Underground hydrogen storage is a critical component of the hydrogen economy, offering large-scale, long-term storage solutions. Among the available geological formations, salt caverns and aquifers are the most commonly utilized. Each has distinct environmental implications, including land use, water contamination risks, and potential for induced seismicity. Understanding these impacts, alongside mitigation measures and regulatory frameworks, is essential for sustainable deployment.

Land use considerations vary between salt caverns and aquifers. Salt caverns require surface facilities for injection and withdrawal, but the subsurface footprint is minimal due to their engineered nature. These caverns are typically created through solution mining, which involves dissolving salt deposits with water. The surface area required is relatively small, primarily for wellheads and compression equipment. In contrast, aquifer storage involves repurposing natural porous rock formations, often necessitating larger surface infrastructure for monitoring and managing gas movement. The land above aquifers may also face restrictions on other uses, such as agriculture or urban development, to prevent interference with storage operations.

Water contamination risks are a significant concern for underground hydrogen storage. In salt caverns, the primary risk stems from brine displacement during cavern creation and operation. If not properly managed, this brine can contaminate freshwater aquifers or surface water bodies. However, salt formations are generally impermeable, reducing the likelihood of hydrogen leakage into surrounding water supplies. Aquifers, by nature, contain water, and injecting hydrogen into these formations risks mixing with groundwater. Hydrogen, being highly diffusive, can migrate through porous rock, potentially affecting water quality or mobilizing contaminants like heavy metals. Proper site selection, with thorough geological characterization, is crucial to minimize these risks.

Induced seismicity is another environmental consideration, though it is less commonly associated with hydrogen storage compared to other subsurface activities like wastewater injection or natural gas extraction. Salt caverns are mechanically stable due to the plastic nature of salt, which creeps under pressure, sealing fractures and maintaining integrity. Seismic events linked to salt caverns are rare and typically minor. Aquifers, however, may pose a higher risk if hydrogen injection alters pore pressure in faulted formations. While large earthquakes are unlikely, even small seismic events can compromise storage integrity or nearby infrastructure. Monitoring and pressure management are key strategies to mitigate this risk.

Mitigation measures for underground hydrogen storage focus on preventing leakage, contamination, and structural failure. For salt caverns, regular integrity testing using sonar surveys ensures the cavern walls remain stable. Brine management plans must be in place to handle produced saline water responsibly. In aquifers, advanced monitoring systems track hydrogen movement and pressure changes, allowing operators to adjust injection rates if necessary. Secondary containment barriers, such as impermeable cap rocks, are essential to prevent vertical migration of hydrogen. Both storage types require robust well construction standards to prevent leaks at the borehole.

Regulatory compliance varies by jurisdiction but generally includes stringent requirements for site characterization, operational monitoring, and emergency response. In the European Union, the Underground Gas Storage Directive outlines safety and environmental standards for hydrogen storage, mandating risk assessments and public consultations. The U.S. Environmental Protection Agency regulates underground injection activities under the Safe Drinking Water Act, requiring permits for aquifer storage to protect groundwater. Salt cavern projects often fall under mining regulations, with additional oversight from energy agencies. Lifecycle assessments (LCAs) are increasingly used to evaluate the full environmental impact of storage projects, from construction to decommissioning.

Lifecycle assessment insights reveal that salt caverns generally have a lower ecological footprint compared to aquifers. The energy-intensive process of solution mining is offset by the long-term stability and high deliverability of salt caverns. Their low permeability reduces the need for continuous monitoring, lowering operational emissions. Aquifers, while initially less energy-intensive to prepare, may require more ongoing energy inputs for compression and monitoring due to hydrogen’s higher diffusivity. Additionally, aquifer storage may involve more land disturbance over time if migration issues necessitate expanded monitoring zones.

In conclusion, underground hydrogen storage in salt caverns and aquifers presents distinct environmental trade-offs. Salt caverns offer greater geological security and lower long-term impacts but require careful brine management. Aquifers provide flexible storage capacity but demand rigorous monitoring to prevent water contamination and pressure-related risks. Mitigation strategies and adherence to regulatory standards are vital for minimizing ecological harm. As the hydrogen economy grows, lifecycle assessments will play an increasingly important role in guiding sustainable storage solutions.