Human activities also greatly harm Earth's subsurface, not just the landmass

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Earth's surface bears the visible scars of human activities, from greenhouse gas emissions to deforestation. However, the impact of human intervention extends far beyond what meets the eye, reaching hundreds of meters to several kilometers beneath the planet's surface into Earth's deep subsurface.

This hidden realm, often overlooked, is now the subject of disturbing research by hydrology experts. Jennifer McIntosh, a professor in the University of Arizona's Department of Hydrology and Atmospheric Sciences, and her colleagues have delved into the ways humans influence Earth's deep subsurface.

"We looked at how the rates of fluid production with oil and gas compare to natural background circulation of water and showed how humans have made a big impact on the circulation of fluids in the subsurface," McIntosh explained.

In a paper published in the journal Earth's Future, they shed light on the significant impact of human activities on the circulation of fluids in this subterranean zone.

Deciphering the hidden world in motion beneath our feet

The study, led by Grant Ferguson, also from the UArizona, compared the rates of fluid production from oil and gas extraction to the natural background circulation of water.

The findings reveal that human-induced fluid fluxes surpass the natural flow, and they are projected to increase further with proposed strategies for combating climate change.

These strategies include geologic carbon sequestration, geothermal energy production, and lithium extraction from underground mineral-rich brine.

"Responsible management of the subsurface is central to any hope for a green transition, sustainable future and keeping warming below a few degrees," emphasizes Peter Reiners, a professor in the UArizona Department of Geosciences and a co-author of the study.

Earth's subsurface fluid fluxes

These fluxes transport water, gases, and other substances through the subsurface, influencing everything from groundwater resources to geothermal energy.

Types of subsurface fluid fluxes

Groundwater flow

Groundwater flow represents the movement of water through porous and fractured rock formations. This flow occurs due to pressure gradients and the force of gravity, allowing water to move from areas of high hydraulic head to areas of low hydraulic head. Groundwater flow plays a vital role in the hydrologic cycle, recharging aquifers and sustaining surface water bodies.

Hydrocarbon migration

Subsurface fluid fluxes also include the migration of hydrocarbons, such as oil and natural gas. These fluids originate from organic-rich source rocks and migrate through permeable rock layers until they encounter traps or seals. Understanding hydrocarbon migration is essential for oil and gas exploration and production.

Geothermal fluid circulation

In regions with high geothermal gradients, subsurface fluids can circulate through fractures and permeable rock formations, absorbing heat from the Earth's interior. This heated fluid can then rise to the surface, creating hot springs, geysers, and fumaroles. Geothermal fluid circulation is a key factor in the development of geothermal energy resources.

Importance of Earth's subsurface fluid fluxes

Subsurface fluid fluxes have far-reaching impacts on various aspects of our planet. They influence the availability and quality of groundwater resources, which are essential for human consumption, agriculture, and industrial activities.

Additionally, these fluxes play a crucial role in the formation and distribution of mineral resources, such as ore deposits and petroleum accumulations.

Moreover, subsurface fluid fluxes contribute to the transport and cycling of chemical elements and compounds within the Earth's crust.

This process has implications for geochemical cycles, rock-water interactions, and the evolution of subsurface environments.

Human intervention in Earth's fluid dynamics

In oil and natural gas production, saline water from the deep subsurface is extracted alongside the targeted resources. This ancient water, often millions of years old, acquires its salinity from the evaporation of ancient seawater or reactions with rocks and minerals.

To maintain reservoir pressures and enhance oil recovery, near-surface water is added to the extracted saline water, and the blended mixture is reinjected into the subsurface, creating a continuous cycle of fluid production and reinjection.

Similar processes occur in lithium extraction, geothermal energy production, and geologic carbon sequestration, where leftover saline water is reinjected into the underground.

"We show that the fluid injection rates or recharge rates from those oil and gas activities is greater than what naturally occurs," states McIntosh.

Rewriting Earth's history through subsurface geology

Using existing data from various sources, the research team found that current fluid movement rates induced by human activities surpass those that occurred before human intervention.

As carbon capture, sequestration, and lithium extraction intensify, the researchers also predicted how these activities might be recorded in Earth's geological record.

Moreover, human activities have the potential to alter not only the deep subsurface fluids but also the microbes that inhabit those depths.

Fluid movements can modify microbial environments through changes in water chemistry or by introducing new microbial communities from Earth's surface to the underground.

Hydraulic fracturing, for example, can trigger sudden blooms of microbial activity in previously dormant deep rock formations.

Earth's subsurface is still mostly a mystery

Despite the significance of these findings, many unknowns remain about Earth's deep subsurface and the extent of human impact on this hidden realm.

"We need to use the deep subsurface as part of the solution for the climate crisis," emphasizes McIntosh. "Yet, we know more about the surface of Mars than we do about water, rocks and life deep beneath our feet."

Continued research and exploration of this enigmatic zone are crucial for understanding the full scope of human influence on our planet and developing sustainable strategies for addressing the climate crisis.

While we strive to mitigate the visible effects of human activities on Earth's surface, we must not overlook the hidden impact that extends deep into the subsurface.

Protecting what lies beneath

As we continue to explore and utilize Earth's deep subsurface in our efforts to combat climate change, we must remain mindful of the hidden impact our actions have on this enigmatic realm.

By studying the fluid dynamics, microbial communities, and geological record beneath our feet, we can develop a deeper understanding of the complex interplay between human activities and the subterranean world.

This knowledge will be crucial in guiding our strategies for a sustainable future, ensuring that our solutions do not create unforeseen consequences in the depths below.

As we navigate the challenges of the climate crisis, we must approach the subsurface with a spirit of curiosity, responsibility, and respect, recognizing that the key to our success may lie in the mysteries that await us in Earth's hidden depths.

The full study was published in the journal Earth's Future.

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