Environmental risk of mining - Environmental Deterioration and Contamination: Problems and their Management - page 123

environmental monitoring during operation and after closure. However, accidents and
catastrophes caused by inappropriate design, maintenance and monitoring of various
operating or abandoned mining facilities such as tailings impoundments have raised
public awareness of the environmental and safety hazards of mining activities all over
the world. The most common method of tailings disposal is placement of tailings slurry
in impoundments formed behind raised embankments. Today, many tailings impound-
ments are lined to prevent seepage, which is rarely the case at historic mine sites. In
addition, modern tailings impoundments are designed to accommodate earthquake
acceleration.
Some of the mine accidents in Europe since 1985 that raised public awareness are
introduced in the next subchapters.
6.1 The Stava tailings dam failure
Mining activities for silver bearing minerals have been carried out in the Stava valley,
in the Italian province of Trento, since the sixteenth century. In the early twentieth
century, the interest shifted to fluorite, an important mineral for the chemical indus-
try. The most important mine was situated at the base of the mountain of Prestavel,
overlooking the entire Stava valley. To extract the fluorite from the rock debris, flota-
tion technology was used. However, this process produced great quantities of tailings
containing toxic and semiliquid waste. To manage the large amount of flotation tail-
ings, two tailings dams were constructed on a nearby slope. The dams were built by
separating the gravel and sand from the water and slime, a cheap construction method
which uses a part of the tailings to be disposed of. On 19 July 1985 the upper dam
of the facility broke first, leading to the collapse of the lower dam. About 200,000 m 3
of tailings flowed 4.2 km downstream at a speed of up to 90 km/h, killing 268 peo-
ple and destroying 62 buildings until they reached the Avisio River. The total surface
area affected was 43.5 ha. An investigation into the disaster found that the dams were
poorly designed and maintained and safe operation was not ensured (Tosatti, 2003;
Sammarco, 2004).
6.2 The Los Frailes tailings dam failure
The tailings dam failure in the South of Spain at Aznalcóllar in 1998 became very
well known in professional circles. Many studies were done and publications were
issued describing the thorough management of the accident and the rehabilitation
of the site (Achterberg et al. , 1999). Los Frailes was an open pit lead-zinc mine at
Aznalcóllar 40 km west of Seville that operated for many years until it finally closed
in 1996. The ore processing wastes (tailings) were placed in two vast shallow ponds
behind an artificial dam. On 25 April 1998, the tailings dam failed and released about
5.5 million m 3 of toxic tailings water and about 1.3 million m 3 of semisolid waste
(Aguilar et al. , 2010). The acidic tailings, which contained dangerous levels of several
toxic metals (Martín et al. , 2007) reached quickly the nearby Agrio River and then its
affluent the Guadiamar River, traveling about 40 km along these waterways before they
could be stopped. The Guadiamar is the main water source for the Doñana National
Park, a UNESCO World Heritage Site.