Dam Burst Contaminates the Pilcomayo River Summary

  • Last updated on November 10, 2022

The rupture of a large commercial mine impoundment released tons of sludge into a headwater tributary of the Pilcomayo River southwest of Sucre, Bolivia, adding more sediment to an already seriously degraded watershed and focusing international attention on the laissez-fare attitude of the Bolivian mining industry to safety and environmental standards.

Summary of Event

Tectonic activity has significantly concentrated metals of economic value within the crustal terranes of western South America, including Bolivia. Consequently, mining within the Pilcomayo River watershed is concentrated in the high altitudes of the folded, mountainous belt of the Bolivian cordilleras, especially near Potisí, southwest of Sucre. Reportedly, in 1996 forty-two mining operations were directly discharging wastes into the streams of the Pilcomayo watershed, continuing a long-standing tradition of no physical or chemical treatment prior to discharge of wastewater from ore-processing facilities. Ecological disasters Water;pollution Disasters;industrial Mining Pollution;water [kw]Dam Burst Contaminates the Pilcomayo River (Aug. 29, 1996) [kw]Contaminates the Pilcomayo River, Dam Burst (Aug. 29, 1996) [kw]Pilcomayo River, Dam Burst Contaminates the (Aug. 29, 1996) [kw]River, Dam Burst Contaminates the Pilcomayo (Aug. 29, 1996) Ecological disasters Water;pollution Disasters;industrial Mining Pollution;water [g]South America;Aug. 29, 1996: Dam Burst Contaminates the Pilcomayo River[09540] [g]Bolivia;Aug. 29, 1996: Dam Burst Contaminates the Pilcomayo River[09540] [c]Disasters;Aug. 29, 1996: Dam Burst Contaminates the Pilcomayo River[09540] [c]Environmental issues;Aug. 29, 1996: Dam Burst Contaminates the Pilcomayo River[09540] [c]Manufacturing and industry;Aug. 29, 1996: Dam Burst Contaminates the Pilcomayo River[09540] Sánchez de Lozada, Gonzalo

Bolivia, a sparsely populated nation of approximately nine million people of mixed ancestry, is one and one-half times the size of Texas. It paid a heavy price at the end of the twentieth century for its unbridled pursuit of metals. Tarnished in reputation, mining for precious and base metals in Bolivia lost some of its luster. On August 29, 1996, an inadequately engineered holding pit for mining wastes, which was adjacent to a headwaters of the upper Pilcomayo River, ruptured near Potisí. This disaster would end four centuries of mining laissez-faire in Bolivia. Previously, mining operations had been heavily subsidized by cheap, indigenous labor and natural infrastructures such as the Pilcomayo River. After the rupture, Bolivian and international mine operators could no longer disregard local and national safety and environmental issues.

Like Bolivia itself, the Pilcomayo River is a lesson in stark physical contrasts. Volumetrically variable, the lengthy waterway is fed by rains and snows from the highest Andean peaks of the Cordillera Real and the Cordillera Oriental, on the eastern edge of the Altiplano, or Bolivian Plateau, and flows southeasterly across the Tropic of Capricorn. Tracing a course of approximately 900 kilometers (about 600 miles), the Pilcomayo merges with the larger Paraguay River opposite Asunción, the capital of neighboring Paraguay. Crossing the folded granitic ridges and valleys of the higher, cooler, Bolivian Andes, the river flows onto and meanders across the Gran Chaco, a flat, torrid, subtropical savanna between northern Argentina and southwestern Paraguay that harbors diverse plant and animal life. Culturally, the Pilcomayo arises within the Bolivian departments of Oruro and Potisí and passes through Chuquisaca and Tarija departments, forming a winding frontier between the lowland Argentine and Paraguayan provinces of Formosa, Boquerón, and Presidente Hayes.

As of the early 2000’s, approximately one million people lived within the river’s upper watershed, with Sucre, the national judiciary center of Bolivia, the largest city relying on its limited resources. Reportedly, no communities had wastewater treatment facilities; untreated wastes were thus directly discharged into the Pilcomayo’s Bolivian reaches. Because of limited rainfall within its evaporative, poorly vegetated upper watershed, the river had a minimal base flow and could be seasonally ephemeral, making it susceptible to irrigation demands, erosion, sedimentation, and pollutants from municipal and industrial sources. Volumetrically, a substantial portion of the river’s perennial base flow within the cordilleran provinces was made up of untreated wastewaters from Bolivian mines and municipalities.

Coupled with similarly laissez-faire governmental policies toward municipal wastes, the Bolivian government’s lack of regulations regarding river dumping resulted in a severe negative impact on the freshwater aquatic ecosystem of the upper Pilcomayo River: It had been severely degraded and was devoid of any commercially exploitable fisheries. Consequently, the 1996 levee failure at the tailings impoundment of the El Porco mine near Potisí only added insult to injury, releasing approximately 350,000 tons of metallic tailings and sludge into the Pilcomayo River in highlands Bolivia, upstream from its border with Argentina and Paraguay. Solids from all human sources, particularly agriculture and mining, only contributed to the large loads of sediments from natural erosional processes within the entire, water-limited basin. Downstream, within the Gran Chaco, metals-laced loadings from mine wastes also clogged the sediment-choked river, making it undependable as a fishery for local residents.

In 1997, Bolivian and Dutch biologists assayed the metals concentrations in fish harvested from the river within Bolivia’s southernmost department, Tarija. Supplied by additional mountain streams, the Tarijan reaches of the Pilcomayo were less impacted by the disastrous discharge. Although the filtering livers of the dissected animals contained higher than normal concentrations of zinc and copper, the edible muscle tissues had not been seriously affected. Fatty tissues and skeletal components, however, had anomalously elevated metals concentrations. Regardless, fear of contaminated fish had a negative impact on the local fishing economy, particularly for ethnic Guaranis, who sold shad to commercial markets in larger communities like Sucre.

Financially, the El Porco metals mine was a joint venture between Compania Minera del Sur (COMSUR, founded in 1962) and Rio Tinto, an international, Australian-based mining corporation. Former two-time Bolivian president Gonzalo Sánchez de Lozada was one of the active partners of COMSUR. Educated in the United States at the University of Chicago, President Lozada engineered extreme economic reforms for the Bolivian economy during his first term (1993-1997). Significantly, he decentralized national government and partially privatized Bolivia’s hydrocarbon industry. Ironically, the El Porco levee break occurred while Lozada was in office.

During the latter half of the twentieth century, COMSUR largely mined lead, gold, silver, tin, and zinc in cordilleran Bolivia and Argentina. The company was wholly owned by Panamanian-registered Minera, S.A., and 11 percent of its equity was held by the International Finance Corporation, a private-sector investment section of the World Bank Group. In 2005, COMSUR was sold to a Swiss financial conglomerate that hoped to improve mining operations and recover wasted metals from tailings at COMSUR’s various Bolivian operations. Rio Tinto controlled 33 percent of the El Porco mine’s holdings.


At the time of the disaster in 1996, uncontrolled water diversions and human-sponsored sedimentation had significantly affected the shad fishery, the most important commercial species within the Pilcomayo watershed. However, the El Porco mine discharge into the Pilcomayo River had a silver lining. National and international diligence were shifted from indifference to concern for local communities and the precious, vital resource of clean water within this water-limited region of Bolivia.

Attention was focused on comprehensive, international watershed management of the Pilcomayo basin between Bolivia, Argentina, and Paraguay, with significant financial and technical assistance from the European Union. A tri-national commission, the National Commission of the Pilcomayo and Bermejo Rivers, undertook significant scientific and engineering studies of the entire tri-national basin to manage its natural and cultural resources with greater regard for environmental impact. In addition, Bolivian authorities forced the closure of nineteen ore-processing operations that had directly discharged metallic wastes into the Pilcomayo River. In 2001, the Bolivian government also promulgated a policy of requiring foreign operators with more than three violations of environmental regulations to forfeit their metals concessions. Ecological disasters Water;pollution Disasters;industrial Mining Pollution;water

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Alcázar, José Luis. “Pilcomayo River to Be Saved from Ruin.” Tierramerica, May 24, 2005. A news account of efforts to overcome the neglect and abuse of the Pilcomayo watershed in west-central South America. Available in Spanish, Portuguese, and English, the news journal is published by the Inter Press Service News Agency with financial support from the United Nations Development and Environment Programs.
  • citation-type="booksimple"

    xlink:type="simple">Cesar, Mike. “Solutions Elusive for Dead Pilcomayo River.” World Rivers Review 14, no. 3 (1999): 10. A journalistic account focusing on the cultural repercussions of the El Porco levee break and attempts to alleviate pollution in the upper watershed of highland Bolivia.
  • citation-type="booksimple"

    xlink:type="simple">Smolders, A. J. P., R. A. C. Lock, G. van der Velde, R. I. Medina Hoyos, and J. G. M. Roelofs. “Effects of Mining Activities on Heavy Metal Concentrations in Water, Sediment, and Macroinvertebrates in Different Reaches of the Pilcomayo River, South America.” Archives of Environmental Contamination and Toxicology 44, no. 3 (2003): 314-323. A scientific paper summarizing a biological and heavy metal toxicological assay of the Pilcomayo River between 1997 and 1999 after the El Porco levee failure.
  • citation-type="booksimple"

    xlink:type="simple">Velasco, Pablo. “The Mineral Industry of Bolivia.” The United States Geological Survey Minerals Yearbook. Denver, Colo.: U.S. Geological Survey, 2001. A brief section providing an economic summary of the mineral resources of Bolivia at the close of 2001.

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