Lopez, and 7 co-authors, reported in 2012 that numerous volcanoes, hot springs, fumaroles, and geothermal wells occur in the Pacific region of Latin America. These systems are characterized by high As concentrations and other typical geothermal elements such as Li and B. The authors reviewed the available data on As concentrations in geothermal systems and their surficial discharges and As data on volcanic gases of Latin America. Data for geothermal systems in Mexico, Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica, Ecuador, Bolivia, and Chile are presented. Two sources of As can be recognized in the investigated sites: Arsenic partitioned into volcanic gases and emitted in plumes and fumaroles, and arsenic in rocks of volcanic edifices that are leached by groundwaters enriched in volcanic gases.
Water containing the most elevated concentrations of As are mature Na–Cl fluids with relatively low sulfate content and As concentrations reaching up to 73.6 mg L−1 (Los Humeros geothermal field in Mexico), but more commonly ranging from a few mg L−1 to tens of mg L−1. Fluids derived from Na–Cl enriched waters formed through evaporation and condensation at shallower depths have As levels of only a few μg L−1 . Mixing of Na–Cl waters with shallower meteoric waters results in low to intermediate As concentrations (up to a few mg L−1). After the waters are discharged at the ground surface, As(III) oxidizes to As(V) and attenuation of As concentration can occur due to sorption and co-precipitation processes with iron minerals and organic matter present in sediments.
Understanding the mechanisms of As enrichment in geothermal waters and their fate upon mixing with shallower groundwater and surface waters is important for the protection of water resources in Latin America. Data on As concentration in volcanic fluids and geothermal systems at several sites in Latin America suggest that there are two different sources of As in volcanic regions. One source is linked to volcanic gases emitted from volcanic craters after partitioning of As between the gas phase and the magma body, and the other is the As contained in the erupted volcanic products that form the volcanic edifice.
Leaching of As from the volcanic rocks occurs when groundwater with dissolved volcanic gases circulates throughout the volcanic edifice. Results for the
As composition of volcanic gases are scarce and the data available give a very wide range of concentrations, from 0.050 to 1.2 mg L−1 for Mexican volcanoes, to 0.013 to 14.7 mg L−1 and 0.031 to 0.40 mg L−1 for Costa Rican and Nicaraguan volcanoes, respectively. The variability of the data even for a single volcano (e.g., see Póas Volcano above) can be explained by multiple factors. One is the difference in sampling and laboratory procedures that diverse researchers have used, and another is the state of activity of the volcano. In the same way that concentrations of carbon dioxide, sulfur dioxide, and other gases change with the state of activity of a volcano (e.g. Giggenbach, 1988; Todesco et al., 2004), it is probable that As concentrations also fluctuate.
In addition, a clear understanding of the way of how As partitions between gas and liquid phases within a magmatic chamber is not possible yet. Further experimental, field, and theoretical research is needed to understand these partitioning processes at high temperatures. Data for the geothermal systems presented in this paper as well as for other geothermal systems of the world (Planer-Friedrich et al., 2006) suggest that Na–Cl mature waters of volcanic systems present the highest As concentrations. Na–Cl waters typically have relatively low sulfate, high salinity, high Na and Cl concentrations, and a pH usually higher than8. High As concentrations are commonly combined with elevated Li and B contents. Na–Cl waters are the result of a long-term water–rock interaction between groundwater enriched in volcanic gases and heated close to the magmatic chamber and the host rock that these waters encounter
along their flow path (Giggenbach, 1988). They are usually discharged at the base or flanks of volcanoes and represent the main water source for many geothermal fields in exploitation.
As indicated by Birkle et al. (2010), the Na–Cl geothermal water is affected by ebullition during its ascent due to changes in hydrostatic pressure and by condensates at shallower levels forming waters that are low in As as observed in the surficial discharges of several geothermal systems in Latin America (e.g., Los Humeros geothermal field, Berlín geothermal field). Mixing of Na–Cl geothermal waters with shallower meteoric waters can produce bicarbonate-rich waters with low to intermediate As concentrations (e.g., Momotombo geothermal field).
Data for the studied sites in Ecuador, Northern Chile, and Ilopango Lake indicate that the fate of As after water discharge from thermal springs depends on the chemical composition of the water and sediments (e.g., presence of ferric iron minerals able to sorb the As) and the concentration of organic matter. Oxidation of As(III) to As(V) occurs as the waters come into contact with the atmosphere. The oxidation process seems to be catalyzed in the presence of organic matter by bacteria attached to macrophytes (Wilkie and Hering, 1998). The presence of bicarbonate, chloride, and silica in solution seems to inhibit the sorption process of As by iron minerals, as observed at La Merced de Nono in Ecuador. Immobilization of As by sorption onto iron minerals in surface
waters is an important natural remediation process for As contamination.
The data reported in this paper (from different provinces in Ecuador, Camarones River in Chile, outlet river from Ilopango Lake in El Salvador) suggest that natural remediation of As contamination is occurring, and explains why As contamination is more important in groundwaters than in surface waters. For the case of caldera lakes rich in As (as in Coatepeque and Ilopango lakes of El Salvador) or for other lakes such as Papallacta and Amuyo Lakes in Ecuador and Chile, respectively, the water/sediment ratio is high and does not allow for natural attenuation.
Significant transfer of As from water to sediments can occur when water circulates along rivers where the water/sediment ratio is lower than in lakes and conditions are more oxygenated, favoring sorption processes in iron minerals. Other processes such as evaporation can cause increases in pH and can maintain high As concentrations in waters in spite of As removal by sorption on stream sediments, as observed in arid and semi-arid climate regions (e.g., Camarones River). More research on As in volcanic and geothermal systems is needed for a better understanding of the processes that determine the concentration of As in groundwater, volcanic gases, and surface waters.
More extensive monitoring of the water quality and health of the inhabitants of the Latin American Pacific Rim is crucial for ameliorating
the effects of arsenic in waters.
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