Volcano Watch — Kīlauea gas emissions far greater than from geo wells

Volcanoes around the world emit a variety of gases in different proportions, with the main components invariably being carbon dioxide, water, and sulfur dioxide. Minor components include, but are not limited to, hydrogen gas, carbon monoxide, hydrochloric acid, hydrofluoric acid, methane, and hydrogen sulfide.

The gases that are of primary interest in relation to volcanic gas emissions and geothermal development are sulfur dioxide and hydrogen sulfide (the gas with the rotten egg odor). The naturally emitted gases originate from the magma stored inside the volcano and are released and degassed during both storage and eruption. By studying the types and quantities of emitted gases, we can estimate the volume of magma stored and erupted and the physical conditions within the magmatic storage system inside the volcano. 

The degassing of magma is an important process which controls many of the characteristics of eruptions, particularly the explosivity of the eruption. 

Estimates of the natural gas emissions from Kīlauea Volcano are based on data collected from fumaroles and eruption plumes by scientists working at the Hawaiian Volcano Observatory under the Volcano Hazards Program of the U.S. Geological Survey, Department of the Interior. When Kīlauea is vigorously erupting, these natural emissions are about 3,700,000 pounds (or 1,850 tons) per day of sulfur dioxide and roughly 60,000 pounds to no more than 120,000 pounds per day of hydrogen sulfide. When the volcano is not erupting, the emissions decrease dramatically to between 400,000 and 700,000 pounds per day of sulfur dioxide and roughly 4,000 pounds to no more than 12,000 pounds per day of hydrogen sulfide, both emitted predominantly from the summit. 

The estimate for hydrogen sulfide emission has a high level of uncertainty because the estimate is derived from the measured sulfur dioxide emission rate and the ratio of hydrogen sulfide to sulfur dioxide in gases collected during eruptions and from fumaroles, rather than being a directly measured amount. Hydrogen sulfide is difficult to measure in sampled gases because it oxidizes after collection to other sulfur compounds. 

Because hydrogen sulfide emitted naturally by Kīlauea is oxidized (combined with oxygen) and hydrated (combined with water) within about 18 hours in the atmosphere, hydrogen sulfide is rarely detected, even near the summit. Vog, or volcanic fog, contains essentially no hydrogen sulfide or sulfur dioxide - they have each reacted in the atmosphere to form sulfate aerosols, such as sulfuric acid and ammonium sulfate. The volcanoes emit these compounds in only small quantities; they mainly form by complex reactions in the atmosphere of the emitted sulfur gases. 

The estimates of hydrogen sulfide emissions from a properly abated geothermal plant, based on operational plants at The Geysers and at Coso in California, range from 40 pounds to perhaps 100 pounds per day for a 25-megawatt plant. The current development plans for Hawai`i, however, call for reinjection of all the fluids and gases, so the emissions would be expected to be less than for these California installations.

Proper regulatory oversight should reduce the chances for large unabated emissions to near zero. For example, due to increasingly strict regulation at The Geysers geothermal field in California (the largest geothermal field in the world), the total hydrogen sulfide emissions decreased between 1976 and 1988 from about 45,600 pounds per day to less than 4,800 pounds per day even though the electric power capacity increased from 500 to nearly 2,000 megawatts. The more recent emission rate is equivalent to 60 pounds of hydrogen sulfide per 25 megawatts of production, well within the estimated range for a properly abated plant. 

In the event of completely unabated emission of geothermal gases, caused, for example, by failure of equipment at the plant or to natural disruption by an earthquake, the rate of hydrogen sulfide emissions can be estimated from data provided to State of Hawai`i regulatory agencies. For example, the HGP-A well in Puna, which produces about 2.5 megawatts, would emit hydrogen sulfide at a rate of about 1,000 pounds per day, until controlled (based on the measured flow rate of 45,000 pounds of steam per hour and a concentration of hydrogen sulfide of slightly less than 0.1 weight percent). Each well has different potential emissions because each well has a different flow rate and probably has slightly different concentration of hydrogen sulfide. For example, the uncontrolled venting at the KS-8 well emitted an estimated 2,500 to 4,300 pounds per day of hydrogen sulfide based on an estimated steam flow of 150,000 to 200,000 pounds per hour and an estimated hydrogen sulfide concentration of 0.07-0.09 weight percent (this concentration was not measured at the KS-8 well, but is assumed to be similar to the measured concentrations for the nearby KS-3 and HGP-A wells). 

In summary, the current eruption at Kīlauea produces thousands of times more hydrogen sulfide than a properly abated 25-megawatt geothermal plant. When Kīlauea is not in eruption, it produces between 30 and 100 times as much hydrogen sulfide as a properly abated 25-megawatt plant.

Volcano Activity Update

The eruption at the episode 50 vent on the west flank of the Pu`u `O`o cone on Kīlauea's East Rift Zone continued through the week. Flows are heading mainly towards the south, but have not progressed beyond the extent of flows erupted earlier from the same vent. The lava pond inside Pu`u `O`o has dropped down to a level about 200 feet below the rim of the cone. The summit tilt has been flat, slightly decreasing throughout the week, indicating that the eruption rate and the resupply rate, from beneath the summit, are comparable. There were no earthquakes this week that had magnitudes greater than 3.0.