In order to know if an aquifer is under the influence of a geothermal source, its temperature (which can be significantly higher in the case of a geothermal influence) can be measured or if there are vapors present in the aquifer, which would mean that the temperatures in that area are very high (near or greater than 212 ºF (100 ºC)).
Of course, Raju Thapa. In the presence of a geothermal source, substances such as arsenic or ammonia (hazardous to health) may appear in the aquifer. In addition, the presence of hydrogen sulfide can be detected by its characteristic rotten egg odor. However, in large quantities, this substance cannot be perceived by field methods and is lethal.
Geothermal water if any may suspect : deep water circulation /exothermic reaction in between minerals and water./, high temperature gradients/, high heat flow value and many more.
Thank you, Sir @Virendra Kumar Saxena for your suggestion. is there any chances that the influence of geothermal water is so minimal that there are no high-temperature gradients (or not detectable) or any other signs, but it will result in high fluoride in groundwater.
In many cases high Fluoride is observed in geothermal water. Geothermal water is defined if temperature of borewell/ spring water is > 5 degree C of atmospheric temperature. Without influence of seasonal temperature variation. If this is the case of bore well and probably getting temperature 45 or more, may possibly suspect deep water circulation probably 1/2 km to 1 km. Probabiliy of underground water- fluorite type mineral can not be ruled out.
If your question is related to groundwater quality degradation due to possible mixing with geothermal water, then collect samples from the groundwater aquifer, analyze them and compare with samples outside of the scope of the geothermal field. Most of heavy metals are aboundent in geothermal water, together with fluoride etc. There are many other geochemical indicators depending on the type of geothermal system that you working on.