Gas-tight pH measurements to assess an effect of CO₂ on groundwater

Acidification of water by dissolving CO₂ may trigger the release of trace metals from rocks. Thus, pH monitoring is considered to be a key to assess effects of CO₂ on groundwater. In this study, we examined a methodology to measure pH of deep-fluid samples using a gas-tight technique. We used a Kuster Flow Through Sampler connected to a custom spectrophotometric pH measurement system at the Illinois Basin – Decatur Project (IBDP) site. The fluid pressure was up to 19 MPa. The Kuster sampler is gas-tight, but cannot maintain pressure at the sampling depth when it is retrieved. In addition, the mechanisms used to transfer fluid from the sampler, called transfer heads, add up to 60 mL of dead space to the 1L sample chamber. To minimize effects of the sampler design on pH measurements, inert gasses (N₂ and Ar) were used to purge the transfer heads and maintain pressure in the sampler during sample handling at the surface. Our results suggest that gas-tight pH measurement is especially important when the partial pressure of gas is more than the atmospheric condition.