Geochemistry of acid mine drainage from a coal mining area and processes controlling metal attenuation in stream waters, southern Brazil.

Veridiana P Campaner,Wilson Machado,Wanilson Luiz-Silva

doi:10.1590/0001-37652014113712

Abstract

Acid drainage influence on the water and sediment quality was investigated in a coal mining area (southern Brazil). Mine drainage showed pH between 3.2 and 4.6 and elevated concentrations of sulfate, As and metals, of which, Fe, Mn and Zn exceeded the limits for the emission of effluents stated in the Brazilian legislation. Arsenic also exceeded the limit, but only slightly. Groundwater monitoring wells from active mines and tailings piles showed pH interval and chemical concentrations similar to those of mine drainage. However, the river and ground water samples of municipal public water supplies revealed a pH range from 7.2 to 7.5 and low chemical concentrations, although Cd concentration slightly exceeded the limit adopted by Brazilian legislation for groundwater. In general, surface waters showed large pH range (6 to 10.8), and changes caused by acid drainage in the chemical composition of these waters were not very significant. Locally, acid drainage seemed to have dissolved carbonate rocks present in the local stratigraphic sequence, attenuating the dispersion of metals and As. Stream sediments presented anomalies of these elements, which were strongly dependent on the proximity of tailings piles and abandoned mines. We found that precipitation processes in sediments and the dilution of dissolved phases were responsible for the attenuation of the concentrations of the metals and As in the acid drainage and river water mixing zone. In general, a larger influence of mining activities on the chemical composition of the surface waters and sediments was observed when enrichment factors in relation to regional background levels were used.

Highlights

The presence of high concentrations of trace elements dissolved in surface and ground waters is one of the main problems associated with acid drainage generated in coal and sulfide mining areas
The results of this study showed that acid drainage generated in the bituminous coal mining activities is a potential source of metals and As to water systems
Precipitation and dilution mechanisms attenuated the concentrations of the Acid mine drainage (AMD) chemical constituents

Summary

Introduction

The presence of high concentrations of trace elements dissolved in surface and ground waters is one of the main problems associated with acid drainage generated in coal and sulfide mining areas. In natural drainage systems affected by AMD, the concentrations of some trace elements (e.g., As, Cd, Cu, Ni, and Pb) can be minimized as a result of various mechanisms inherent to the water system. In the Pedras stream/Leonor stream drainage systems, while most elements remarkably showed water enrichment factor lower or close to 1 (just As presented an EFw exceeding 2 at stations A-3 and A-19, and As at station A-19; Fig. 4), sediment enrichment factor values exceeding 2 were found for most elements (As, Cd, Co, Ni, and Zn) These elements always showed lower EFs in the station S19, which receives the discharge from the uncontaminated section of the Pedras stream (Fig. 1). To the Laranjinha river/ Mina stream drainage systems, the anthropogenic effect was stronger on the chemical composition of sediments than on the waters in the Pedras stream/

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