Estudo ambiental no município de Paracatu, MG: quantificação, especiação e transporte de elementos tóxicos

Abstract

Paracatu River, whose basin is predominantly located in the northwest of Minas Gerais State, is the largest tributary in the drainage area of the San Francisco River. This region has considerable gold (Morro do Ouro - Paracatu), lead and zinc reserves (Vazante), which are exploited by mining companies and represent the main economic activity in this region, along with agriculture and livestock. Due to high levels of toxic elements in this region, studies aiming to assess the environmental impact caused by mining activities, prospecting and irrigated agriculture have great local relevancy. This work determined 14 elements, including As, Cd, Cr, Hg, Pb and Zn, in surface water, groundwater and sediment samples from various parts of Paracatu city. Besides that, wereconducted studies of speciation of As in waters and Hg in sediments, since species may differ in toxicity, solubility and availability to the environment. Additionally, As and Hg were determined in vegetables consumed by inhabitants. It was observed that approximately 13% of the determinations in surface waters and 6% of the determinations in groundwaters are above the limits for human consumption established by CONAMA. About 75% of concentrations in sediments for elements that have a reference value established by CONAMA had exceeded level 1. Most vegetables samples presented As and Hg concentrations within the normal range for plants and do not offer risks of intoxication. Although, the concentrations are low, there is a correlation among concentrations in water, sediment and leaves. The proposed methodology for speciation of arsenic was based on selective hydride generation, using citric acid (C6H8O7) for arsenite determination. For total As determination, the samples were pre-reduced with thiourea and quantified byHG AAS, in this case, using 10% HCl v/v as carrier acid. The arsenateconcentration was calculated as the difference between total arsenic andarsenite. This methodology was validated and considered adequate for analysis of the total As and arsenite in waters. Water samples showed arsenic concentrations ranging from < 1,3 [g L-1 to 113 [g L-1. It must be emphasized that there was a reduction of the total As concentration in all surface water and groundwater samples from the 2nd collection. In the 1st sampling, it was observed a predominance of pentavalent species, less toxic. But, in the 2nd sampling, trivalent species predominated in most samples. However, some elements have been found in high concentrations in the sediments, the results suggest that, except for Zn, other elements have low mobility mainly due to the retention together with Al, Fe and Mn oxides in elevated contents in this region.