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The Geochemistry of Acid Waters of the Rio Agrio, Patagonia, ArgentinaDr. Chris Gammons Dr. Steve Parker
Figure 1. The Rio Agrio ~ 20 km downstream of Copahue Volcano. This river is naturally-acidic due to emissions of volcanic gas near its headwaters. Montana Tech Professors Chris Gammons (Geological Engineering) and Steve Parker (Chemistry and Geochemistry) are studying the acidic waters of the Rio Agrio watershed, in northern Patagonia, Argentina (Fig. 1). What makes this watershed unique is that it owes its high acidity and dissolved iron content to natural volcanic processes. As such, it makes an interesting comparison to bodies of water in Montana and elsewhere that are acidic due to pyrite oxidation and mine drainage.
Figure 2 A) View of Copahue Volcano with snowfields and volcanic ash (gray)
Figure 2 B) Hot springs at the head of the Rio Agrio. The hot, acidic water is piped to a nearby village where it is mixed with ash, and locals enjoy bathing in the acid mud
Figure 2 C) The summit crater lake at Copahue Volcano. This water has a pH of less than 2, and is milky white due to an abundance of colloidal elemental sulfur 
Figure 2 D) Chris Gammons (left), with Scott Wood (middle) and Chris Shope (right), wearing gas masks at the summit of Copahue. 
The Rio Agrio begins as a series of hot springs high on the flanks of Copahue Volcano (el. 2980m, Figs. 2A, 2B). Copahue is an active volcano that last erupted in 2000. The hot spring waters are extremely acidic (pH between 0 and 2), due to abundant degassing of SO2, HCl and HF off of an active magma chamber located beneath the volcano. The summit of the volcano has a small crater lake with no outlet, which is also extremely acidic (Fig. 2C). Researchers visiting these sites must wear gas masks to protect their lungs from the acid fumes (Fig. 2D).
In March of 2003, Gammons and a colleague from the University of Idaho, Prof. Scott Wood, along with MT Tech graduate student Chris Shope, visited Patagonia and sampled the Rio Agrio from its headwaters on Copahue Volcano to a point ~ 50 km downstream where it ceased to be acidic due to dilution from tributary streams. Their findings were recently published in the journal Chemical Geology (Gammons et al., 2005a). The upper river drains into a very large, glacially-carved lake called Lago Caviahue (Fig. 3A, 3B). This lake is up to 80 m deep, and has a pH almost exactly the same as the Berkeley pit-lake in Butte (pH ~ 2.6). Samples taken at different depths in the lake (Fig. 3B) show that it is acidic from top to bottom. Despite this fact, recent research by German scientists has shown that the sediment in the bottom of Lago Caviahue is thriving with sulfate-reducing bacteria (Wendt-Potthoff and Koschorreck, 2002). This is in stark contrast to the sediment in the bottom of the Berkeley pit-lake, which has no bacteria of this type. This difference in microbiology is probably due to the much higher concentrations of toxic metals (Cu, Zn, Cd) in the mining lake as opposed to the naturally-acidic lake in Argentina.
In March of 2004, Gammons and Steve Parker returned to the Rio Agrio to examine 24-hour changes in the chemistry of the river (Parker et al., in review). The team sampled the Rio Agrio at three locations over a complete diurnal cycle (Figs. 3C, 3D). The results were very interesting, and showed large cyclic changes in the concentration and speciation of iron and other solutes in the river water. These fluctuations are caused by a variety of geochemical and microbial processes that operate on a day-night cycle. Similar daily cycles were recently documented in mining-impacted streams in Montana by Gammons, Parker, and colleagues with the U. S. Geological Survey (Gammons et al., 2005b, 2005c).
Figure 3 A) View of Lago Caviahue (middle distance) from the summit of Copahue Volcano.
Figure 3 B) Collecting water samples from a boat on Lago Caviahue
Figure 3 C) The "Salto del Agrio" waterfall on the lower Rio Agrio, near one of the diurnal sampling sites. This water has a pH of ~ 3.
Figure 3 D) Steve Parker on the lower Rio Agrio, near another one of the diurnal sampling sites.
The results of this ongoing research on the geochemistry of acidic waters has special relevance given the recent discovery of the mineral jarosite by mechanical rovers on the surface of Mars (Madden et al., 2004). On Earth, jarosite is typically found near pyrite-rich mine waste that is actively oxidizing and releasing acid to the environment. For example, jarosite is abundant on mine dumps near Butte, and gives much of the barren land behind the Berkeley Pit its sickly, yellowish-olive color. Some researchers feel that Mars may once have contained acidic rivers and oceans. If so, it is more likely that the high acidity was due to volcanic processes, such as what is happening today in the Rio Agrio, than due to oxidation of pyrite, as is the case in Butte. Whatever the answer, research on river systems such as the Rio Agrio gives us fascinating insight into one of Earth’s more exotic and unique environments.
References:
Gammons C. H., Wood S. A., Pedrozo F., Varekamp J., Nelson B., Shope C. L., and Baffico, G. (2005a) Hydrogeochemistry and rare earth element behavior in a volcanically acidified watershed in Patagonia, Argentina. Chemical Geology 222, 249-267.
Gammons C. H., Nimick D. A., Parker S. R., Cleasby T. E. and McCleskey, C. B. (2005b) Diel behavior of Fe and other heavy metals in a mountain stream with acidic to neutral pH: Fisher Creek, Montana, USA. Geochimica et Cosmochimica Acta 69, 2505-2516.
Gammons C. H., Wood S. A. and Nimick D. A. (2005c) Diel behavior of rare earth elements in a mountain stream with acidic to neutral pH. Geochimica et Cosmochimica Acta 69,3747-3758.
Madden M.E.E., Bodnar R. J., Rimstidt J. D. (2004) Jarosite as an indicator of water-limited chemical weathering on Mars. Nature 431, 821-823.
Parker S. R., Gammons C. H., Pedrozo F., Wood S. A. (in review) Diel changes in metal concentrations in a geogenically acidic river: Rio Agrio, Argentina. [Submitted to Journal of Volcanology and Geothermal Research].
Wendt-Potthoff, K., Koschorreck, M., 2002. Functional groups and activities of bacteria in a highly acidic volcanic mountain stream and lake in Patagonia, Argentina. Microbial Ecology 43, 92-106. |