Multi-site monitoring of dissolved CO2 in the Critical Zone using low-cost measuring platforms: Preliminary findings from an investigation along the Jemez River, New Mexico, USA

Reducing uncertainty in CO₂ fluxes from watersheds requires an increase in spatiotemporal coverage of dissolved CO₂ (PCO₂) data. Importantly, increasing longitudinal coverage at sites of known hydrothermal CO₂ discharge is critical in constraining the influence of hydrothermal systems on water quality and watershed carbon budgets. To meet the need for increased spatiotemporal coverage of PCO₂, a low-cost PCO₂ monitoring platform was developed and deployed at three sites within the Jemez River watershed, New Mexico, where large PCO₂ inputs to stream reaches are known to be present. The monitoring platform consists of a waterproofed, low-cost gas analyzer interfaced with an Arduino-based data logger. This study presents preliminary data collected at three sites along the Jemez River during March 2018. Over the monitoring period, PCO₂ concentrations systematically decreased from headwater (PCO₂ > 10,000 ppmv) to downstream sites (PCO₂ < 800 ppmv). Minimal daily variability was observed at the upstream site while the two downstream sites both exhibited daily PCO₂ cycles. Large decreases in PCO₂ from headwater to downstream sites represent significant degassing of hydrothermal CO₂. PCO₂ dynamics within the main stem of the Jemez River are likely more dominated by stream metabolic processes as evidenced by increased amplitude of daily CO₂ cycles at downstream sites. High PCO₂ stream reaches are systematically diluted at downstream sites by snow melt runoff and baseflow contributions from non-hydrothermal groundwater. However, despite rapid degassing and dilution of PCO₂ concentrations, some hydrothermal CO₂ may be transported significant distances away from hydrothermal discharge areas (i.e. high soil flux, fumaroles). Increased monitoring is needed at upstream reaches of the watershed to determine: 1) specific stream reaches which are sources of hydrothermal CO₂ and 2) temporal variability of upstream PCO₂, which for the Jemez River, imparts significant effects on downstream water quality. Globally, advective transport of hydrothermal CO₂ away from hydrothermal discharge areas may account for a large fraction of the total CO₂ flux from watersheds containing hydrothermal upflow zones.