THE EFFECT OF PARTICLE SIZE DISTRIBUTION ON THE GEOCHEMISTRY OF STREAM SEDIMENTS FROM THE UPPERPECOS RIVER, SAN MIGUEL COUNTY, NEW MEXICO
Virginia McLemore, Lynn Brandvold, Anwar Hossain and Tim Peace
Abstract: Stream-sediment samples were collected at six sites along the Pecos River in May 1994, in order to study the particle size distribution and its relationship to metal concentrations. Previous studies have shown that stream sediments from the Pecos River are elevated in Cu, Pb, and Zn below the abandoned Pecos mine and the Alamitos Canyon mill site. Zinc was also elevated in the area of the Lisboa Springs Fish Hatchery. Higher concentrations of Cu. Pb, and Zn are typically found in the smallest size fraction (silt and clay; 0.063 mm) which typically represents less than 10% of the stream-sediment sample by weight. The sediment samples were sieved into six size fractions (1-2 mm, 0.5-1 mm, 0.25-0.5 mm, 0.125-0.25 mm, 0.063-0.125 mm, and <0.063 mm) and each size fraction was analyzed by flame atomic absorption spectroscopy (FAAS) for Cu. Pb, Zn, Cd, Fe, Mg, and Mn. In three samples exhibiting low total metal values, metal concentrations (Cu, Pb, Zn, Cd) increase with decrease in particle size fraction. The sample from above Lisboa Springs Fish Hatchery contains elevated concentrations of Zn, which suggests that precipitate leached and suspended material eroded from the upper Pecos River, upper Willow Creek, and the Pecos mine waste dump could be deposited at the diversion dam above the fish hatchery. During sporadic periods of turbulent flow, in the fall when the Pecos River is at low flow, suspended material elevated in Zn could be entering the fish hatchery and contributing to recent fish kills. In three samples with elevated metal values, the highest metal concentrations (Cu, Pb, Zn, Cd) correspond to the smallest (fine sand, silt and clay; 0.063-0.125 mm, <0.063 mm) and largest (coarse and very coarse sand; 0.5-1 mm, 1-2 mm) size fractions. This suggests that the metals are traveling mainly as suspended material and as larger mineral or other grains weathered from the mine waste pile and the tailings pile.
New Mexico Bureau of Mines and Mineral Resources, Socorro, NM 87801
New Mexico Institute of Mining and Technology, Socorro, NM 87801