Base Metal and Trace Element Concentrations in Stream Sediments Along the Pecos River, From the Southern Pecos Wilderness to Brantley Dam, North of Carlsbad, Eastern New Mexico

Elizabeth A. Carey, Lynn A. Brandvold, and Virginia T. McLemore

Abstract. Since reclamation of the Pecos mine waste pile, area roads and campgrounds, and the Alamitos Canyon mill was begun in 1991, there has been continued monitoring of metal and trace element concentration in stream sediments and water along the Pecos River, extending from the upper Pecos River to below Brantley Dam, north of Carlsbad. In 1992, 1996, and 2000, surface water and stream sediments were sampled and analyzed for the entire reach of the Pecos River. Between 1992-1996 the upper reach of the river between the Pecos Wilderness and Villanueva was sampled on 8 occasions as part of a multi-disciplinary study. These efforts were initiated so that point and non-point sources of contamination associated with the areas of concern could be identified and the effects of reclamation examined. The Pecos mine generated approximately 70,000 m3 of waste rock, which was piled at the mine site on a slope above the river. The crushed ore from the mine was transported by aerial tramway to the Alamitos Canyon mill 18 km south of the mine. Both mine waste and mill tailings have been sources of acidic drainage contributing elevated concentrations of metals and other trace elements. This paper provides the most recent year's sample data for the stream sediments and compares the data to the earlier year's data. In this study, the <63µ fraction was digested with aqua regia and analyzed for Cd, Cr, Cu, Fe, Mn, Pb, and Zn using flame atomic absorption spectroscopy. Geochemical trends over the period 1992-2000 confirm a decrease in concentrations with time since reclamation began, especially in the immediate vicinity of the Pecos mine and below the confluence of Alamitos Creek which drains the mill site area. In 2000, a sharp increase was noted in Cu and Pb levels in stream sediments immediately below the fish hatchery, 19 km below the Pecos mine.