Volcano Watch — Pondering the pond: what Halema‘uma‘u water chemistry tells us

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Crater lakes occur in many volcanoes around the world, including Poas (Costa Rica), Kawah Ijen (Indonesia), and Whakaari/White Island (New Zealand). But water in Halema‘uma‘u is a first in written history for Kīlauea, spurring interest both locally and globally.

The water has risen about 1 meter (1 yard) per week since first spotted on July 25, 2019. Initially, HVO was limited to remote observations of lake size, color, and surface temperature. As the lake grew, HVO began formulating a plan to sample the water. The lake's chemistry could reveal where the water was coming from and what it might mean for degassing and potential hazards at Kīlauea's summit.

After much discussion, HVO and other USGS scientists, Hawai‘i Volcanoes National Park staff, and helicopter and Unmanned or Unoccupied Aircraft Systems (UAS) pilots decided that a UAS was the best option for sampling and would be attempted.

Our opportunity came just after the lake's 3-month anniversary on October 26, 2019, when a USGS UAS successfully collected about 750 milliliters (25 ounces) of water from the lake. The sample was then shipped to mainland USGS laboratories for sophisticated analyses.

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Results thus far indicate an acidic lake, with a pH of 4.2 (neutral is pH 7). Interestingly, most volcanic crater lakes have a pH of less than 3.5 (more acidic) or higher than 5 (less acidic), which places the Halema‘uma‘u lake's pH squarely in the uncommon middle range. However, that might result from the lake's young age and ongoing growth.

Mathematical modeling performed prior to the lake's appearance predicted that groundwater could flow into Halema‘uma‘u once the area had cooled enough after the 2008-2018 lava lake drained away. So, it was not entirely a surprise when water began to pond in the crater.

But, it's important to note that Halema‘uma‘u is where most summit sulfur dioxide (SO₂) degassing takes place, and that SO₂ dissolves readily in water.

As water flows underground toward the now-cooling crater, it dissolves SO₂ rising from magma below. This leads to high concentrations of sulfate ions in the lake (53,000 milligrams per liter) and a tendency towards a more acidic pH.

However, that acidic water reacts chemically with Kīlauea's basaltic rock, which makes the lake less acidic (raises the pH) and results in high concentrations of magnesium in the water. (Basalt is classified as a mafic rock due to its large amount of magnesium and iron [Mg and Fe] in addition to other elements, such as sodium and potassium.) The ratios of magnesium to sodium and of sodium to potassium in the lake water are similar to those ratios in Kīlauea's basalt, which is further evidence of chemical reactions between the water and rocks.

Calcium concentrations are not very high in the water sample; calcium is instead combining with sulfate ions to form solid minerals that precipitate from the water. Iron is also likely forming various minerals, contributing to the lake's yellowish colors.

Complex gas/rock reactions result in Kīlauea's lake water being chemically different from groundwater in a research well south of Halema‘uma‘u and from rainwater. Testing of oxygen and hydrogen that form the water molecules indicate that the lake water was originally rain that percolated into the subsurface where it became groundwater and the chemistry changed.

The Halema‘uma‘u lake is still rising. The current pH reflects the balance between incoming groundwater and the degree of SO₂ degassing from below. If the lake level stabilizes, or the amount of SO₂ changes, the pH may also change. At Mount Pinatubo (Phillipines), after its cataclysmic 1991 eruption, a crater lake formed with a nearly-neutral pH but became more acidic with increased SO₂ degassing and later volcanic activity.

Chemical analyses confirm that the Halema‘uma‘u crater lake dissolves magmatic SO₂. This implies that HVO's measured SO₂ emission rates (about 30 tonnes per day) underestimate the total outgassed SO₂ at Kīlauea. Without the lake, SO₂ emissions from the summit would likely be higher.

This finding is important given that an increasing SO₂ emission rate can indicate shallowing magma. Now, lake sulfate concentrations may be a new clue to changing conditions at Kīlauea. Accordingly, we hope that this first water sample is not our last.

Volcano Activity Update

Kīlauea Volcano is not erupting and its USGS Volcano Alert level remains at NORMAL (https://volcanoes.usgs.gov/vhp/about_alerts.html). Updates for Kīlauea are now issued monthly.

Kīlauea monitoring data have shown no significant changes over the past week. Rates of earthquake activity or seismicity across the volcano remain largely steady. Sulfur dioxide emission rates are low at the summit and below detection limits at Pu‘u ‘Ō‘ō and the lower East Rift Zone. The water pond at the bottom of Halema‘uma‘u continues to slowly expand and deepen.

Mauna Loa is not erupting. Its USGS Volcano Alert level remains at ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption is certain.

During the past week, about 180 small-magnitude earthquakes (nearly all smaller than M2.0) were detected beneath the upper elevations of Mauna Loa. Deformation measurements show continued summit inflation, consistent with magma supply to the volcano's shallow storage system. Fumarole temperature and gas concentrations on the Southwest Rift Zone remain stable.

Mauna Loa updates are issued weekly. For more info on the status of the volcano, please go to: https://volcanoes.usgs.gov/volcanoes/mauna_loa/status.html.

Ten earthquakes with three or more felt reports occurred in Hawai‘i this past week: six quakes with magnitudes between 1.2 and 4.9 generally located 26 km (16 mi) west of Pepe‘ekeo and at 30 km (19 mi) depth on Nov. 11 between 5:24 and 8:34 a.m. HST; two quakes with magnitudes 1.2 and 2.7 located 24-28 km (15-17 mi) east of Hōnaunau-Nāpō‘opo‘o at shallow depths on Nov. 9 at 6:09 and 1:39 a.m. HST; a magnitude-3.4 quake 10 km (6 mi) north of Waimea at 23 km (14 mi) depth on Nov. 9 at 5:48 a.m. HST; and a magnitude-2.8 quake 4 km (2 mi) southeast of Hōnaunau-Nāpō‘opo‘o at 10 km (6 mi) depth on Nov. 8 at 10:56 a.m. HST.

HVO continues to closely monitor both Kīlauea and Mauna Loa for any signs of increased activity.