Geothermal energy rocks--but don't drink the water!


In a study of water quality in Napa and Sonoma counties, researchers found that some public water supplies used for drinking water and irrigation are being subjected to mixing with hydrothermal fluids originating from deeper aquifers. About 14 percent of the Napa-Sonoma aquifers used for drinking water supply had concentrations of arsenic above health-based thresholds. The contamination of public supply aquifers may have serious ramifications because arsenic has been linked to several forms of cancer.

Matthew Forrest, a recent graduate of the Center for Marine Biodiversity and Conservation Ph.D. program at Scripps, was the lead author of a scientific paper, which was part of an ongoing USGS study by Scripps alumnus Justin Kulongoski, a research hydrologist with the USGS California Water Science Center. It is the first study to use a broad suite of statistical methods to analyze and characterize mixing between relatively shallow groundwater and deeper hydrothermal fluids. The study also represents the first attempt to quantify the proportions of hydrothermal fluids contaminating public supply wells in the United States, and the results showed that some wells contained in excess of 30 percent hydrothermal fluids.

The study, “Hydrothermal contamination of public supply wells in Napa and Sonoma Valleys,” is published in the June 2013 issue of the journal Applied Geochemistry.”

“This paper is significant for using new statistical approaches and extensive ground water testing to calculate the amount of mixing between hydrothermal fluids and shallow ground water,” said Scripps geologist and study co-author Richard Norris, Forrest’s advisor.  “What impresses me is that some of the hot sources have introduced enough arsenic into public water supplies to constitute a health hazard.”

Hydrothermal fluids are generated when meteoric water (groundwater that has recently been in the atmosphere and returned to the earth as precipitation) infiltrates deep into the crust where it encounters a heat source, such as a magma chamber. As the water warms, it becomes less dense and rises back towards the surface, often ascending along fault or fracture zones. In the Napa and Sonoma area, residual heat is thought to be emitted from shallow magma chambers that were part of ancient volcanic activity in the region. Faults associated with the Hayward Fault Zone system may act as conduits, bringing the hot fluids towards the surface where they can mix with groundwater.

When rocks and sediments are exposed to water at high temperatures, certain elements are leached into the water, which explains the high concentrations of contaminants in hydrothermal fluids. Arsenic solubility increases with increasing water temperature, such that hydrothermal fluids often have high arsenic concentrations, as do older groundwaters that have had extended exposure to arsenic-bearing minerals. Hydrothermal fluids commonly contain concentrations of arsenic that exceed U.S. Environmental Protection Agency (EPA) maximum contamination levels (MCL) for drinking water (10 µg/L).

Four-fifths of the hydrothermal wells, and five of nine contaminated public wells surveyed in the study were found to have arsenic concentrations exceeding the U.S. EPA MCL for drinking water. Arsenic toxicity is a global health problem affecting millions of people. Chronic arsenic exposure can cause serious effects of the neurologic, respiratory, hematologic, cardiovascular, and gastrointestinal systems, and may lead to skin, lung, and bladder cancer.

Hydrothermal fluids also commonly exceed recommended water quality criteria for fluoride. About 75 percent of the hydrothermal wells had concentrations of fluoride above health based thresholds (MCL = 4 mg/L), however only 1 percent of the Napa-Sonoma aquifers used for drinking water supply had fluoride concentrations greater than 4mg/L. This is important because long-term intake of drinking water with elevated fluoride concentrations can lead to dental and skeletal fluorosis. Skeletal fluorosis causes pain and damage to bones and joints and dental fluorosis can cause pitting or mottling of tooth enamel in children.

The statistical methodology used in this study accurately characterized the chemical and isotopic differences between hydrothermal fluids and groundwater in the Napa and Sonoma valleys, and provided powerful techniques to help identify contamination in public supply wells used for drinking water and irrigation. While contamination from hydrothermal fluids may be considered “natural” (as opposed to contamination directly generated by anthropogenic sources), elevated levels of arsenic and fluoride in drinking water are potentially hazardous regardless of their sources.

“Results from this study are important because they provide a means to identify the source of groundwater contamination, and quantify the amount and extent of mixing in the aquifer system,” said Kulongoski. “This data, combined with regional water quality assessments, provide vital information about the sources and distribution of contaminants in groundwater in Napa and Sonoma counties.”

– Matthew Forrest with Robert Monroe

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