Long-term monitoring of the geochemistry of surface water and stream-sediment samples from the upper Pecos River, from the southern Pecos Wilderness to Brantley Dam, north of Carlsbad, eastern New Mexico: year nine


Since reclamation of the Pecos mine waste piles, area roads and campgrounds, and the Alamitos Canyon mill area began in 1990-1991, there has been continued monitoring of base metals and trace element concentrations in stream sediments along the Pecos River, extending from the upper Pecos River to below Brantley Dam, north of Carlsbad. In 1992, 1996, and 2000, water and stream sediments were sampled and analyzed for the entire reach. Between 1992-1996, the upper reach of the river between the Pecos Wilderness and Villanueva was sampled on eight occasions as part of a multi-disciplinary study. Five areas of concern included the Pecos mine waste pile (a volcanogenic massive-sulfide deposit containing Pb, Zn, Cu, Ag, and Au), the Alamitos Canyon mill tailings (El Molino site 18km south of the mine), roads and campgrounds north of Terrero (mine waste material from the Pecos mine waste pile was used as fill), and the Lisboa Springs Fish Hatchery (where several fish kills in 1991-1995 have been attributed to drainage from the mine site and contaminated campground sites). Geochemical analyses of surface-water samples indicate that drainage from the Pecos mine is not significantly affecting the composition of the surface water in the area, except in the immediate vicinity of the mine and mill site. Elevated concentrations of Cu, Pb, Zn, and Cd occurred in stream sediments below both the Pecos mine and Alamitos Canyon mill sites, before reclamation began. Collectively, multi-disciplinary studies suggest that Cu, Pb, Zn, and other metals were eroded and leached from the Pecos mine waste pile and the tailings piles in Alamitos Canyon. These geochemical trends confirm a decrease in concentrations with time since reclamation began, especially in the immediate vicinity of the Pecos mine (where Cu levels decreased from 310 to 92 ppm; Cd from 17 to 4.7 ppm; Pb from 300 to 160 ppm; and Zn from 3100 to 2080) and below the confluence of Alamitos Creek (Cu decreased from 270 to 30 ppm; Pb from 590 to 93 ppm; and Zn from 990 to 287 ppm). The overall metal concentrations dramatically decrease in stream sediments below Pecos Village, mostly due to dilution of sediment derived from the red bed sedimentary units.