Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Blaine McCleskey, research chemist with the U.S. Geological Survey.

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Arsenic is a geogenic, or naturally occurring, chemical element in surface- and groundwaters that is of great public-health concern. Thermal waters around the world are known to contain elevated concentrations of geogenic arsenic, and the thermal water in Yellowstone is no exception! Since arsenic is a known carcinogen, the United States Environmental Protection Agency (EPA) set the arsenic standard for drinking water at a maximum of 10 µg/L (micrograms per liter). Thermal waters in volcanic areas are known to contain arsenic concentrations that are often 1-3 orders of magnitude higher than the EPA drinking water standard, and these elevated concentrations can impact downstream water resources worldwide. For example, elevated arsenic concentrations in thermal waters were measured in El Tatio, Chile (up to 48,000 µg/L); Los Humeros, Mexico (up to 74,000 µg/L); Lassen Volcanic National Park, California (up to 27,000 µg/L); and Kamchatka, Russia (up to 10,000 µg/L).

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Arsenic concentrations are also elevated in Yellowstone thermal waters, with most concentrations in the range of 5 to 4,500 µg/L. Higher concentrations (17,000 µg/L) have been measured in the Ragged Hills area of Norris Geyser Basin, but these levels are rare in Yellowstone because arsenic minerals (like orpiment and realgar) can form in most areas, and that limits the amount of arsenic in thermal waters. Despite the elevated arsenic concentrations, visitors who pay attention to park rules and regulations should not worry about their safety as it relates to arsenic. For example, it is not allowed to bathe, soak, or swim in water entirely of thermal origin, and traveling off boardwalks or designated trails in hydrothermal areas is prohibited. In addition, nearly all the arsenic remains in the water during boiling, so the risk of inhalation of arsenic is minimal.

Yellowstone contains more than 10,000 thermal features, like hot springs, pools, geysers, mud pots, and fumaroles, that have diverse chemical compositions, including varying amounts of arsenic. Arsenic in Yellowstone thermal surface waters is primarily derived from deep thermal fluids, where water-rock interactions control surficial compositions. These complex water-rock interactions depend on subsurface geology (rhyolites, basalts, and sedimentary deposits), temperature, boiling, changes in gas contents, precipitation and dissolution of secondary arsenic minerals, and incorporation into sinter. Arsenic geochemistry is complex because it can exist in several forms and can undergo complex chemical reactions—even reactions that are controlled by the microbes that live in thermal waters! As a result, the three major thermal water types in Yellowstone have distinctly different arsenic concentrations.

• Neutral-chloride waters, like those that come from geysers like Steamboat, generally have elevated arsenic concentrations (1,000 – 3,000 µg/L). These thermal features are prevalent in Upper, Midway, Lower, Shoshone, and West Thumb Geyser Basins, where geysers and pools often have large discharges. The arsenic concentration in a water sample collected from Old Faithful Geyser, in Upper Geyser Basin, contained 1,500 µg/L.
• Acid-sulfate waters, like those from Mud Volcano, typically have relatively low arsenic concentrations (< 50 µg/L). These thermal waters are formed by combining shallow groundwater with high-temperature thermal gasses that contain little arsenic. Acid-sulfate features typically don’t have large discharges flowing away from the features. Sulphur Caldron, a large acid-sulfate pool, contains < 20 µg As/L.
• Carbonate-rich waters that readily form travertine, like at Mammoth Hot Springs, have moderate arsenic concentrations (50 – 500 µg/L). Two of the largest-discharging springs in Yellowstone, the Boiling River in Mammoth and an unnamed large spring in Snake River Hot Spring area, contain about 440 and 300 µg/L, respectively.

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What is the fate of arsenic in thermal waters? Most of the water discharged from Yellowstone’s thermal features ultimately ends up in a nearby river. As result, the arsenic concentrations in the main rivers draining Yellowstone are also elevated because very little arsenic is lost over long distances. By employing the same methods that are used to monitor Yellowstone’s rivers for thermal input, the arsenic concentration and flux can also be quantified. Downstream from thermal areas, the summertime river arsenic concentrations are elevated in the Firehole River (~380 µg/L), Gibbon River (~140 µg/L), Madison River (~250 µg/L), Yellowstone River (20-30 µg/L), and Gardner River (~110 µg/L). The total arsenic flux from Yellowstone is also large (~180,000 kilograms/year), and arsenic is transported several hundred kilometers downstream from Yellowstone. As a result, arsenic impacts downstream water resources, requiring additional treatment at some drinking water treatment plants.  There are popular swim areas in Yellowstone, including the Firehole and Boiling River Swim Areas. To minimize the risk of illness from swimming and soaking in Yellowstone rivers, the National Park Service recommends that you avoid swallowing river water and any activities that cause water to enter your nose. If you submerge your head, wear nose plugs or hold your nose shut.

The chemistry of Yellowstone thermal waters is complex, and the resulting compositions are a reflection of the high-temperature water-rock interactions.  In addition to arsenic, several other solutes of concern are elevated in Yellowstone thermal waters, including mercury, fluoride, and antimony, and researchers are also investigating the source and fate of these chemicals. They may not pose an immediate threat to health in the Yellowstone area despite the elevated concentrations, but they can teach us quite a lot about the geology and geochemistry of the subsurface hydrothermal systems!