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

Since reclamation of the Pecos mine and campgrounds began in 1990-1991, there has been continued monitoring of base metal 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. This study reports on the most recent sampling and analysis of the stream sediments and compares the data to earlier data. These efforts were initiated so that point and non-point sources of contamination associated with the mine could be identified and effects of reclamation examined. The Pecos mine is a volcanogenic - massive sulfide deposit that was mined on and off for forty two years beginning in 1902 and generating on the order of 70,000 m3 of waste rock. The waste rock was piled at the mine site and has been a source of acidic drainage contributing elevated concentrations of metals and other trace elements. In this study, the <63 micron fraction was digested with aqua regia and analyzed for Cu, Pb, Zn, Mn, Cd, Fe and Cr using flame atomic absorption spectroscopy. Mercury concentrations were determined using microwave digestion followed by cold vapor atomic absorption. With three sets of data now in place (1992, 1996, 2000), geochemical trends confirm a decrease in concentrations with time since reclamation began, especially in the immediate vicinity of the Pecos mine. 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.