waste is typically such aheterogeneous mixture [meaning the onsettime of acidification, the buffering capacityof the alkaline minerals, and other factorsare notoriously hard to pin down]. I’d beconcerned if this was the only method ofmonitoring the waste.”

Says Kirk, “Oxygen measurement isone of several methods to track oxidationin sulfide-bearing mine waste, and I’d liketo see a probe like this used in conjunctionwith temperature measurements [anothergood way to assess the rate of oxidation]and water quality, particularly pH and sul-fates. Tailing material in particular is typi-cally quite homogeneous, but only interms of its size. As you move through lay-ers of rock during mining operations, younaturally get variations in sulfide level andneutralization potential, so I think youneed more than one method of monitoringto provide a complete picture.

“People tend to view acid drainage asinevitable, but I disagree,” Kirk adds. “Ithink the advances that are coming out ofinstitutions like CSIRO and other agen-cies, in both storage and monitoring, aretremendously promising.”

Lance Frazer

Environmental Health Perspectives • VOLUME 109 | NUMBER 10 | October 2001 A 489

Innovations • Probing the Depths of a Solution for Acid Mine Drainage

Sugge s t ed Read ingPatterson BM, Davis GB. Remote in situ monitoring of gaseous and dissolved oxygen in

sulphidic minewaste repositories. In: Proceedings of the Second AMEEF InnovationConference, 4–5 August 1999, Fremantle, Western Australia. Melbourne, Australia:Australian Minerals and Energy Environment Foundation, 1999; 58–60.

Davis GB, Patterson BM, Barber C, Johnson CD. Evaluating in situ remediation usinginnovative monitoring technologies. In: Proceedings of the 1997 InternationalConference on Groundwater Quality Protection: Remedial Technology andManagement Policy for NAPL Contamination, 1–3 December 1997, Taipei, Taiwan;45–62.

Skousen J, Ziemkiewicz P. Acid mine drainage control & treatment. Morgantown, WV:National Mine Land Reclamation Center, 1995.

Patterson BM, Davis GB, Johnston CD. Automated in situ devices for monitoring ofVOCs and oxygen in water and soil environments. Contaminated site remediation:challenges posed by urban and industrial contaminants (Johnston CD, ed). In:Proceedings of the 1999 Contaminated Site Remediation Conference, 21–25 March1999, Fremantle, Western Australia. Wembley, Australia: Centre for GroundwaterStudies, CSIRO Land and Water, 1999; 227–234.

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Power probe. A new mine probe can detectoxygen, a key ingredient in the formation of acidmine waste, enabling early prevention measures.