Arsenic Geochemistry of Stream Sediments Associated with Geothermal Waters at La Primavera Geothermal Field, Mexico

David M. Welch, Gregory P. Miller, David I. Norman, New Mexico Institute of Mining and Technology; and Lynn A. Brandvold, New Mexico Bureau of Mines and Mineral Resources


As part of a reconnaissance study on arsenic speciation of geothermally impacted stream waters; stream sediments, algae and plants were collected at 20 sample locations along the Rio Salado Watershed and its tributaries at La Primavera, Mexico. Nitric acid extractions were conducted on the sediments to determine the total amount of arsenic available to the environment and its partitioning into different size fractions. Sediments and algae were digested using 0.1M phosphoric acid and speciated by ion exchange chromatography to determine the relative proportions of As (III), As (V), monomethylarsonate (MMA), and dimethylarsinate (DMA) present. Sediment analyses of TOC, Fe, and Mn were also done to evaluate their relationship to arsenic. In addition to sediments, waters were collected and analyzed for pH, Eh, temperature, conductivity, and general chemistry. Arsenic speciation was also done on the waters.

Total arsenic in sediments available to the environment ranges between 3 and 16 ppm and is highest in the silt-clay fraction. However, some samples show a bimodal distribution with high concentrations in the coarser size fractions that is related to TOC, Fe, and Mn. Arsenic occurs predominately as As (V), with lesser amounts of As (III); MMA and DMA were present in minor amounts. As (III) is proportional to As (V) with a correlation coefficient greater than 0.93. Arsenic concentration is positively correlated with Mn and TOC and shows no correlation with Fe. Algae contain between 18 and 68 ppm arsenic on a dry weight basis and the arsenic is predominately As (V). Generally, algae with the highest concentrations of arsenic were found at the source springs where waters are hottest.

Keywords: arsenic, arsenic speciation, sediment, algae