The Pohoiki Warm Ponds Got Hotter, Our Explanation Why

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The newly formed tide pools at Pohoiki have increased in temperature in mid-2020, with springs as hot as 106°F. Post written with Philip Ong. The temperatures of the tide pools weren’t something I was monitoring previous to a few months ago, as the heat felt pretty consistent following the 2018 eruption of Kīlauea to those temperatures I remember prior. However, in early August I noticed a temperature difference on my visit following the park’s reopening after its first COVID-19 closure. The 2018 eruption of Kīlauea on the Lower East Rift Zone (LERZ) left lasting effects on the Lower Puna coastline, with over 3 miles of the “Red Road” that ran between Pohoiki and Kapoho covered by lava dozens of feet thick. Pohoiki is precariously positioned on the southern edge of lava flows that claimed the east side of the Issac Hale State Park (known as “Bowls” and “Shacks”). Following the end of the eruption, sand and rock driven by ocean currents inundated Pohoiki Bay, blocking the region’s only boat ramp with tons and tons of sediment. Sand accumulation created new tide pools along the old coastline that was previously exposed to the open ocean, now trapping heated spring water and forming several new natural volcanically-heated ponds. The thermally-heated, spring-fed tide pools have long been associated with the shoreline of Kīlauea, including one well known pond at Pohoiki (referred to here as the ‘Old Warm Pond’) that predates the eruption, and another at Ahalanui Beach Park. There has always been some temperature fluctuation in these tide pools depending on a few factors, including the tides and rainfall. However, the recent changes exceeded what could be expected in my own personal experience, as well as in the experience of those familiar with the area that have spoken with me. The ponds are notably hotter now, and have been consistently that way for a few months. The newly created warm ponds did not see significant changes for almost 2 years after the 2018 eruption of Kīlauea, with the temperature of the pools holding around or below the standard body temperature of 98.6°F. Starting in mid-2020, the ponds started to increase in temperature, and are now consistently measuring above 100°F, with the highest temperature I have recorded to date of 106°F at the surface above the springs. RESULTS Results in the attached table represents six field visits to Pohoiki to record temperature between August and November, 2020. There is still some fluctuation in temperatures in the ponds at Pohoiki on different days, which is consistent with how the temperature behaved before the eruption, and continues to do two years after the eruption. Tidal conditions for each sampling date are noted but do not seem to predictably affect the observations. A baseline temperature maximum of 98.6°F pre-temperature increase is sourced to USGS as “spring temperatures range to 37°C” (Janik, et al., 1994), personal experience, and is consistent with temperatures ascertained in conversation with people that frequent the area. DISCUSSION The data presented in the attached table does not capture the rise in temperature that we suspect occurred between May and early August, 2020, but it does show that the temperature has leveled off and held at temperatures well above those previously recorded by USGS. High and low tides do make for some variance in the surface temperature, but do not explain the general increase in temperature of the springs. To understand the reasoning behind the change in spring temperatures at Pohoiki we first examine what is known about the thermal springs along the coastline. “Springs along the coast from Pohoiki to Cape Kumukahi are a significant thermal anomaly with a surprising coherence that indicates there is a significant regional flow of thermal water that discharges along the coast (Janik et al., 1994).” “The warm springs downgradient from the lower ERZ do not contain thermal fluids like those found in the deep geothermal wells in the rift zone. Rather, their chemistry is that of diluted seawater except for silica and bicarbonate levels, and silica and chloride content increase with increasing temperature. A plausible explanation for the spring chemistry is that seawater in saturated rock below the freshwater lens south of the lower ERZ is heated to about 165°C, boils to 100°C and loses steam, and then mixes with the overlying freshwater lens and flows toward the shoreline, where it is further mixed with seawater from the ocean as it discharges.” (Janik et al., 1994, Scholl et al., 1995) In other words, the flow of spring water at Pohoiki predominantly consists of brackish salt-water. Chemical analysis prior to 2018 has shown a different signature from the groundwater sampled from wells within the East Rift Zone around Pāhoa, suggesting the springs at Pohoiki are fed by a separate water table which sits deeper underground and draws from heated, slow-moving sea-water on the south side of the East Rift Zone (Janik, et al., 1994). Water feeding the springs has been modeled to be roughly 18-20 years old based on tritium decay, and matches rainfall that came down up to the ~500ft elevation above Pohoiki based on deuterium and oxygen isotopes and collected rainfall data (Scholl et al., 1995). We suggest that the recent rise in heat at springs of Pohoiki is the effect of the volcanic eruption in 2018. After the eruption, temperatures of the tide pools did not immediately deviate noticeably from previous levels observed, even though the area has undergone significant geologic changes. Yet now, two years later a trend change arrives. Interestingly, the tritium decay models suggest that additional heat associated with the eruption is propagating through the brackish water faster than the water itself is moving through the ground. If our explanation is correct, we would expect the spring-water temperature at Pohoiki to remain at the recent elevated temperatures for years, potentially longer before diminishing very slowly over time. Remnant magma insulated beneath the ground in the LERZ will remain hot for decades, and the 2018 eruption added significant fresh magma into the area for the first time since the 1955 Lower Puna and 1960 Kapoho eruptions did similar. With the recent increase in temperature at the end-point, it would be interesting to see an updated hydrologic geochemical and isotope study post-2018 from the USGS or other researchers to expand upon, or correct our interpretation, of the process for temperature increase that occurred between July and August in the springs around Pohoiki. CONCLUSION The vast majority of the known springs between Pohoiki and Cape Kumukahi were covered by lava in the 2018 eruption, leaving only those at Pohoiki uncovered. Rapid sediment inundation created several new anchialine ponds in 2018 that were previously exposed to open ocean at Pohoiki. Temperatures of the tide pools were roughly at or below body temperature until a recent increase in heat from the springs feeding into the ponds in mid-2020. We believe the most likely explanation for the delayed rise in temperature at the springs around Pohoiki is that added heat from the 2018 eruption has been conducted into the deeper water table south of the LERZ, has propagated through that brackish level of the water table faster than the water itself, and has finally emerged at Pohoiki after an almost two year journey. LIMITATIONS To fill in gaps in observations before the change in temperature I rely upon personal experience and the reports from reliable sources at Pohoiki that have more experience with the area on a daily basis than I do. USGS reports that previously published list the tide pools of Pohoiki prior to the eruption in 2018 list temperatures of 34-35°C (~93-95°F), and having a maximum temperature of 37°C (98.6°F) (Sorey & Colvard, 1994). METHODS Temperature Polling, temperatures recorded are the maximum found that day with a General Infrared Thermometer (IRT205). All of the temperatures listed were taken at the hottest location on the surface of the water, above where water feeds in. The fluctuations in tide make the precise spot slightly variable in a few of the tide pools. REFERENCES (Janik, et al., 1994) Cathy J. Janik, Manuel Nathenson, Martha A. Scholl. (1994) Chemistry of spring and well waters on Kilauea Volcano, Hawaii, and vicinity. Published by U.S. Department of the Interior/ U.S. Geological Survey (OFR 94-586). https://pubs.er.usgs.gov/publication/ofr94586 (Scholl et al., 1995) M.A. Scholl, S.E. Ingebritsen, C.J. Janik, and J.P. Kauahikaua. (1995) An Isotope Hydrology Study of the Kilauea Volcano Area, Hawaii. Published by U.S. Department of the Interior/ U.S. Geological Survey (WRI 95-4213). https://pubs.usgs.gov/wri/1995/4213/report.pdf (Sorey & Colvard, 1994) Michael L. Sorey, Elizabeth M. Colvard. (1994) Potential Effects Of The Hawaii Geothermal Project On Ground-water Resources On The Island Of Hawaii. Published by U.S. Department of the Interior/ U.S. Geological Survey (OFR 94-4028). https://pubs.er.usgs.gov/publication/wri944028