Sampling for Multi-Phase Characterization of Deep, Gas-Rich Groundwater Systems

Groundwater under pressure often contains dissolved gases at significantly elevated concentrations, which can complicate sampling. Water chemistry can be altered as acid gases such as carbon dioxide come out of solution. Aquifer parameters can be impacted as bubbles form in the aquifer framework, “vapor locking” the area around a pumping well. Finally, the dissolved gases may be toxic and/or explosive, which can create a hazard for samplers or pump crew.

Since water chemistry can begin to change immediately upon exsolution of dissolved gases, prompt field chemistry is critical. Alkalinity, pH, and other project-specific parameters should be determined within minutes of depressurization.

Dissolved gas profiles can be very important for flow modeling, as gas exsolution can reduce the effective hydraulic conductivity by 30-50% or more around pumping wells.

Sampling dissolved gases at the surface raises a number of issues. In most cases, the sample has traveled through a significant length of plastic tubing to reach the surface. A flow-through gas separation system may preferentially collect some gases due to faster exsolution rates. In pumping a deep well, gases at the surface may have originated in water that has not yet left the borehole.

In-situ methods remove tubing from the equation, and are generally a better choice for control of pressure. However, some are not completely isolated from atmosphere, and most have small sample volumes.

An innovative sampler design allows a sample to be collected and maintained at reservoir pressures and isolated from the atmosphere until analyzed, minimizing the alteration in water chemistry associated with exsolution of gases. A large sample volume permits more analyses to be performed, and may entirely replace pumping for some applications. Discharge of the sample into an evacuated container allows capture of exsolving gases, which may then be decanted for analysis.