Arsenic Speciation in Geothermally Impacted Groundwater and Surface Water

Gregory P. Miller, David M. Welch, David I. Norman, New Mexico Institute of Mining and Technology
Lynn A. Brandvold, New Mexico Bureau of Mines and Mineral Resources
Rosa Marie Prol-Ledesma, University of Mexico, Mexico City


Arsenic occurrence and speciation was investigated in streams and sediments impacted by geothermal waters at the La Primavera Caldera, Guadalajara, Mexico. The high-arsenic sodium-bicarbonate waters flow from springs in 30,000-145,000 year old volcanic rocks, These waters discharge at 55-65 ºC with arsenic concentration approaching 1 mg/L. The springs form the headwaters of the Rio Caliente-Rio Salado-Lake Chapala watershed. Water is withdrawn from the watershed for agricultural and domestic use in the Guadalajara area.

Water and sediment samples were collected from 20 stations over 10 km of stream reach during baseflow. Filtered and unfiltered waters were separated in the field into As(3+), As(5+), monomethylarsonate and dimethylarsinate fractions by ion chromatography. Partial extractions were conducted on sediments to determine 1) speciation of As within sediments, 2) relative amounts of As associated with Fe and Mn oxides, organics and adsorbed to grain surfaces, and 3) As contained in various size fractions. Field parameters of pH, temperature, conductivity, and redox potential were measured. Streamflow measurements were made to evaluate mass balance.

Total arsenic mass is conserved in streams. All springs and surface waters are dominated by As(5+), As(3+) is uncommon. Methyl arsenic compounds comprise up to 25% of the total arsenic present. Methyl arsenic in spring discharges suggests that methylation occurs in the subsurface. Chloride concentrations in springs indicate limited influence from underlying 270 ºC geothermal waters. Reduced sulfur was not found in the water. In sediments a dimodal variation in As concentration with grain size was noted, and is proportional to TOC.

Keywords: As, As speciation, geothermal springs, TOC, La Primavera Caldera, sediment As speciation