Heavy metals in acid streams from lignite mining areas

Abstract

The drainage water from three abandoned lignite mining fields (Haunstrup, Søby, and Skraastrup) in central Jutland, Denmark is extremely acid (pH, 2.4–4.4), and the concentrations of heavy metals are markedly elevated compared with background levels and with concentrations in two regenerated lignite pits (Karstoft, Nt. Vium). Iron occurs with the highest concentrations in the drainage water and the concentrations of the other elements are in the order Mn > Zn>> Ni > Pb > Cu > V > Cr and > Cd. Concentrations of all analysed elements decline downstream from the mining areas. Iron shows the steepest gradient, due to flocculation within a short distance from the mining areas. Except for Fe, simple dilution with unpolluted water is sufficient to account for the downstream gradients. The mining areas in the river Skjern system have only a local effect on concentrations of heavy metals. In the river Vorgod, concentrations increased by the following factors: Mn, 7; Fe, 6; Zn, 4; Ni, 2; and Cr, 1.5 after confluence with drainage water from Haunstrup, and in the river Rind Mn, 3; Zn, 2; and Ni, 1.2 increased after the outlet from Søby. The export of heavy metals from the lignite mining areas compared with the transport in the river Skjern is about 31% for Fe, 24% for Zn, 18% for Mn, 13% for Ni, and below 10% for the other elements. Treatment of the drainage water from Haunstrup with lime has halved the export of Fe and increased the pH from about 3.0 to 4.0, whereas the concentrations of Cd, Cr, Cu, Mn, Ni, and Zn have remained almost unchanged. The diversity of submerged macrophytes is very low in the acidic water near the mining areas. High acidity, high levels of heavy metals and low alkalinity are associated, and all may play a role in suppressing the macrophyte flora.