Abstract

Seasonal and spatial variations in the concentrations of trace elements, pH and Eh were found in a creek watershed affected by mine drainage and leachate from several waste rock dumps within the As-Pb-rich Indae mine site. Because of mining activity dating back to about 40 years ago and rupture of the waste rock dumps, this creek was heavily contaminated. Due to the influx of leachate and mine drainage, the water quality of upstream reach in this creek was characterized by largest seasonal and spatial variations in concentrations of Zn(up to 5.830 mg/l), Cu(up to 1.333 mg/l), Cd(up to 0.031 mg/l) and SO₄^2-(up to 173 mg/l), relatively acidic pH values (3.8-5.1) and highly oxidized condition. The most abundant metals in the leachate samples were in order of Zn(0.045-13.909 mg/l), Fe(0.017-8.730 mg/l), Cu(0.010-4.154 mg/l) and Cd(n.d.-0.077 mg/l), with low pH(3.1-6.1), and high SO₄^2-(up to 310 mg/l). The mine drainage also contained high concentrations of Zn, Cu, Cd and SO42- and remained constantly near-neutral pH values(6.5-7.0) in all the year. While the leachate and mine drainage might not affect short-term fluctuations in flow, it may significantly influence the concentrations of chemicals in the stream. The abundance and chemistry of Fe-(oxy)hydroxide within this creek indicated that the Fe-(oxy)hydroxide formation could be responsible for some removal of trace elements from the creek waters. Spatial and seasonal variations along downstream reach of this creek were caused largely by the influx of water from uncontaminated tributaries. In addition, the trace metal concentrations in this creek have been decreased nearly down to the background level at a short distance from the discharge points without any artificial treatments after hydrologic mixing in a tributary. The nonconservative(i.e. precipitation, adsorption, oxidation, dissolution etc.) and conservative(hydrologic mixing) reactions constituted an efficient mechanism of natural attenuation which reduces considerably the transference of trace elements to rivers.

Keywords Trace elements, Mine drainage, Leachate, Waste rocks, Natural attenuation