Mineral-rich geothermal water can alter the surrounding soil and vegetation.

Geothermal fluids brought to the surface through production wells may contain dissolved gases, minerals and other substances that can affect the environment if released.

These substances may include gases such as carbon dioxide (CO₂), methane (CH₄), hydrogen sulphide (H₂S) and ammonia (NH₃), as well as dissolved minerals containing elements such as arsenic (As), mercury (Hg) or antimony (Sb). If released in significant quantities, they can affect air, water and soil quality, while carbon dioxide and methane also contribute to greenhouse-gas emissions. Geothermal developments with potentially significant emissions or mineral concentrations are monitored and controlled to limit their environmental impact.

After use, geothermal fluids are commonly reinjected into the reservoir through injection wells, helping to maintain reservoir pressure and limiting the discharge of mineral-rich water at the surface. Binary-cycle plants operate with a closed secondary working-fluid loop and have essentially no direct gaseous emissions, while emissions from other plant types can be reduced using gas-abatement systems.

Long-term withdrawal of geothermal fluids can lower reservoir pressure and, in some geological settings, cause compaction and ground subsidence. At Wairākei in New Zealand, decades of geothermal production have caused substantial local subsidence, although reinjection and reservoir management can reduce this effect. Fluid injection can also induce seismic activity, particularly during the stimulation of enhanced geothermal systems. In Basel, Switzerland, hydraulic stimulation of a deep geothermal reservoir in 2006 triggered numerous microearthquakes and an earthquake of local magnitude 3.4. The project was suspended and ultimately abandoned in 2009 after a comprehensive seismic risk assessment.