74
L.D. Lacerda and W. Salomons
300 000 tons of Hg from 1540 to 1900, mostly in the silver production in Spanish
colonial America and in the North American gold rush in the second half of the
last century (Nriagu 1994). Also, significant mining occurred in Southeast Asia
and Australia (Bycroft et al. 1982; James 1994). In recent years, this technique has
been resuscitated in the tropics, where it represents a simple, cheap, and reliable
technique for gold production, ready to be used by a growing population lacking
proper jobs in most developing countries in the tropics (Lacerda et al. 1995).
Present global annual emission of Hg to the environment from this activity may
reach up to 500 tons (Lacerda and Marins 1997).
A typical characteristic of these small, artisanal mining operations is the low
efficiency of the process due to the intuitive use of Hg. For example, Hg emission
factors (the amount of Hg released to the environment to produce 1 kg of gold)
in Amazon gold mines may range from 1.1 to 5 (Mallas and Benedicto 1986;
Pfeiffer and Lacerda 1988), although the average emission factor is approximately
1.3 (Lacerda et al. 1995). In colonial Spanish America and in last-century North
American gold mines, these emission factors typically ranged from 1 to 2 (Fisher
1977; Nriagu 1994). Most of the Hg is emitted to the atmosphere during the
burning of the Hg-Au amalgam. However, from 30 to 45% of the total Hg emitted
ends up in river banks, soils, and drainages. As a result, Hg-rich tailings are a
common feature left by ancient and recent gold and silver mining operations
using Hg amalgamation.
A major problem of such tailings is their long time span as a significant source
of Hg to the environment. Tailings deposited over 100 years ago and containing
up to 500 ~lg Hg g~1 are still present in central Canada (Lane et al. 1988). In the
Carson River Valley, Nevada, over 100 000 tons of tailings containing nearly
7000 tons of Hg and with concentrations of up to 4900 ~Lg g ~I are continuously
reworked along the valley (Lechler and Miller 1993). Other important old tailings
from gold and silver mining particularly enriched in Hg are the Falson-Natomas
region, California (Prokopovich 1984), the Lerderberg River gold fields, Australia
(Bycroft et al. 1982), and Nova Scotia, Canada (Lane et al. 1988). Small tailing
deposits, however, are scattered world-wide. These old mining tailings, together
with those produced by the "New Gold Rush", represent a significant long-term
environmental threat. However, it should be realised that the impact of mercury
stored in tailings deposits is only part of the whole issue of mercury release from
gold and silver mining operations (Lacerda and Salomons 1998).
3
Mercury Distribution in Gold Mining Tailings
In most gold and silver mining areas, Hg is deposited and accumulated in mine
tailings. An important question is how Hg is eventually transported to, and spread
through pristine adjacent environments (Allan 1995). Although only few studies
have dealt with this subject, a general pattern of Hg distribution in tailings can be
stated, based on data gathered from old and recently deposited tailings.
L.D. Lacerda and W. Salomons
300 000 tons of Hg from 1540 to 1900, mostly in the silver production in Spanish
colonial America and in the North American gold rush in the second half of the
last century (Nriagu 1994). Also, significant mining occurred in Southeast Asia
and Australia (Bycroft et al. 1982; James 1994). In recent years, this technique has
been resuscitated in the tropics, where it represents a simple, cheap, and reliable
technique for gold production, ready to be used by a growing population lacking
proper jobs in most developing countries in the tropics (Lacerda et al. 1995).
Present global annual emission of Hg to the environment from this activity may
reach up to 500 tons (Lacerda and Marins 1997).
A typical characteristic of these small, artisanal mining operations is the low
efficiency of the process due to the intuitive use of Hg. For example, Hg emission
factors (the amount of Hg released to the environment to produce 1 kg of gold)
in Amazon gold mines may range from 1.1 to 5 (Mallas and Benedicto 1986;
Pfeiffer and Lacerda 1988), although the average emission factor is approximately
1.3 (Lacerda et al. 1995). In colonial Spanish America and in last-century North
American gold mines, these emission factors typically ranged from 1 to 2 (Fisher
1977; Nriagu 1994). Most of the Hg is emitted to the atmosphere during the
burning of the Hg-Au amalgam. However, from 30 to 45% of the total Hg emitted
ends up in river banks, soils, and drainages. As a result, Hg-rich tailings are a
common feature left by ancient and recent gold and silver mining operations
using Hg amalgamation.
A major problem of such tailings is their long time span as a significant source
of Hg to the environment. Tailings deposited over 100 years ago and containing
up to 500 ~lg Hg g~1 are still present in central Canada (Lane et al. 1988). In the
Carson River Valley, Nevada, over 100 000 tons of tailings containing nearly
7000 tons of Hg and with concentrations of up to 4900 ~Lg g ~I are continuously
reworked along the valley (Lechler and Miller 1993). Other important old tailings
from gold and silver mining particularly enriched in Hg are the Falson-Natomas
region, California (Prokopovich 1984), the Lerderberg River gold fields, Australia
(Bycroft et al. 1982), and Nova Scotia, Canada (Lane et al. 1988). Small tailing
deposits, however, are scattered world-wide. These old mining tailings, together
with those produced by the "New Gold Rush", represent a significant long-term
environmental threat. However, it should be realised that the impact of mercury
stored in tailings deposits is only part of the whole issue of mercury release from
gold and silver mining operations (Lacerda and Salomons 1998).
3
Mercury Distribution in Gold Mining Tailings
In most gold and silver mining areas, Hg is deposited and accumulated in mine
tailings. An important question is how Hg is eventually transported to, and spread
through pristine adjacent environments (Allan 1995). Although only few studies
have dealt with this subject, a general pattern of Hg distribution in tailings can be
stated, based on data gathered from old and recently deposited tailings.