Chemical reactions deep underground affect water quality, but monitoring methods are laborious, expensive, and limited in scope. A team of scientists led by Penn State University has discovered that seismic waves can explore groundwater and protect its resources. The research is published in the journal Proceedings of the National Academy of Sciences.
About a third of the US population gets its drinking water from groundwater, so it was important and necessary for scientists to protect this valuable resource. At the moment, they do not know exactly where the water is and how it moves in the bowels of the Earth. In the new study, scientists used human-generated seismic waves, which are similar to those from an earthquake.
Traditional geochemical surveys of water include drilling a borehole up to 10 cm into the ground, collecting soil and rock samples, and grinding and analyzing the chemical composition of the samples in the laboratory. This process is expensive and time-consuming, and only reveals geochemical information for that particular point.
The researchers lowered instruments that can send and receive signals, as well as capture high-resolution images of the borehole to a depth of 35 m.
Using a seismic logging tool, the researchers mapped the subsurface layers of the earth. The team lowered the log into the borehole and took measurements as it surfaced. Higher velocities indicated that the waves were passing through hard bedrock or where the pores in the weathered rock were filled with water. Slower velocities indicated that the waves were passing through weathered rock with air-filled pores or soil at the surface.
The research team assimilated the information into a physical rock model that determined variations in composition, porosity, and saturation to explain the measured velocities.
Scientists have found that simple chemical reactions between water and clay cause small changes that seismic waves can “see.” The changes helped researchers understand where water opens the pores in the bowels.
The researchers also found tiny bubbles of gas in the groundwater, which they speculate to be underground carbon dioxide produced by “microbial respiration” and mineral reactions in the interior. Soil microbes produce carbon dioxide as a byproduct of respiration, just as humans do when they exhale. When water passes through the soil en route to the subsoil, it can carry this carbon dioxide with it.
According to scientists, carbon dioxide in water does not pose a health hazard. Researchers can “see” it with the help of seismic waves, without even knowing about its existence in the bowels of the earth.