Further insights into interconnections between the shallow and deep systems from a natural CO2 reservoir near Springerville, Arizona, U.S.A.

摘要

If carbon dioxide (CO2) sequestration into deep geologic reservoirs is to be accepted by the public and environmental regulators, the possibility of upward leakage into shallow groundwater should be acknowledged and those processes well-understood. Studies of natural CO2 reservoirs and their connection (or lack thereof) with the shallow subsurface is one way to explore these issues. A natural reservoir near Springerville, Arizona, U.S.A. has leaked CO2 to the surface along a fault zone for thousands of years, creating large travertine deposits. In recent times, the CO2 leak rates have declined significantly yet the shallow aquifer is still highly enriched in dissolved CO2. In previous studies, using water level data and simulations we demonstrate that the fault zone provides hydrologic communication between the shallow aquifer and the deeper reservoir. It is reasonable to assume, therefore, that the source of the CO2 in wells completed within the fault zone is the deeper CO2 reservoir. We present water chemistry data to demonstrate the geochemical impact of this CO2 on shallow groundwater quality. Interestingly, arsenic concentrations are elevated, but other trace metals concentrations are not. Previous studies [1,2,3] showed that CO2 originating from magmatic and deep crustal origin is migrating upward along the fault and dissolving into the shallow groundwater. Saline waters are also mixing, to a much lesser degree, but their source was unknown. Here we present strontium isotope data that clearly shows the source to be water/rock interactions with reservoir rocks, which include Paleozoic carbonates, evaporites and shale units.