BIOGEOCHEMICAL, HEALTH, AND ECOTOXICOLOGICAL PERSPECTIVES ON GOLD AND GOLD MINING - CHAPTER 10 ppsx

Gold Mine Wastes: History, Acid Mine Drainage, and Tailings Disposal Of the major metal mining industries, gold mining is the most waste intensiveDa Rosa and Lyon 1997.. Dumping of mine

PART 3

Effects of Gold Extraction on Ecosystems

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Gold Mine Wastes: History, Acid Mine

Drainage, and Tailings Disposal

Of the major metal mining industries, gold mining is the most waste intensive(Da Rosa and Lyon 1997) Refined gold consists of but 0.00015% of all raw materialsused in the gold-mining process It is estimated that it takes 2.8 tons of gold ore toproduce the gold in a single wedding band, the rest being waste (Da Rosa and Lyon1997) After waste rock is removed and the ore extracted, the ore is processed toseparate the gold from the valueless portion of remaining rock which is known as

erals Tailings can also contain chemicals used in ore processing Amounts oftoxicants in tailings — including arsenic, lead, cyanide, and sulfuric acid — aredeleterious to fish and other wildlife Tailings are usually stored in piles on land or

in containment ponds, but sometimes are pumped back into the underground spacefrom which the ore was mined Dumping of mine tailings directly into rivers orother water bodies is no longer allowed in the United States, but occurs with somefrequency elsewhere, especially in developing countries (Da Rosa and Lyon 1997) This chapter presents an overview of gold mining and gold mining wastes, withemphasis on acid mine drainage effects and mitigation, and tailings disposal into

10.1 OVERVIEW

The mining process consists of exploration, mine development, mining or tion, mineral processing or beneficiation, and reclamation for closure (USNAS1999) Modern exploration involves various types of sophisticated geochemicalsampling, geophysical techniques, satellite remote sensing, and other methodologiesfor identifying deeply buried mineral deposits After mining rights are acquired,exploration continues with testing, usually drilling, which disturbs surface and sub-surface environments, although effects are usually minor The area required for a

extrac-various ecosystems Later chapters deal with gold mining wastes of arsenic (Chapter

11), cyanide (Chapter 12), and mercury (Chapter 13)

tailings Mine tailings and waste rock contain heavy metals and acid-forming

min-164 PERSPECTIVES ON GOLD AND GOLD MINING

large mine and its facilities, including waste dumps and tailings ponds, sometimesexceeds 1000 ha, and in the United States often involves a combination of federaland private lands for a single mine When an economic deposit has been identifiedfrom the exploration and the required permits are obtained, the deposit is preparedfor extraction This involves installation of power, roads, water, and physical supportfacilities including offices, fuel bays, and materials handling systems Surface loca-tions are marked and prepared for storage of overburden materials, tailings, andother wastes (USNAS 1999) In the United States, any citizen can locate and file amining claim on public land — usually administered by the U.S Bureau of LandManagement — entitling the prospector to mineral rights of a certain tract, usually

20 acres (9.1 ha) One part of the claim stipulates that mining operations must notinterfere with fish migration and spawning seasons (Petralia 1996) Proper design

of a tailings disposal system is essential to the economic success of the operation

as well as to the preservation of wilderness, hunting, fishing, trapping, and agriculture(Ripley et al 1996)

Near-surface deposits in open-pit mines are prepared for production by removingthe overlying waste material (USNAS 1999) Deeper deposits involve construction

of shafts and tunnels Mine development has the potential for significant mental damage Most mines use the same basic operations in extracting ores: drilling,blasting, loading, and hauling After blasting, the fragmented rock is transported to

environ-a minerenviron-al processing fenviron-acility Continued mining environ-activities result in growing wenviron-astedumps Mineral processing or beneficiation usually involves crushing and grindingthe ore, separating the valuable minerals by physical and chemical methods, andtransporting the concentrate to a smelter or refinery The waste or unwanted minerals(tailings) are stored in tailings ponds near the mine site Tailings usually containsmall amounts of gold not completely recovered during beneficiation, undesirabletoxic minerals, waste rock minerals, and residual chemicals Environmental damagemay be substantial if stored wastes from tailings dams, ponds, leached rock, or leachsolutions are discharged or otherwise released (Ripley et al 1996; USNAS 1999;Fields 2001) Reclamation returns the mining and processing site to beneficial useafter mining In some cases, however, complete reclamation may not be possibleand long-term monitoring will be necessary Current reclamation practices includereducing slope angles on the edges of waste rock dumps and heaps to minimizeerosion; capping these piles and tailings with soil; planting grasses or other vegeta-tion that will benefit wildlife or grazing stock and help prevent erosion; directingwater flows to minimize contact with potential acid-generating sulfides in the dumps,heaps, and piles; and removing buildings and roads (USNAS 1999)

Adverse effects of gold extraction include land disturbance, erosion, and thedisruption of riverine ecosystems (Ripley et al 1996) Discharges of water containingsuspended solids and runoff from disturbed land affects local streams throughincreased turbidity and reduced light penetration, channel alteration, and alteredstream flow rates and course Heavily mined streams had a reduction in algal speciesdiversity and avoidance by predatory fish Sediment deposition adversely affectedfish behavior, inhibited reproduction, and lowered dissolved oxygen levels Physio-logical effects of suspended solids on Arctic grayling (Thymallus arcticus) areextensive and include abnormal gill development, reduced feeding activity, and2898_book.fm Page 164 Monday, July 26, 2004 12:14 PM

altered pigmentation patterns If left untended, sedimented streams in the Yukon area

of Canada may take as long as 20 years for recovery of water quality and 30 to

70 years for habitat restoration (Ripley et al 1996)

10.1.1 Lode Mining

Where the gold is still held in the host rock, it is known as lode gold and itsextraction is called lode or hardrock mining Commercial operations tunnel into themountain or dig a tunnel or shaft to extract the ore, perhaps blasting out the surround-ing material The ore-bearing rock is then crushed to free the gold (Petralia 1996).The average tenor of gold ore is 0.2 to 0.3 troy ounces per metric ton (Stone 1975).Profits depend upon the amount of ore, current price, and the costs associated withmining, treating, transporting, and marketing Access is probably the most importanteconomic factor, and excessive costs of road building can make a fairly rich oredeposit uneconomical Permissible lode mining claims — as filed with a countyclerk — are usually limited to 1500 ft (457 meters) along the vein and not morethan 300 ft (91 meters) on each side of the vein (Stone 1975)

Lode mining accounts for about 97% of the ore tonnage extracted by hardrockmining in the United States (Da Rosa and Lyon 1997) Lode mining may take theform of strip mining, open-pit mining, and underground mining Strip mining is thestripping away of layers of soil and waste rock over a mineral deposit Open-pitmining involves excavating the surface in a concentrated location to access theunderlying mineral ore body, including gold To reach these deposits, the pit is dug

in a progressive series of stages The walls are usually terraced, 13 to 20 metershigh, and the steps are 5 meters wide Open pit mines can exceed 1.6 km acrossand 1000 meters in depth Open-pit mines create large quantities of waste rock,usually stored on the surface in piles exceeding 100 meters in height These wastesare usually not returned to the pit when the mine closes Underground mine operatorsdig shafts for access and ventilation and horizontal tunnels (adits) for access anddrainage to reach the ore The extracted ore is carried to the surface through theshafts and adits by truck, rail car, and other conveyances The development of newtechnologies for moving vast amounts of earth and for extracting gold from low-grade ores has created large quantities of new and potentially toxic mining wastes(Da Rosa and Lyon 1997)

The main environmental effects of lode gold mining are related to the discharge

of liquid effluents that adversely impact aquatic life (Ripley et al 1996) In Canada,

in 1986, for example, 35 million m3 of water used in auriferous-quartz mining wereultimately discharged to water courses together with about 16 million m3 of minewater Discharges were generally alkaline with pH 7.5 to 8.0, but sometimes theywere acidic with pH range 1.7 to 4.9 (Ripley et al 1996) Gold mine tailingsfrequently exceeded maximum allowable concentrations set by various regulatoryagencies for cyanide (Eisler 1991) and metals (Eisler 2000) At Yellowknife, Canada,gold mine tailings effluents contained, in mg/L, 84.0 for total cyanide (vs 2.0 formaximum allowable concentration); for other components in the waste stream thesevalues were 4.7 for arsenic (1.0), 5.0 for copper (0.6), 0.4 for nickel (0.2), and 20.0for zinc (1.0; Ripley et al 1996)

166 PERSPECTIVES ON GOLD AND GOLD MINING

In 1992, about 75% of the lode gold mines in Canada operated underground(Ripley et al 1996) The gold in these auriferous-quartz deposits is usually recoveredusing crush and grind, cyanide leach, zinc precipitation, or carbon in pulp extractionprocesses followed by refining Some operations roast the ore prior to cyanidation inorder to free gold particles enclosed in arsenopyrite for leaching, with subsequentrelease of arsenic (Ripley et al 1996) Arsenic wastes and wastes from the cyani-

as tailings over another area of bed (Harvey and Lisle 1998) Placer mining in activestreams may adversely affect habitat for benthic macrobiota and spawning habitat

of aquatic animals (USNAS 1999)

Placer gold mining in the United States began in the eastern states during thelate 1700s and in the southern Appalachian region in the early 1800s (West 1971).After the richer deposits were exhausted, interest turned to New Mexico where goldplacer mining was documented in 1828 In early 1848, a major strike was made onthe American River, California, and triggered the first of the great domestic goldrushes In Alaska, gold mining was reported as early as 1848 In Canada, gold wasfound in the Yukon Region in 1878 Rich finds were reported in the CanadianKlondike region of the Yukon in 1897 to 1898 Gold was mined in Nome, Alaska,

in 1898, and in Fairbanks in 1962 (West 1971)

The occurrence of valuable substances (including tungsten, rare earths, garnets,precious stones, gemstones) in gold placers is well known (Buryak 1993) Althougheconomically feasible to extract these materials together with gold, with an overallreduction in mining costs, the practice is not common

Panning and Sluicing

Panning and sluicing are simple forms of placer mining that depend on low-costlabor (Krause 1996; Da Rosa and Lyon 1997) Many of the early gold prospectorsmined by panning, which involves swirling streambed gravels and sands in a shallowmetal pan to trap the denser gold particles Another placer mining technique is topour the stream gravel into a long trough or sluice that contains a series of riffles2898_book.fm Page 166 Monday, July 26, 2004 12:14 PM

dation process are discussed in more detail in Chapters 11 and 12, respectively

along the bottom The denser gold particles are trapped in the riffles while the lessdense sediments are washed away.

Hydraulicking

As the amount of gold which could be recovered easily by stream panningdwindled, a new form of capital-intensive placer mining was practiced Commonlycalled hydraulicking and first used in California in 1853, this technique involvedspraying gravel banks of rivers with pressurized water and capturing the runoff inlong sluices to recover the gold particles (Nriagu and Wong 1997; Da Rosa andLyon 1997; USNAS 1999) The high-pressure nozzles used in hydraulic operationsconsumed water at the rate of about 20,000 m3 per hour, washing out large portions

of the river banks (Da Rosa and Lyon 1997) To obtain the large quantities ofwater needed, mining companies constructed dams and more than 8200 km of waterdelivery systems to transport the water from reservoirs to mining sites The largeamounts of sediment mobilized by hydraulicking choked natural streambeds withmud and sand, causing flooding that impacted agricultural crops, fisheries, anddrinking water for livestock and humans In 1882, agricultural interests in Marysville,California — after a series of hydraulicking-induced floods — initiated legal actionagainst mining companies In 1884, Judge Alonzo Sawyer ruled in favor of farminginterests by enjoining the mining companies from discharging debris into the floodedwaterways and its tributaries The Sawyer decision started the decline of the hydrau-lic mining period in California In 1893, the U.S Congress passed the CaminettiAct which provided for restricted hydraulic mining in California under the control

of the California Debris Commission and required hydraulicking operations toimpound all debris (Da Rosa and Lyon 1997) Today hydraulicking is practiced inonly a few places in the United States, and these operations need to comply withstate and federal water quality discharge requirements (USNAS 1999)

Dredging

Placer dredging consists of digging underwater deposits by a rotating cutterheadand suction line or by rotating a cutting bucket line (Nriagu and Wong 1997) Thedredged material is delivered onto a floating platform into a revolving screen orshaking table, and disaggregated using a jet of water The fluid mixture falls throughperforations in the screen or table onto a series of sluices equipped with gold-savingriffles, mats, and mercury Primitive forms of dredging were used in West Africa inthe 1700s and the first steam engine for dredge service was constructed in England

in 1795 (Nriagu and Wong 1997) The first successful bucket line dredge in theUnited States was operated in 1896 in southwestern Montana (West 1971) Placermining in most areas of western North America benefitted from the introduction ofthe dredge in 1898, making possible consolidation of many small claims into largeleases (Nriagu and Wong 1997) In Alaska, gold dredging began in 1903; by 1914,

42 dredges were in operation, with a peak of 49 reached in 1910 (West 1971).California had 63 operating dredges in 1910 Dredging was interrupted by WorldWar II in 1941; after the war, in 1945, dredging costs were prohibitively high andonly a few of the deactivated dredges were returned to service (West 1971)

168 PERSPECTIVES ON GOLD AND GOLD MINING

In 1986, however, the world’s largest bucket line offshore dredge began tions on 85 km2 (21,000 acres) of the State of Alaska through leases brokered bythe U.S Minerals Management Service (Barker et al 1990) The operation producedabout 1.1 tons (36,000 ounces) of gold in 1987 worth US $34.5 million; a similarresult occurred in 1988 Fine gold is also mined along the coast and sea floor offNome, Port Clarence, Tuksuk Channel, Cook Inlet, Yakutat, and other locations(Barker et al 1990)

opera-Suction dredging and associated activities have various effects on stream systems, and most are not well understood (Harvey and Lisle 1998) Suction dredging

eco-is common during the summer in many river systems in western North America andreportedly adversely affects aquatic and riparian organisms, channel stability, anduse of river ecosystems for other human activities Suction dredging is subject tofederal and state regulations, but additional regulations seem needed to protectthreatened or endangered aquatic species in dredged areas, incubation of embryos

in gravel substrates, or spawning runs followed by high flows (Harvey and Lisle1998) Suction dredge gold mining in a northern California stream in 1983 did notsignificantly affect mean numbers of benthic invertebrates or diversity indices; how-ever, some taxa were adversely affected at selected sites (Somer and Hassler 1992).Dredging dislodged aquatic insects that were eaten by young coho salmon (Onco- rhynchus kisutch) and steelhead trout (Oncorhynchus mykiss) Sedimentation ratesand organic fractions were elevated downstream from the dredging In 1984, cohosalmon and steelheads were observed spawning in areas that had been dredged in

1983 (Somer and Hassler 1992)

10.2 ACID MINE DRAINAGE

Gold mines in the United States and Canada — some more than 100 years old,some recently closed, and some still active — are leaking metal-rich acidic waterinto the environment, resulting in hundreds of millions of dollars in remediationcosts annually (Da Rosa and Lyon 1997; USNAS 1999; Fields 2001).This acidicdrainage, often referred to as acid mine drainage or AMD, is derived from sulfide-containing rock excavated from an underground mine or open pit The sulfur reactswith water and oxygen to form sulfuric acid (H2SO4) Iron pyrite (FeS2) is the mostcommon rock type that reacts to form AMD, but marcasites and pyrrhotites alsocontribute significantly On exposure to air and water, the acid will continue to leachfrom the source rock until the sulfides are leached out — a process that can last forcenturies The sulfur is released by weathering, oxidation, and erosion, with con-current production of sulfuric acid The rate of acid production from inorganicoxidation of iron sulfides is enhanced by various species of acidophilic bacteria,especially Thiobacillus ferrooxidans The acidity of the water and its proximity tometal in the ore may generate waters of low pH that are high in copper, cadmium,iron, zinc, aluminum, arsenic, selenium, manganese, chromium, mercury, lead, andother elements released from the ores with increasing acidity The resulting solution

is sufficiently acidic to dissolve iron tools in underground mines and kill migratorywaterfowl that shelter overnight in pit lakes AMD seeps out of tailings, overburden,2898_book.fm Page 168 Monday, July 26, 2004 12:14 PM

and rock piles being processed for gold removal If left unchecked, it can contaminategroundwater AMD is often transported from the mining site by rainwater or surfacedrainage into nearby watercourses where it severely degrades water quality, killingaquatic life and making water virtually unusable (Da Rosa and Lyon 1997; USNAS1998; Fields 2001)

Anthropogenic AMD dates back to at least the Middle Ages, but new techniques

in gold mining have produced a virtual flood of acid water throughout the AmericanWest, Canada (Fields 2001), and elsewhere (Cidu et al 1997; Ogola et al 2002).Naturally occurring acid rock drainage can produce a trickle of acidic waste thatstains rock faces red from iron Mining, however, accelerates the process by exposingvery reactive components — potentially unstable thermodynamically with respect

to oxygen — to surface atmospheres (Fields 2001) Underground gold mines ture ore bodies with adits, mine tunnels, and shafts that allow air and water to enterand react with sulfide materials that are exposed inside the mine (Da Rosa and Lyon1997) AMD can leach from underground mine openings into streams and aquifers

punc-In open-pit mines, sulfide minerals on the exposed sides of the pit excavation aremoistened by precipitation or by groundwater seeps, generating intense AMD flows(Da Rosa and Lyon 1997)

10.2.1 Effects

Aquatic ecosystems are considered the most sensitive to the effects of AMDwaters, toxic heavy metals, and sediments from mining Collectively, these contam-inants cause disrupted reproduction, altered feeding, inhibited growth, habitat loss,decreased respiration, death, and chronic degradation of the aquatic environment(Da Rosa and Lyon 1997) Massive fish kills are reported after a major spill or suddenstorm which adds additional pollutants to streams In many AMD-impacted streams,there is no life for several kilometers downstream of a mine except for the mostacid-resistant species Land animals, such as mink (Mustela vison) and otters (Lutra

spp.), dependent on aquatic systems for food and habitat are also affected by AMD,with population declines reported near affected streams (Da Rosa and Lyon 1997) AMD is usually first recognized when streams or pools appear orange (Da Rosaand Lyon 1997) Acid waters dissolve and mobilize many metals, including iron, copper,aluminum, cadmium, and lead These, especially the iron, precipitate with decreasingacidity and coat stream bottoms with an orange-, red-, or brown-colored slime or cement(Da Rosa and Lyon 1997) The cement physically embeds gravels, impairing streambedhabitat for fishes and macroinvertebrates (USNAS 1999) When the spaces betweengravels are embedded with fine-grained sediments or floc, egg survival of trout, salmon,and other benthic spawners is threatened by lack of oxygen (USNAS 1999) Below a pH level of 4.0, most aquatic organisms die (Da Rosa and Lyon 1997).Many streams receiving AMD are 10 to 100 times more acidic (pH 2 to 3) than theconcentration lethal to most species of aquatic plants and animals, except selectspecies of acid-tolerant bacteria Heavy rains can flush large amounts of acidifiedmine wastes into streams, causing massive fish kills The main physiological mech-anisms for fish death in acid water are osmoregulatory failure and impaired oxygenuptake At pH 3.5 to 4.0, only about half the frog and salamander embryos tested

170 PERSPECTIVES ON GOLD AND GOLD MINING

had survived Most freshwater fish species were unable to survive when water pHwas less than 4.2 At pH levels less than 4.5, most benthic species of animals died

At sublethal pH levels less than 5.0, most aquatic plants were impaired and tolerant plants tended to dominate Heavy metals and sediments associated withAMD exacerbated the toxic effects of low acidity (Da Rosa and Lyon 1997) One gold mine in California discharged AMD into the Sacramento River forabout 100 years until mining was halted in 1963 Fish kills of hundreds of thousands

acid-of salmon and trout have been documented at this site since the 1920s Unlessremediation is implemented, low AMD pollution may persist for hundreds of years(Da Rosa and Lyon 1997) A gold mine that opened in 1988 in the Black Hills ofSouth Dakota began generating AMD in 1992 In 1994, and again in 1995, AMDflooded offsite into a nearby creek, creating a low pH (2.1) environment lethal tofish and invertebrates (Da Rosa and Lyon 1997) At Spirit Mountain, Montana, AMDcontaminated the drinking water supply of about 1000 nearby residents with lead,arsenic, and cadmium (Fields 2001) When consumed in high doses, sulfates mobi-lized during AMD can cause diarrhea and other gastrointestinal problems, especially

in children (Da Rosa and Lyon 1997)

10.2.2 Mitigation

Acid will continue to be generated until the iron sulfides are leached from themine waste material, or until steps are taken to completely seal off the sulfide rocksource from oxygen and water (Da Rosa and Lyon 1997)

Methods for prevention of acid drainage include those that prevent acid ation from starting and those that treat the acid generation at the source so that nodrainage occurs (Da Rosa and Lyon 1997; USNAS 1999; Fields 2001) Prevention

gener-of acid generation usually includes capping and sealing acid-generating rock toprevent air and water from reaching the rock and initiating the generation of acid

In dry climates, a less effective seal and a good vegetative cover may allow transpiration of most of the water infiltrating into the pile Another variation oncapping — and one practiced widely outside the United States — is to bury acid-generating materials in water to prevent contact with air This is accomplished byplacing the waste in a closed body of water or by covering the top of a tailings pondwith water once tailings deposition is completed Subaqueous tailings disposal ofacidic mine wastes is used at several Canadian mines wherein wastes are dischargedunder water into a prepared impoundment or a natural body of water, such as a lake

evapo-or the ocean floevapo-or — although discharge into natural waters is prohibited in theUnited States Some mine operators, both domestic and foreign, place potential acid-generating materials into pits that are expected to fill with water Once the pit lake

is formed, the material is no longer exposed to air However, covering the rocks andtailings may not prevent oxygen from reacting with sulfides in the rocks becausesubstantial amounts of oxygen can be trapped in waste rocks and tailings andoxygenated water can infiltrate the area from other sources To reduce the availability

of sulfides to both water and air, new techniques are under investigation, includingautoclaving and encapsulating the rock in materials such as silica

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The use of chemical additives to prevent acid generation when applied to wasterock or spent ore piles is economically feasible (USNAS 1999; Fields 2001) Themost common method for treating in place to prevent acid drainage is to add lime

or other neutralizing materials to acid-generators The neutralizing materials need

to be in sufficient concentration to counteract all the acid-generating potential Thelong-term effectiveness of this type of mixing is unknown, and the relative rates ofacid generation and neutralization are not well documented Other cost-effectiveprocesses to prevent acid drainage include separation of acid-generating portions ofthe ore from other components, and these portions can be treated more efficientlythan the larger volume of spent ore material (USNAS 1999; Fields 2001) Acid drainage that contains metals is a potential long-term water quality issue

at some mine sites The factors that create acid drainage and that minimize its impactsare well understood; however, few long-term monitoring data are available to predictthe extent of damage at a specific mine site Further, it is difficult to predict whenacid drainage will start, the degree of acidity, and the total amounts of metals involved(USNAS 1999) One procedure used to predict AMD is acid–base accounting, which

is based on estimations of acid-generating and acid-neutralizing materials in thewaste rocks (Da Rosa and Lyon 1997) Minerals containing sulfur, especially pyrites,have the potential to generate acidity when exposed to water and oxygen Buffering

or neutralizing-acid minerals include carbonates, especially CaCO3 The erating and acid-neutralizing potentials are expressed as numerical values and arecompared to predict the potential for generation of AMD However, acid–baseaccounting does not include the potential role of bacteria and other variables inproducing AMD In one case, a gold mine near Elko, Nevada, has been combatting

acid-gen-a serious AMD problem since 1990, when surfacid-gen-ace wacid-gen-ater dracid-gen-ainacid-gen-age from the mine’swaste rock piles began generating acid (plus mercury and arsenic), contaminating3.2 km of a nearby stream Acid–base accounting tests conducted by the mine owners

on rock samples indicated that no potential acid problems were expected (Da Rosaand Lyon 1997) Accordingly, kinetic testing is often used to supplement acid–baseaccounting and is based on acid generation from materials in a controlled chamberenvironment of air, water, and bacteria In contrast to acid–base accounting, kinetictests on mine wastes use a larger sample volume, and tests are run for extendedperiods of time, often months (Da Rosa and Lyon 1997)

In some mines, remediation efforts can be concentrated on specific areas withinthe mine Using these techniques, problem areas can be identified and contaminatedflows isolated or diverted (Hazen et al 2002) For example, in one multiple-levelunderground mine in Colorado that was in gold production between 1870 and 1951,hydrometer measurements using water isotopes of hydrogen and oxygen were used

to identify problem areas Measurements showed that discharges from a central levelportal increased by a factor of 10 during snowmelt runoff, but zinc concentrationincreased by a factor of 9.0 Less than 7% of the peak discharge of zinc was fromsnowmelt; the majority was from a single internal stream with high zinc (270 mgZn/L) and low pH (3.4) New water contributed up to 79% of the flow in this highzinc source during the melt season Diversion of this high zinc source within themine decreased zinc flow by 91% to 2.5 mg/L (Hazen et al 2002)

172 PERSPECTIVES ON GOLD AND GOLD MINING

An alternative to chemical treatment of AMD is bioremediation, a set of passivetreatment techniques which use bacteria or other organic agents (Da Rosa and Lyon1997) For example, bactericides to inhibit iron-oxidizing bacteria, such as Thioba- cillus ferrooxidans, have been used successfully to reduce the costs of treating acidicrunoff from reactive waste rock piles Another successful technique is the construc-tion of wetlands to route mine effluents through areas stocked with metal-absorbingaquatic plants, such as cattails (Typha latifolia) These plants can also serve as agrowing base for bacteria that function as metal collectors (Da Rosa and Lyon 1997)

10.3 TAILINGS

Most of the early metal mines in the United States dumped tailings directly intostreams (Da Rosa and Lyon 1997) Damage from this practice was extensive andpersisted unchecked until the 1930s At that time, tailings were impounded, althoughsome mines continued to flush tailings into public waterways until enforcement ofthe Clean Water Act in the 1970s forced them to stop Tailings are now stored in apond or impoundment behind an earth-fill dam However, impoundment embank-ments can fail if improperly designed or located on an unstable foundation (Da Rosaand Lyon 1997) In the Philippines, for example, a tailings impoundment failed inMarch 1996 discharging 4 million tons of tailings — containing copper, lead,mercury, cadmium, and other contaminants — into a nearby river, blocking 26 km(Fields 2001) Mines in some countries continue to discharge tailings into nearbywaterways In Indonesia, a single gold mine has discharged 120,000 tons of tailingsdaily since 1972 (43.8 million tons annually) into a nearby river system, flooding morethan 30 km2 of rainforest and agricultural lands (Da Rosa and Lyon 1997) In general,metal wastes from gold mining — specifically, cadmium, zinc, and copper — exceedall current guidelines promulgated by regulatory agencies for freshwater and marinelife protection via the medium, and health of humans, other mammals, and birdsfrom ingestion of contaminated diets (Eisler 2000)

Selected examples follow on results of field studies and laboratory investigations

of tailings waste disposal into freshwater, marine, and terrestrial ecosystems

photosynthesis-inhib-Field Investigations

Gold mining in the Black Hills of South Dakota has remained an active industrysince gold was first discovered there in 1874, with most of the mining associated2898_book.fm Page 172 Monday, July 26, 2004 12:14 PM

with gold veins and placers in the northern Black Hills (May et al 2001) At leastfive large gold mines involving 800 ha are still operating Gold recovery from lodemines was originally accomplished through mercury amalgamation, with an esti-mated 15 kg of mercury lost daily to Whitewood Creek in the northern Black Hills.The use of mercury was discontinued in 1971 and replaced with processes relying

on cyanidation In addition to mercury, daily averages of 140 kg of cyanide, 100 kg

of zinc, and 10,000 kg of arsenopyrites were also released into Whitewood Creekevery day In the 103 years between 1876 and 1978, a total of about 100 milliontons of finely ground gold mill tailings were discharged into Whitewood Creek.Recent analysis of water samples from the impacted areas indicates levels of concern

in water for arsenic (>50 µg/L) and selenium (>5 µg/L) Sediment concentrations,

in mg/kg DW, for arsenic (1951), cadmium (3), copper (159), mercury (0.6), nickel(64), lead (176), and zinc (250), were considered sufficiently high for potentialadverse ecological effects, including metals accumulation into the benthic food chainfrom sediment-released metals May et al (2001) recommend more research on thedynamics of metals transport from sediments and accumulation in food webs, andexperimental studies of effects of metals-contaminated invertebrate diets on salmo-nids and other fishes typical of the study area

In Montana, restoration of Whites Creek — home of the West Slope cutthroattrout (Oncorhynchus clarki) in the Big Belt Mountains — began in 1995 (Skidmore1995) This trout stream had been devastated by historic gold mining activities over

a 60-year period that ended in the 1940s The surrounding area had been stripped

of its gold by dredges and ground sluicing A combination of tailings, piled highwithin the narrow valley, and hydraulic mining produced an eroding and unstablestream that threatened the survival of the last remaining native population of WestSlope cutthroat trout

In Alaska, most of the gold is recovered from placer deposits, and tailings areassociated with turbidity and toxic metal problems (Pain 1987; Yeend 1991; LaPerriereand Reynolds 1997) The gold frequently lies in gravel over the stream bedrock Toreach the gold, the vegetation, soil, and gravel over the deposit are removed and thegold separated from the gravel, usually by washing the deposit through a sluice from

a nearby stream The most obvious damage around a placer mine is the physicaldestruction to the vegetation and stream banks In one creek, where mining hadceased 60 years earlier, only about 25% of the bank supported plants The lack ofground cover makes the banks unstable and liable to erode into the stream duringstorms The water immediately below the mine contains a high proportion of fineclay particles and sand Some of these particles are trapped in holding pools, asrequired by permit But the smaller particles frequently remain in suspension andescape into the stream Sediments in water can be divided into components, includingsettleable or nonsettleable solids, total solids, total suspended solids, total dissolvedsolids, and fixed and volatile components Excessive sediments in water may alterthe physical and chemical properties of the receiving water body, with adverse effects

on the native plants and animals Turbidity is an approximation of the amount ofsuspended solids in water Increasing turbidity, for example, restricts photosynthesis,thereby limiting the base of the food chain Increasing sedimentation may decreasealgal productivity through smothering and scouring Current regulations mandate

174 PERSPECTIVES ON GOLD AND GOLD MINING

that discharged water must be treated to reduce settleable solids to <0.2 mL/L Alaskahas set limits for turbidity of stream water and the permissible amount of suspendedsediments Turbidity is measured in nephelometric units (NTUs) NTUs are based

on the amount of light scattered by a water sample and calibrated against a standard

To protect stream life, turbidity should legally not exceed 25 NTUs above ground, but these values were often 100 to 1000 times higher downstream of manyactive mines The increased sediment load reduces the amount of oxygen in thewater as microorganisms break down the organic material from the soil Increasedsediment loadings also increase alkalinity, sequester nutrients by binding them intochemical complexes, blanket the stream bed effectively destroying the benthos, andproduce or prevent plant photosynthesis (Pain 1987; Yeend 1991; LaPerriere andReynolds 1997) At 25 to 50 NTUs, the light reaching a depth of 10 cm is about 60%that at the surface Between 500 and 1000 NTUs — common levels in heavily minedstreams — only 0.3 to 5% of the incident radiation penetrates to 10 cm Theoretically,

back-an increase of 5 NTUs cback-an reduce photosynthesis in shallow streams by as much as13% An increase of 25 NTUs, the accepted standard in Alaska, could reduce pro-duction by 50% Heavy mining increases turbidity to an average of 1700 NTUs andcompletely inhibits primary productivity (Pain 1987; LaPerrriere and Reynolds 1997) High concentrations of trace metals (Cd, Pb, Zn, Cu) and arsenic associated withgold also enter the waste stream; all of these are known to produce toxic effects insalmon and trout at concentrations near background levels (Pain 1987; LaPerriereand Reynolds 1997) In Fairbanks, for example, some groundwaters are so contam-inated with arsenic from gold mining activities 30 years earlier that they are con-sidered unsafe for drinking Bacteria associated with arsenic in the water drainingfrom lode and placer gold mines oxidize iron and sulfur and probably accelerate therate at which trace metals leach from the sediment (Pain 1987)

Recommendations for habitat restoration in Alaska caused by placer mine ities (Pain 1987) include:

activ-1 Mechanical replacement of gravels and soils and restoration of the normal stream channel.

2 Reduction in the amounts of sediments released.

3 Encouraging the growth of vegetation along affected banks.

4 Maintaining structural integrity of settling ponds to prevent spills.

5 Recycling the wastewater through sluices that contained the gold-bearing gravels

6 Adding chemicals that cause the fine particles to aggregate and sink (a technique

at least 4000 years old) Ancient texts from India about 2400 BCE suggest adding vegetable substances to water for clarification purposes Since 1889, U.S water companies have treated drinking waters with chemical clarifiers; however, these may be too expensive, too inefficient, and may not work at low temperatures typical of northern Alaska Pain (1987) suggests that polyethylene oxide, a water soluble resin, seems promising for aggregation.

In Canada, there are an estimated 6000 abandoned mine sites that pose potentialtailings hazards to aquatic ecosystems In October 1990, 300,000 metric tons ofwater-saturated gold mine tailings spilled into the Montreal River in northernOntario, Canada, via a small creek, as a result of the collapse of a tailings dike2898_book.fm Page 174 Monday, July 26, 2004 12:14 PM