LOW-COST MONITORING OF CO2 CONCENTRATIONS AND FLUXES WITHIN THE CRITICAL ZONE: INSIGHTS FROM THE SPRINGFIELD PLATEAU AQUIFER, NORTHWEST ARKANSAS

Direct measurements of CO₂ concentrations and CO₂ fluxes are fundamental in characterizing pathways and rates of carbon exchange within Earth’s Critical Zone. These data are also paramount in validating land-atmosphere modeling approaches. As feedbacks between environmental factors and carbon pools may be non-linear and occur over a range of time-scales, adequate assessment of CO₂ variability requires high-resolution temporal monitoring. Development of relatively low-cost measurement and data logging systems would, in turn, greatly aid increasing coverage in direct measurements of CO₂ variability through space and time. Here we present two lost-cost methods and associated results of bench top reference experiments and field deployments for 1) direct measurement of dissolved CO₂ and 2) measurement of CO₂ fluxes from soil and water surface boundaries. Field deployment sites were located within the Springfield Plateau aquifer Critical Zone, northwest Arkansas. Bench top reference tests showed that 1) direct measurements of dissolved CO₂ concentrations by application of a semi-permeable membrane were within analytical error of reference gas cylinder concentrations with application of a novel water vapor correction and 2) fluxes measured using a novel, portable accumulation chamber method were within 15% of reference values, which is similar to the accuracy of commercial portable flux measurement systems. Ranges of CO₂ concentrations at soil, surface water, and spring discharge sites spanned three orders of magnitude with nearly all sites expressing some degree of diurnal variability and covariation with other environmental variables measured on site. Notably, a surface water site deployed over several months began to show increased CO₂ diurnal cyclicity inferred here to be associated with increased stream metabolic function from January to April 2017. Extended discussion of the factors driving CO₂ patterns at individual sites is beyond the scope of this study; however, the methods presented here demonstrate ready-to-use robust, reproducible methods for measuring CO₂ variability. These monitoring methods are applicable beyond the study area across Earth’s surface where CO₂ fluxes may be highly variable even at sub-diurnal time-scales.