Laatste update: 15/06/09
Source: several articles by A.J.P. Smolders et. al.
The Pilcomayo River
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The Pilcomayo River in South America is a tributary to the large La Plata system and has a river basin area of 272,000 km²,
of which 98,100 km² are located in Bolivia. The river originates in the Bolivian Andes (Cordillera Oriental, 5000 m above sea
level) and cuts down through the Andes for 700 km, through an inaccessible terrain with rapids and narrow canyons, until it
reaches the Chaco Plains near the town of Villa Montes (400 m above sea level). Subsequently, the river flows southwards
through the semi-arid Chaco Plains where it forms the borderline between Argentina and Paraguay before it joins the
Paraguay River just north of the town of Asuncion.
The upper parts of the Pilcomayo river have as yet not been regulated by dams or other hydro-technical works. Therefore,
the river is strongly influenced by the hydrological cycle, which is driven by the differences in rainfall between the dry (May–
October; winter) and the wet seasons (November–April; summer).(Smolders et al. 2000; Smolders et al. 2002). This has
important consequences for the functioning of the river ecosystem.
In the Paraguay–Argentine territory large areas of land are inundated by the river during the rainy season; these floodplains
serve as important feeding-grounds for many migratory fish species. This lower part of the river (from Villa Montes to
Asuncion) has a total length of almost 1000 km.
The lower reaches are alternately affected by inundation and drought, while the soils susceptibility to erosion in the Andean
region results in a very high load of suspended matter, that consists of a very fine gray, quartz-rich sand, in the rainy season
(Guyot et al., 1990; Smolders et al., 2002). The largely natural sediment loads have created an alluvial fan system which is
still active (Iriondo 1993). The sediments deposited in the Chaco Plains cause siltation of the river bed and subsequently
result in a gradual withdrawing of the river and the river floodplain in upstream direction (Iriondo 1993; Medina Hoyos 1998;
Smolders et al. 2002). Alluvial sedimentation has interrupted the former direct connection between the upper reaches of the
Pilcomayo river and the Paraguay river.
The mining region near the city of Potosí forms part of the Bolivian Pilcomayo catchment. Mining activities started in the
sixteenth century at Cerro Rico de Potosí. Until the early nineteenth century, this hill remained one of the most important
silver sources of the world. From then on, tin mining gradually became more important and remained so until 1985.
Due to the crash of the tin market and the governmental restructuring of mining operations in 1985, the Crushing-Grinding-
Flotation method was introduced to recover Pb and Zn. As a result, the contamination of the environment by heavy metals
has increased because the grain size of the mine tailings was greatly reduced from several millimeters in the conventional
gravity-concentration method to 0.3–0.05 mm in the flotation method. Moreover, flotation reagents (such as xanthates and
cyanides) are now added during the process (Medina Hoyos 1998; Anonymous 1999). The flotation process also requires
a high pH, therefore lime is added to increase the pH of the tailings. As the applied technology is not optimized, the residual
tailings contain high levels of unrecovered metals.
At present, there are as many as 40 processing plants (ingenios) operating in or immediately adjacent to the city of Potosí
(Anonymous 1999). These plants all work with the flotation process and discharge the entire processing tailings, without
any treatment, into local streams and canals (Medina Hoyos 1998; Hudson-Edwards et al. 2001). At their origin, near the
mines, these streams are strongly influenced by acid mine drainage from mines and waste piles and have a pH range of 1–
4 and extremely high concentrations of dissolved heavy metals (Anonymous 1999). Near the city of Potosí, slurries
containing highly alkaline water (pH 10–12) and toxic tailing levels are emitted in the rivers as they pass the ingenios.
Ultimately, these streams drain into the Tarapaya River, which is a tributary to the Pilcomayo River.
of which 98,100 km² are located in Bolivia. The river originates in the Bolivian Andes (Cordillera Oriental, 5000 m above sea
level) and cuts down through the Andes for 700 km, through an inaccessible terrain with rapids and narrow canyons, until it
reaches the Chaco Plains near the town of Villa Montes (400 m above sea level). Subsequently, the river flows southwards
through the semi-arid Chaco Plains where it forms the borderline between Argentina and Paraguay before it joins the
Paraguay River just north of the town of Asuncion.
The upper parts of the Pilcomayo river have as yet not been regulated by dams or other hydro-technical works. Therefore,
the river is strongly influenced by the hydrological cycle, which is driven by the differences in rainfall between the dry (May–
October; winter) and the wet seasons (November–April; summer).(Smolders et al. 2000; Smolders et al. 2002). This has
important consequences for the functioning of the river ecosystem.
In the Paraguay–Argentine territory large areas of land are inundated by the river during the rainy season; these floodplains
serve as important feeding-grounds for many migratory fish species. This lower part of the river (from Villa Montes to
Asuncion) has a total length of almost 1000 km.
The lower reaches are alternately affected by inundation and drought, while the soils susceptibility to erosion in the Andean
region results in a very high load of suspended matter, that consists of a very fine gray, quartz-rich sand, in the rainy season
(Guyot et al., 1990; Smolders et al., 2002). The largely natural sediment loads have created an alluvial fan system which is
still active (Iriondo 1993). The sediments deposited in the Chaco Plains cause siltation of the river bed and subsequently
result in a gradual withdrawing of the river and the river floodplain in upstream direction (Iriondo 1993; Medina Hoyos 1998;
Smolders et al. 2002). Alluvial sedimentation has interrupted the former direct connection between the upper reaches of the
Pilcomayo river and the Paraguay river.
The mining region near the city of Potosí forms part of the Bolivian Pilcomayo catchment. Mining activities started in the
sixteenth century at Cerro Rico de Potosí. Until the early nineteenth century, this hill remained one of the most important
silver sources of the world. From then on, tin mining gradually became more important and remained so until 1985.
Due to the crash of the tin market and the governmental restructuring of mining operations in 1985, the Crushing-Grinding-
Flotation method was introduced to recover Pb and Zn. As a result, the contamination of the environment by heavy metals
has increased because the grain size of the mine tailings was greatly reduced from several millimeters in the conventional
gravity-concentration method to 0.3–0.05 mm in the flotation method. Moreover, flotation reagents (such as xanthates and
cyanides) are now added during the process (Medina Hoyos 1998; Anonymous 1999). The flotation process also requires
a high pH, therefore lime is added to increase the pH of the tailings. As the applied technology is not optimized, the residual
tailings contain high levels of unrecovered metals.
At present, there are as many as 40 processing plants (ingenios) operating in or immediately adjacent to the city of Potosí
(Anonymous 1999). These plants all work with the flotation process and discharge the entire processing tailings, without
any treatment, into local streams and canals (Medina Hoyos 1998; Hudson-Edwards et al. 2001). At their origin, near the
mines, these streams are strongly influenced by acid mine drainage from mines and waste piles and have a pH range of 1–
4 and extremely high concentrations of dissolved heavy metals (Anonymous 1999). Near the city of Potosí, slurries
containing highly alkaline water (pH 10–12) and toxic tailing levels are emitted in the rivers as they pass the ingenios.
Ultimately, these streams drain into the Tarapaya River, which is a tributary to the Pilcomayo River.
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