Microseismic Events Cause Significant pH Drops in Groundwater

Earthquakes cause rock fracturing, opening new flow pathways which can result in the mixing of previously isolated geofluids with differing geochemistries. Here, we present the first evidence that seismic events can significantly reduce groundwater pH without the requirement for fluid mixing, solely through the process of dynamic rock fracturing. At the Grimsel Test Site, Switzerland, we observe repeated, short‐lived groundwater pH drops of 1–3.5 units, while major and minor ion groundwater concentrations remain constant. Acidification coincides with reservoir drainage and induced microseismic events. In laboratory experiments, we demonstrate that fresh rock surfaces made by particle cracking interact with the in situ water molecules, likely through creation of surface silanols and silica radicals, increasing the H+ concentration and significantly lowering groundwater pH. Our findings are significant; pH exerts a fundamental control on the rate and outcome of most aqueous geochemical reactions and microseismic events are commonplace, even in seismically inactive regions. Plain Language Summary Earthquakes can change groundwater composition by opening new fractures, mixing different water sources. In this paper, we describe observations of changes in water pH from small earthquake events triggered by the drainage and refilling of a reservoir. The reservoir overlies a tunnel system created for underground research at Grimselpass, Switzerland. The rocks are cut by several open fractures which transport surface or reservoir water into the tunnels, and which are long enough to have hosted small earthquakes. Tiny earthquake events caused no changes in groundwater pressure or chemistry, but resulted in groundwater locally becoming significantly more acidic (equivalent to the difference between tapwater and vinegar). To explain this observation, we conducted experiments to grind and break rock fragments in water of similar chemistry to the water at Grimsel. The first experiments ground particles and the second experiments broke rock fragments, using a hydraulic press. Both experiments confirmed that reactions between water and fresh mineral surfaces can make water more acid. Small earthquakes are common in the Earth's crust, even in areas that do not regularly experience large earthquakes. Our results suggest that in silica rich rocks, short‐term acidification of groundwater may arise as a result of cracking, due to small earthquakes. Key Points Microseismic event