Water chemistry and estimation of background levels of elements in surface water bodies from a protected area in the vicinity of Fe deposits, Southeastern Amazon

Abstract

This paper aimed to provide the knowledge of water chemistry and quality and background values of the elements in surface waters around the Fe-ore deposits of N3 and N4WSul of Serra dos Carajás, Brazil. Water samples were collected from 17 monitoring points monthly/quarterly between 2013 and 2016 and the elemental analyses were carried out using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The waters are slightly acidic to slightly alkaline in nature and are classified into “good” and “optimum” water quality category. Seasonal variation impacted water quality, with significantly higher content of metals (such as Fed, Fet, and Ald) during the rainy period that is due to more intense surface-runoff and leaching. Comparing with CONAMA 357/05/WHO limits, nonconformities were mainly observed for pH, Fe and Mn, and in some cases for Ald, Znt, Cud, Cdt and Znt, and were significantly higher in the N4WSul area than in the N3. Although, Fe and Mn mostly enter the water bodies from catchment weathering, the poor correlation between Fe and Mn indicated that their source and transport processes are different. In fact, elevated Mnd concentrations associated with low DO content in the dry period are explained by a reductive dissolution of manganese oxides via bacterial decomposition of OM. The background threshold values (BTVs) of elements were estimated by the upper 95% simultaneous limit (USL95; using ProUCL software) and the median ± 2 Median Absolute Deviation (mMAD) method, while the former was considered as the most suitable method for defining BTVs. The BTVs for Al(d), Fe (d), Mn and Zn are mostly exceeding the maximum limits stipulated by the legislation. However, since these areas are located in a protected region, these values are seen as representative of the natural conditions of the study area and reflect geogenic influence.