Biogeochemical Controls on Antimony and Arsenic Mobility in  Siliceous Hydrothermal Systems

Basic El Tatio Geology & Hydrogeology

The geothermal geyser field at El Tatio is located near the Chile-Bolivia border in the northern Antofagasta Province. The field is geologically associated with the Altiplano-Puna Volcanic Complex in the north-south trending Tatio Graben, bounded on the east by the Serrania de Tucle-Loma Lucero horst and on the west by El Tatio volcanics (Lahsen and Trujillo, 1976; de Silva and Francis, 1991). The geothermal field is ~30 km², but the majority of the thermal features are found within 10 km², in three distinct basins (upper, middle, and lower basin).

Most of the springs are near local boiling (86 ^oC) at circumneutral pH. Siliceous sinter deposits are found throughout the field, sometimes as large cones at geyser vents or as terraces at springs. Runoff from El Tatio forms the headwaters of the Calama Basin, where recharge is dominantly from mountain snowmelt. There have been only a few investigations of the hydrothermal geochemistry of El Tatio (Ellis, 1969; Mahon, 1971; Cusicanqui et al., 1975; Giggenbach, 1978; Giggenbach and Stewart, 1982). Based principally on isotopic evidence, the hydraulic system has been described as a short deep flow path with recharge in the nearby Andes Mountains along the border with Bolivia and Argentina (Giggenbach, 1978; Magaritz et al., 1990).

Google Earth image of El Tatio geyser field (upper left) and surrounding volcanic region.

Drag your mouse over the image, and you will be linked to a closer image.

The geysers at El Tatio are in a remote, high altitude site (>4300 m), in a largely uninhabited portion of northern Chile (II Region) in the Atacama Desert. Precipitation in this desert is limited by the rain-shadow along the Andes Mountains, cold ocean current upwelling, and atmospheric subsidence (Hartley and Chong, 2002). The high altitude and climatic conditions make El Tatio one of the most extreme environments on Earth, and it is also one of the world’s largest geyser fields. The value of scientific research within this unique setting is difficult to quantify, but the proposed research will certainly shed light on topics of integrated geologic, hydrologic, and geomicrobiological interest.

There are few safeguards at El Tatio, and there are no friendly park rangers to keep people from wading in the boiling pools – which apparently tourists do with alarming frequency. There is a visitor station, complete with flush toilets. People who desire to camp at El Tatio are asked to stay near the station, and there is a small kitchen for campers. The station is managed by locals from nearby villages.

The nearest medical facilities are located ~150 km away in Calama. To adjust to working at high elevation, our team stays either in Chiu Chiu (1.5 hr drive from El Tatio, ~3000 m elevation), or at San Pedro de Atacama (2 hr+ drive from El Tatio, and lower elevation.

Tourists can drive up on their own, or they can join one of the many eco-adventure groups out of San Pedro de Atacama. The tour groups leave in the early morning, when it is still dark, and arrive at El Tatio at sunrise when the geysers are spectacular.

This is a blown-up, tilted Google Earth image (from above) of the El Tatio basins, roads, and Rio Tatio flowing through the basins (greenish area). The basin are considered "middle", "upper", and "lower". In the image, the upper basin is the largest.

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Photos by A.S. Engel, unless noted in caption.

El Tatio is a natural laboratory to study fundamental relationships between metals, precipitates, and microbes. Our project takes advantage of the unique natural setting at  El Tatio. The results of this project will have significance beyond the restricted realm of high altitude, hydrothermal microbial systems.

We are examining in detail the influence of thermophilic microbes on As and Sb speciation and mobility, the relationships between silica precipitation, biogeochemical speciation and partitioning to biomass or geyserite, and the influence of mineral precipitation on metal bioavailability and microbial community viability.

The very high concentrations of As and Sb, the virtually unrestricted access to the field site, and the extreme environmental conditions allow us to directly observe and measure important geochemical transitions that are typically either impossible to access in situ, or are occurring at much lower concentrations.

Although the specific environment of El Tatio is unusual, the results of our work have immediate transfer value to a variety of environments and situations, such as metal contamination associated with mining.

Taking a well-deserved break after finishing water sampling efforts. Pictured here (left to right): Dr. Pablo Pasten (Universidad Catolica, Santiago), Dr. Phil Bennett (Univ. Texas at Austin), Marco Alsina (grad student with Pasten), and Jeff Landrum (grad student with Bennett).
Photo by A.S. Engel.