Arsenic and trace metals in river water and sediments from the southeast portion of the Iron Quadrangle, Brazil

Abstract

The Iron Quadrangle has been one of the most important gold production regions in Brazil since the end of the seventeenth century. There, arsenic occurs in close association with sulfide-rich auriferous rocks. The most abundant sulfide minerals are pyrite and arsenopyrite, yet trace metal sulfides occur in subordinate phases as well. Historical mining activities have been responsible for the release of As and trace metals to both aquatic and terrestrial environments close to mining sites in the region. Therefore, this study was aimed to evaluate the distribution and mobility of As, Cd, Co, Cr, Cu, Ni, Pb, and Zn in streams in the southeast portion of the Iron Quadrangle between the municipalities of Ouro Preto and Mariana, the oldest Brazilian Au mining province. Total concentrations of some trace metals and arsenic in water were determined. The four-stage sequential extraction procedure proposed by the commission of the European Communities Bureau of Reference (BCR) was used to investigate the distribution of these elements in stream sediments. Arsenic concentration in water was > 10 μg L⁻¹ (maximum limit permitted by Brazilian environmental regulations for water destined for human consumption) at all sampling sites, varying between 36.7 and 68.3 μg L⁻¹. Sequential extraction in sediments showed high concentrations of As and trace metals associated with easily mobilized fractions.

Trace Elements in River Waters and Sediments Before and After a Mining Dam Breach (Bento Rodrigues, Brazil)

“…11 The Carmo River runs for a length of 134 km, and near its headwaters are its main tributaries, the Funil Creek and Tripuí Creek, which have their sources in the municipality of Ouro Preto, located in the southern portion of the IQ. Its mouth is in the municipality of Rio Doce, at its confluence with the Piranga River where the Doce River is formed.…”

“…Furthermore, these sampling points are located along the Carmo River, which drains the cities of Ouro Preto and Mariana, and also drains the Passagem Mine (municipality of Mariana ) that is geologically rich in arsenic minerals. 11 Segura et al 2 collected samples of tailings mud 23 days after rupture of the dam of Fundão (November 28, 2015), where it was determined that Fe (~20000 to 30000 mg kg -1 ) and Mn (~100 to 300 mg kg -1 ) were the main elements, and trace elements (Pb, As, Sr, Zn, Cu, Ni and Co) with concentrations in the slurry ranging from < 0.46 mg kg -1 to ~6.5 mg kg -1 . During its flow through the river the mud was mixed with the bottom material (sediment), which contains high quantities of trace elements (Table 2), forming a mud layer.…”

“…[6][7][8] The historic mining activities developed in this region have been responsible for the release of chemicals elements into aquatic and terrestrial environments. [9][10][11] Large quantities of arsenic in soil (200-860 mg kg -1 ), sediments (22-3200 mg kg -1 ) and groundwater (0.4-350 µg L -1 ) have been found in this region. 9 Besides of the arsenic, quantities of Cd (5.9-6.9 mg kg ) and Zn (33.1-77.7 mg kg -1 ) in sediments were also found in this region.…”

“…9 Besides of the arsenic, quantities of Cd (5.9-6.9 mg kg ) and Zn (33.1-77.7 mg kg -1 ) in sediments were also found in this region. 11 Chemicals elements can be present in the sediments in the following chemical fractions: soluble, exchangeable, bound to organic matter, bound to iron, aluminum and manganese oxides/hydroxides, carbonates, phosphates, sulfates (or other secondary minerals), or bound to silicates. 12,13 Therefore, determinations of total chemicals elements concentrations in environmental samples provide little detailed information on their potential effect as pollutants.…”

“…20 The procedure simulates the mobilization and retention of these species in the natural environment, using alterations in environmental conditions such as pH, redox potential and degradation of organic matter. 11 A series of reagents is applied to the sample, increasing the extraction force in each stage in order to dissolve the metals present in different phases of the sediments. 28 The advantage of using the BCR sequential extraction method is the provision of detailed information on the origin, mode of occurrence, physical-chemical and biological availability, mobilization and transport of trace metals.…”