Geothermal energy is a new, renewable energy whose utilization needs to be strengthened. However, researchers and practitioners have to face up to various environmental problems related to geothermal systems. In this review, harmful constituents derived from geothermal systems are focused on with their formation mechanisms, types, speciation, and environmental effects being generalized. Geothermal water reinjection and undisturbed downhole heat exchange are incapable of completely eliminating the environmental and health threats posed by exploitation of geothermal energy. Moreover, deterioration of the quality of other types of waters in and around a geothermal area resulting from natural discharge of geothermal waters should not be neglected either. Water treatment is one of the vital ways for prevention and remediation of environmental pollutions induced by geothermal discharge. Thus, promising research directions in the area, which are worth being developed in the future, are summarized.