Distribution and partitioning of copper, lead and zinc in stream sediments above and below an abandoned mining and milling area near Pecos, New Mexico, USA

Abstract

A reconnaissance study of base metals and mercury in stream sediments from along the Pecos River from North-eastern to South-eastern New Mexico showed elevated levels of copper, lead and zinc at two areas: below an abandoned mine waste dump and below an inactive mill. Zinc was also present at elevated levels in the area of a fish hatchery. During subsequent sampling trips, stream-sediment samples were collected from tributaries as well as the main stem of the river from the Pecos Wilderness boundary south to Villanueva State Park, a distance of approximately 88 km. Sediments were sieved to obtain the 2 mm to 63 µm and <63 µm size fractions and analysed for copper, lead and zinc using aqua regia dissolution and flame atomic absorption spectrometry. The smallest size fraction, which typically constituted 10% or less of the mass of the sample, contained the highest concentrations of copper, lead and zinc with the exception of Willow Creek at the mine waste dumps and San Miguel, a site downstream. Partial dissolution techniques were employed on the size fractions from selected sites to estimate exchangeable, organic and oxide-bound metal forms. Copper was the only metal with a significant association with the organic fraction (about 26% with more than half the sites having 20% or greater). The largest amounts of copper and lead (about 56%) were in a form solubilized by aqua regia but not by the three extraction techniques. In contrast, the largest amount of zinc (about 52%) was associated with the iron and manganese oxide fraction. A significant amount of lead (about 38%) was associated with the oxide fraction with very little associated with either exchangeable or organic fractions. Zinc fractionation was low for the exchangeable and organic forms with the exception of significant amounts of exchangeable zinc at sites below the mine waste dump.