Geology Reference
In-Depth Information
(3.111)) or, in conjunction with other nutrients, to stimulate bacterial
reduction of Cr
VI
, could provide a means of retention of Cr
III
in solution
in preference to either precipitation of Cr(OH)
3
or reoxidation to Cr
VI
under the prevailing alkaline conditions of high-lime COPR sites.
Farmer and co-workers
93
found that
o
3% of dissolved chromium
was present as Cr
VI
in groundwater at one Glasgow borehole site with
a high dissolved organic carbon content of 300 mg L
1
. Here the
chromium occurred predominantly as Cr
III
in association with organic
material of high molecular weight, with an implied reduction of Cr
VI
by
humic substances, perhaps via carboxylate groups, and the formation of
Cr
III
-humic complexes.
Although the drinking water supply of people in the Glasgow area is
not threatened by the Cr
VI
-contaminated groundwater, there are in-
stances elsewhere of such problems. For example, a massive plume of
Cr
VI
-contaminated groundwater in the Mojave Desert, with concentra-
tions as high as 12 mg L
1
, was reported to be within 40 m of the
Colorado River, which supplies drinking water to California, USA.
100
This contamination arises from when an electric utility used Cr
VI
to
control corrosion and mould in water-cooling towers and then dumped
untreated wastewater into percolation beds in the 1950s and 1960s.
Current short-term emergency pumping, transport, and offsite treatment
of the contaminated groundwater will have to be replaced by a long-
term solution close to the river, possibly featuring in situ groundwater
treatment or the installation of a permeable reactive barrier in which
iron would reduce Cr
VI
to Cr
III
.
3.3.2.6 Aquatic Contamination by Gold Ore Extractants. The separate
use of mercury and cyanide has led to the contamination of freshwater
systems.
3.3.2.6.1 Mercury in the Amazon Basin. With the price of gold
soaring in the late 1970s and early 1980s, there developed a modern-
day gold rush in South America,
101
which reached its peak during 1990-
1993.
102
In the relatively inaccessible Amazon area in Brazil, there were
up to a million people operating on an informal basis. The technique
which is used to extract the placer gold which has accumulated in many
stream and alluvial deposits is based on amalgamation with mercury.
Typically, high-pressure water hoses are used to dislodge alluvium,
which is then taken into a large sluice by a motor suction pump. The
sluices are lined with sacking into which mercury is added to amalga-
mate the gold particles, which lodge in the sacking. In other versions, no
mercury is used in the sluices. Instead, material is collected from the
