Geologic CO2 input into groundwater and the atmosphere, Soda Springs, ID, USA

Abstract

A set of CO2 flux, geochemical, and hydrologic measurement techniques was used to characterize the source of and quantify gaseous and dissolved CO2 discharges from the area of Soda Springs, southeastern Idaho. An eddy covariance system was deployed for ~one month near a bubbling spring and measured net CO2 fluxes from −74 to 1147gm−2 d−1. An inversion of measured eddy covariance CO2 fluxes and corresponding modeled source weight functions mapped the surface CO2 flux distribution within and quantified CO2 emission rate (24.9t d−1) from a 0.05km2 area surrounding the spring. Soil CO2 fluxes (<1 to 52,178gm−2 d−1) were measured within a 0.05km2 area of diffuse degassing using the accumulation chamber method. The estimated CO2 emission rate from this area was 49t d−1. A carbon mass balance approach was used to estimate dissolved CO2 discharges from contributing sources at nine springs and the Soda Springs geyser. Total dissolved inorganic carbon (as CO2) discharge for all sampled groundwater features was 57.1t d−1. Of this quantity, approximately 3% was derived from biogenic carbon dissolved in infiltrating groundwater, 35% was derived from carbonate mineral dissolution within the aquifer(s), and 62% was derived from deep source(s). Isotopic compositions of helium (1.74–2.37 Ra) and deeply derived carbon (δ13C≈3‰) suggested contribution of volatiles from mantle and carbonate sources. Assuming that the deeply derived CO2 discharge estimated for sampled groundwater features (~35t d−1) is representative of springs throughout the study area, the total rate of deeply derived CO2 input into the groundwater system within this area could be ~350t d−1, similar to CO2 emission rates from a number of quiescent volcanoes.