ENVIRONMENTAL GEOCHEMISTRY IN THE FOUR CORNERS REGION FROM SEDIMENTS OF SMALL LAKES

The Four Corners and San Juan coal-fired power plants near Farmington, NM are among the largest such generating facilities in the western U.S. Coal combustion can be a significant source of airborne trace-element pollution. We are examining the geochemistry of sediment from small lakes both upwind and downwind of the plants to develop a history of trace-element deposition over the last few hundred years. We have currently sampled 2 upwind lakes: Carnero Lake in the Mogollon Rim area near Springerville, Arizona, and Red Lake, a small reservoir built ca. 1895 near Navajo, New Mexico. Sediment accumulation in Carnero Lake is apparently very slow, because the thickness of soft, organic-rich sediment is < 0.5 m. This material overlies dense, light gray, clay-rich sediment that likely represents Pleistocene deposition. This stratigraphy and slow sedimentation rate are similar to that documented in lakes in the Chuska Mountains, New Mexico. Trace metal analyses of the cores show no obvious elevation of the Pb, Cd, As or Hg content of the sediment, but the slow sedimentation means that historic sediment was likely not recovered and that the modern record cannot be resolved. In contrast, cores taken from Red Lake are >1 m in length, implying an average sedimentation rate of about 1 cm/yr and the potential for high resolution of historic changes. These cores are primarily moderately organic silt and clay with lower sandy zones, and bottom in denser organic-poor mud. Analyses for ²¹⁰Pb dating are currently being run at LANL. Two lakes downwind of the power plant will also be sampled and analyzed for metal content and ²¹⁰Pb chronology to determine how trace element concentrations vary over time, in relation to onset of power plant operation and other local and regional human activities.