Mound springs of the western U.S. are associated with Quaternary travertine and lacustrine carbonate deposits, and record long-lived interactions of deeply-sourced (“endogenic”) fluids with the near-surface hydrologic regime. Springs occur along faults and fracture zones associated with both local and regional extension (e.g., Rio Grande rift; Basin and Range; Colorado Plateau; Arizona transition zone; Hurricane fault, Utah). Upwelling waters emerge as springs along basin margins and mix with aquifer waters in the shallow hydrologic system. Isotopic characteristics (He and C) are suggestive of a mantle component for the CO₂. Geochemical mixing models indicate that only a small component of saline, radiogenic, hydrothermal fluid is needed to produce observed spring chemistries. Diffuse CO₂ degassing into regional streams and aquifers can be quantified using hydrochemical models (our flux estimate for the Colorado Plateau is on the order of 10¹⁰ mol C/yr), as can impairment to water quality via high solute loads and the presence of elevated trace metal concentrations.

We use interdisciplinary datasets integrating tectonic setting, water and gas chemistry, and microbial community analysis for a suite of CO₂-rich springs of the western U.S. The spring vent environments exhibit a range of temperature, pH and salinity but share key geochemical similarities to chemolithotrophic microbial ecosystems found in oceanic hydrothermal systems (black and white smokers). Cloning and sequencing of amplified 16S rRNA genes using universal primers identifies organisms with >95% similarity to marine denitrifiers and thermophiles, as well as novel forms. Bacterial communities are similar among sampled locales in CO, AZ and NM, and include Gamma-proteobacteria sequences that exhibit strong similarity to halophilic and marine bacteria representatives from cold seeps, hydrothermal vents, saline lakes, and Arctic brine ice. Our archaeal sequences are dominated by Crenarchaeota, often detected in marine and terrestrial volcanic environments. These results suggest that the springs, while individually ‘unique' relative to surrounding habitats, are in fact consistent in pattern, warranting evaluation as both portals into a deep subsurface biosphere, as well as their negative impact on water quality.