Geochemistry of suspended particles in a mine-affected mountain stream

The particles in a mine drainage and the creek into which it drains were examined using microbeam and filtration methods. The composition of the particles in the drainage and creek were distinctly different, though both carried a chemical signal from the mine. High concentrations of particles made up of Fe and S were seen in the drainage, especially during low flow. The molar Fe:S ratio of these suspended particles was 3.5:1. The influx of the mine drainage into the creek increased the relative number of aggregates and particles with associated Fe. However, few discrete Fe–S particles were seen in the creek, where solid-phase Fe and S were consistently associated with aluminosilicate minerals. Instream precipitation is predicted and appears to be an important additional source of Fe and Al oxyhydroxides to the particles. Some of the Fe and Al associated with particles in the creek was acid-soluble, but most of the acid-soluble Zn, Mn, Cu, Ca, Mg and Si were transported in the <0.45 μm fraction at a site downstream of the mine drainage. One third of the suspended particles had associated P. These suspended particles take part in the complex geochemistry of this system, and represent an important pool as a potential sink and source of metals and other elements.