Robert Stallard. U.S. Department of the Interior U.S. Geological Survey

Transcription

1 Mining in humid tropical landscapes examined using gold as a model An Examination of Dispersal Processes and Mercury in Living Landscapes Caroní River Basin, Venezuela Robert Stallard Work and personnel supported by: Forest Service, United States Department of Agriculture (USFS) Corporación Venezolana de Guayana (CVG) Geological Survey of the United States Department of the Interior (USGS) U.S. Department of the Interior U.S. Geological Survey

2 Ecosystem Services & Industrial Landscapes Water supply hydropower, navigation, and public consumption requires infrastructure. Water quality low levels of contamination and sediment forests do well here. Biodiversity terrestrial & aquatic. Carbon storages Forest / Soils Carbon replacement Hydroelectricity. Intangible beauty Ecotourism.

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7 Guri Dam Corporación Venezolana de Guayana (CVG) Electrificación del Caroní (EDELCA) Stage 1, , 105 m, 2.1 GW Stage 2, 1986, 162 m, 10.2 GW, ~#3 ~# 8 in lake volume, m 3 Spillway, 27,000 m 3 s -1. Several Downstream dams are being added to the 100 m drop downriver.

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10 Extinction: Carrizal Seedeater?

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22 Mining and the Tropics 1 The warm temperatures and abundant moisture in the humid tropics create conditions whereby the impact of mining can be widespread, substantial, and sometimes unique to the tropical setting. Mining mobilizes geologic material at rates that greatly exceed natural processes, dramatically increasing the supply of sediments to rivers to the detriment of ecosystems, lakes, and artificial reservoirs downstream.

23 Mining and the Tropics 2 Ores and fresh bedrock are chemically unstable in the earth-surface environment. The oxidation of sulfide ores produces sulfuric acid and enhanced leaching of often-toxic trace metals into surface and ground waters. The extraction process also may use toxic chemicals, which if not recovered also enter surface and ground waters. Restoration of landscape, protection of surface and ground waters, and recovery of wastes require funds, enforced laws, and a stable system of governance.

24 Mining and the Tropics 1 The list of major geologic commodities extracted in tropical settings includes, iron, aluminum, gold, diamonds, tin, cobalt, nickel, and rare earths. This discussion focuses on gold in Amazonia, for which a nearly complete suite of issues related to mining is encountered. This discussion focuses on gold in Amazonia, for which a nearly complete suite of issues related to mining is encountered.

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28 Gold Properties & Ore Dense malleable solid metal specific gravity 19 Chemically inert, found in its attractive brilliant metallic form, but often gold particles are quite fine and the gold must be separated from its ore. Hard-rock gold is typically associated with hydrothermal activity but not necessarily with sulfide ores. Gold is so inert an so malleable that it survives weathering and erosion Sedimentary deposits of fine gold with coarser aluminosilicates are called placer deposits. Gold is deposits are formed in zones of deep and long-term, humid-tropical weathering, where it appears that humic materials may have a role in translocating the gold downward in soil profiles. This is referred to as a supergene deposit.

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37 Sediment and Reservoirs Reservoir lifetimes are inversely related to sediment input. The estimated Guri lifetime of 10,000 years could be reduced to 100 years by a 100-fold sediment input.

38 Gold Extraction Gold rapidly dissolves in liquid mercury to form an amalgam, which can be boiled off, leaving residual gold. Effective for placer gold. Gold recovery is only 40% efficient. Gold reacts with cyanide in the presence of oxygen. Gold cyanidation (MacArthur-Forrest Process 1887) involves oxidizing gold in the presence of cyanide ions and oxygen under alkaline conditions (to prevent degassing of HCN), extraction of the dicyanoaurate complex anion, then isolation and hightemperature reduction to form metallic gold. Gold cyanidation is an industrial process outside the realm of the artisan miner. Although cyanide conjures up gas chambers and rat poison, it degrades in the environment and is therefore much less dangerous than mercury, which persists and is toxic in all forms.

39 Renegade or Artisan Gold Mining The renegade or artisan miner receives close to the world market price for gold attractive work. Simple panning has a small impact if mercury is not used. Dredging and sluicing stir up vast quantities of sediment. Mercury is commonly used to increase gold extraction. Despite this, extraction is about 40% efficient. All the mercury is lost to the environment: Half is Hg 0 gas and half is metallic liquid. The liquid mercury continues dissolving gold.

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42 Mercury, Gold, and the Tropical Environment Mercury from gold extraction enters the tropical environment in three forms: mercury metal [Hg 0 ], divalent mercury [Hg 2+ ], and dimethyl mercury [Hg(CH 3 ) 2 ]. All forms are poisonous; dimethyl mercury is the worst. 3. Sunlight and ozone convert Hg 0 to Hg 2+. Hg 0 Hg 2+ +2e - 4. Hg 2+ in air is deposited on leaves and dissolves rapidly in clouds and rain. 2. Fire is used to separate mercury from gold by distillation, often in open air. 1. Mercury metal is used to dissolve gold out of river sediment. Metallic mercury vapor Gold 8. People are poisoned. Hg(CH 3 ) 2 5. Hg2+ is then deposited on soil and enters steams. 6. The ratio Hg 2+ :Ca 2+, which may be higher in tropical soils and streams, might promote mercury uptake if Hg 2+ follows Ca Microorganisms in anoxic soils, sediments, and waters convert Hg 2+ into dangerous Hg(CH 3 ) 2, which bioaccumulates.

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44 Channel Dispersal and Mercury Problems: Great density and liquid nature restrict mobility will go to lowest parts of a channel to sit and react. Volatile on hot exposed floodplains. Very difficult to assess inventories. Dissolved mercury and methyl-mercury are very hard to measure. Recovery: Probably near site of introduction. Continues to dissolve gold recovery could be self financing.

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56 We Know Fish and people eating fish on the Caroní have seriously elevated concentrations of methyl mercury in their bodies (one USGS researcher was poisoned by eating a single meal of peacock bass). Unfortunately, the life cycles and eating habits of aquatic tropical organisms are poorly understood. Dissolved mercury downriver from mining areas is <1/10 the level that is considered by the EPA to be hazardous to aquatic life.

57 We do not Know Whether this is a natural situation, such as the bioconcentration of mercury in swordfish. If the mercury is not natural, whether the source is: Gold mining Dam construction and associated inundation of forest (as is seen in Canada and Scandinavia). Global inorganic mercury pollution (coal burning) and local conversion to methyl-mercury.

58 Restoration Though Industrial Mining Renegade mining leaves behind 40% of the gold plus discarded liquid mercury continues dissolving gold. Industrial mining uses cyanidation. Corporations can be required to use best practices to minimize local and regional impacts and undertake restoration (assuming a system of well written contracts and enforced laws). Recovery of liquid mercury from renegade mining would mitigate a major problem and provide additional gold recovery.

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61 Best Practices Sometimes best practices for some industries agree with goals of conservation and sustainability in a changing world. Here: Hydroelectricity replaces fossil fuels in well designed systems (complicated issue). Sediment reduction for reservoirs requires minimized road building, mining, and deforestation. Food safety for reservoir-based fisheries requires minimum introduction of bioconcentrating toxins (mercury, PCB s, some pesticides). Ecotourism requires attractive landscapes.

62 Investigation Time Scales Local process studies. Demonstration sampling, initial site surveys one time each. Water & sediment events and biweekly Rain and air bimonthly Bioinventories seasonal Soil seasonal Human seasonal Longitudinal dispersion monthly or seasonal: low, rising / high, falling. Guri and lower Caroní monthly.

63 Geochemical Sampling Regions 1 Local processes -- studies that identify processes controlling dispersal of mercury near sites of acute contamination. Probably best done as paired contaminated / clean watersheds Icabaru headwaters for Caroní system. Maximum impact for Caroní River system. Km 88 not in Caroní watershed. probably most contaminated region. Much of are controlled by Placer Dome Inc. who is required to clean up preexisting contamination.

64 Geochemical Sampling Regions 2 Upper Caroní River -- seasonal longitudinal study -- pseudo-lagrangian study to examine dispersal of sediment along mainstem. This presumes that impact of mining is sufficient to be seen as sediment contamination. Fish should be harvested too.

65 Geochemical Sampling Regions 3 Guri / Lower Caroní mass balance. The flooding of the Guri reservoir is a potential non-mining mercury source. Sample discharges and biota of upper Caroní, Paragua, Guri Dam, Caroní mouth. Questions: Is Guri flooding a source of methyl mercury? Is the Guri Reservoir a sink for mining contamination from Upper Caroní and Paragua? Is mining in the lower Caroní a significant mercury source? Importance: Large human population Logistically easy -- sustainable on the long term.

66 Sampling of Fluvial Transport in the Caroní Basin Longitudinal mass-balance study starting in areas with extensive use of mercury. Identification and establishment of control sites with no known contamination. Clean rivers in National Parks Rain-fed lakes. Mass-balance study of the Guri Reservoir. We established the efficacy of the approach, but changes in Venezuela eliminated funding.

67 Geochemical Sampling Water (Hg species, ph, Eh, conductivity, temperature, O 2, DOC, alkalinity, Cl, SO 4 2, Na +, K +, Mg 2+, Ca 2+, Si(OH) 4, Fe, Al): Local studies -- rivers, tailings pools, soil water, rain. Longitudinal studies -- rivers Guri / Lower Caroní -- rivers and rain Sediment and soil (Hg species, texture, coatings, bulk chemistry (including IEC and C org ) mineralogy, Au, Ag)

68 Biological Sampling Humans. Fish and their food chains. Terrestrial and aquatic indicator organisms that are common in many settings: Earthworms soil and sediment feeders large and not very mobile abundant in alluvium. Bromeliads good atmospheric samplers. Identify the important organisms and conditions that promote methylation virtually unknown.