A low-cost, automated, and adaptable system for field-based measurement of pCO2, pCH4, and associated variables in groundwater

Abstract

The Groundwater Automated Chemical Examiner (GrACE) is an automated analytical system of integrated sensors for the field-based measurement of chemical and physical properties of groundwater. The GrACE system was designed to explore parameters influencing the dissolved concentrations of carbon dioxide (pCO₂) and methane (pCH₄) in groundwater, incorporating one of two equilibrator designs to measure dissolved gas partial pressures. The pCO₂ sensor has an operating range of 200–100,000 μatm with an accuracy and precision of 17% and 27.5%, respectively, while the pCH₄ sensor has a range of 800–8000 μatm and an accuracy and precision of 3% and 3.2%, respectively, over a month-long measurement campaign without recalibration. The e-folding response times to changes in pCO₂ and pCH₄ are 41.7–50.3 and 103–173 s, respectively. Field trials using the GrACE system found fluctuations in CO₂ and CH₄ concentrations of over 1270 and 12 μmol L⁻¹, respectively, in a 24-h period strongly correlated with variability in groundwater volume. These results reveal that the response times of the analytical system are > 30 times shorter than short-term environmental variations measured here. Complementary measurements of water temperature, electrical conductivity (EC), pH, turbidity, water depth, and dissolved oxygen (pO₂) are also conducted by the GrACE system. The GrACE system uses open-source code and relatively low-cost, low-power 5 V sensors, several of which are “plug-and-play,” enabling (i) modifications to the design presented here for changing experimental conditions and (ii) deployments in remote locations lasting weeks to months using solar powered batteries so both short-term (hours to days) and longer-term (seasonal) processes can be explored.