The contamination with arsenic in Santana led to the perception that the brownish waters of the Amazon River carry arsenic, at a rate higher than 235,000 tonnes per year, and it seems important that this arsenic should be studied in detail. The anomaly was caused by the discharge of tailings of a pellet plant in an excavated sump. These tailings were composed of iron- and arsenic-rich minerals, crystallized at above 900°C in the metallurgical process, which become unstable when submersed in fresh water at pH 5.0-6.0. In the sump, iron, arsenic, manganese, and other metals were slowly solubilized from the minerals, entering the groundwater. When iron got in contact with the water organic compounds, it formed a gel, absorbing As, Mn, and other metals, as it normally happens during weathering in the Amazon. The sump has been built in practically impermeable clayey soil, which made the gel move slowly, giving time to form hard concretions of limonite, characteristic of the Amazonian soil. Iron, manganese, arsenic and other metals were retained in the concretions, and the arsenic-bearing iron gels went for a maximum of some hundreds of meters away from the sump. Soon after the identification of the contamination, the water of the sump was, under close control, discharged into the Amazon River, and the tailings were removed to high ground. In sequence, as it was impossible to properly isolate the tailings, they were sold and shipped away. In the meantime, there were complaints that persons have been affected by arsenic-contaminated water that flowed from a broken dam (that never existed) into a small creek that crossed a community, and that people that consumed the water had cancer. A large medical team was called by the local government, thousands of persons were examined, diseases were found, and none related to arsenic. Analysis of blood and hair presented arsenic in limits worldwide accepted as normal. As samples of the Amazon showed a conspicuous presence of arsenic, they led to the study of the river waters for arsenic. Information is presented to correct misunderstandings present in the literature. Two important points are commented on to understand the event. One is the geological affinity of arsenic with iron. The other is that the toxicity of a solid is determined by how much of the toxic element is liberated for contamination; the standards used for the proper definition of toxicity of a solid are presented.
