Collecting baseline data is an important component of any T&M Plan. A rigorous shallow subsurface monitoring plan establishes seasonal variations in a carbon sequestration project Area of Review (AOR) and provides useful data for monitoring during CO₂ injection. However, the regulatory guidance does not always clearly define the technologies and tools needed for a successful monitoring plan. This article explores how a comprehensive monitoring approach can reduce future problems during CO₂ injection, and the importance of a comprehensive shallow subsurface monitoring program in defending against false positives that could potentially shut down a carbon sequestration project and lead to considerable financial loss.

Identifying and Mitigating Risk in Your Carbon Sequestration Project

Perhaps the largest risk involved with carbon sequestration projects is CO₂ leakage caused by mechanical or geologic failure, which then negatively impacts an underground source of drinking water (USDW). Even if a leak occurs, understanding the original geochemistry of an aquifer is essential to determining whether an aquifer or USDW has become impaired. Impairment implies environmental change significant enough to increase the risk to drinking water receptors. Baseline monitoring can help identify such impairment by providing a reference point for comparison.

Financial Pitfalls and Reducing Client Liability

Evidence of a leak can lead to project shutdown or even cessation, resulting in significant financial loss. Carbon sequestration project developers and owners who adopt comprehensive baseline monitoring programs compile multiple lines of data and evidence to determine whether subsurface changes are the result of a CO₂ leak or normal natural variation.

Site Characterization

Carbon sequestration projects that incorporate the collection of baseline data establish important geochemical, geological, and hydrologic criteria used to evaluate the complicated nature of the shallow subsurface. Understanding the system's natural variability before injection begins is important for deciphering seasonal changes during injection that may have nothing to do with aquifer impairment.   

The determination of monitoring well location, depth, and quantity for the monitoring of groundwater quality is based on site-specific information. Collaborating on a comprehensive design for the monitoring station locations is important to correctly capture the possible sensitive receptors in the area, cover the largest area that is practicable, and identify a background monitoring location that is outside of the predicted plume and pressure front area.

Components of a Robust Shallow Subsurface Monitoring Program

The United States EPA 40 CFR 146.90 outlines the T&M requirements for the owner or operator of a Class VI well. Additionally, states with primacy (e.g. Wyoming, North Dakota, and Louisiana) have thorough T&M plans that must meet or exceed the EPA underground injection control (UIC) T&M requirements. The rules in these regulatory agencies state that baseline data must be collected, though the type and duration of the baseline data collection are somewhat ambiguous. How does an owner/operator obtain meaningful baseline data, and why is it important to inform future T&M?

Below is a description of the main types of baseline monitoring data:

Groundwater

• The collection of discrete samples to observe for geochemical changes and potential CO₂ migration in the subsurface. Highly precise data but limited lateral coverage.
• Tools: Multiparameter water quality meter, bladder/submersible pump

Continuous Data Recorded at Telemetry Stations

• Less comprehensive data but it provides continuous (minute to hourly) coverage that can provide clues regarding seasonal fluctuations or episodic events.
• Tools: Continuous monitoring probe

CO₂ Efflux

• Provides lateral coverage of the shallow subsurface soil environment. Measures CO₂ efflux from soil to air, providing a clearer view of seasonality.
• Tools: Soil CO₂ efflux measurement

Soil Vapor

• Provides general CO₂ measurements that can be useful in leak detection over a wider area.  Effective in identifying changes in the shallow subsurface as CO₂ rises through the water table and expands as it reaches the vadose zone.
• Tools: Soil vapor probes for continuous monitoring and soil vapor sample points for discrete sample collection.

Air Monitoring

• Eddy covariance provides air monitoring over a large area from a single system.  Measures CO₂ flux, where positive fluxes indicate greenhouse gas source emissions while negative fluxes indicate sequestration.
• Tools: Eddy covariance tower is installed on reasonably flat terrain and is free of obstructions.

Ecosystem Stress Monitoring

• Provides large coverage using satellites that track changes in vegetation growth. 
• Tools: Uses remote sensing satellite technology

Conclusions

Understanding the shallow subsurface conditions at a carbon sequestration site is an important component of building a sustainable project. Just as understanding and accurately measuring reservoir conditions is essential to updating a reservoir model, accurately measuring and understanding shallow subsurface conditions is essential to protecting USDWs and the health of the surrounding environment. Every project is different and may not need every component listed above. Depending on project location, risk tolerances, interest in obtaining California Air Resource Board (CARB) certification, and other factors, choosing an appropriate baseline monitoring program that maximizes coverage but remains cost-effective is key to project success.

Are you looking for expertise on the shallow subsurface components needed for your carbon sequestration project? Trihydro’s CCUS team can help.