This depends upon many factors including:

1. The level of chemicals in the groundwater; whether naturally occurring or introduced. (NOTE: The Maximum Contaminant Level (MCL) for drinking water was established by the EPA and can be found on their website at: http://water.epa.gov/drink/contaminants/index.cfm .  It is important to note, however, that not all chemicals, compounds or elements have an MCL.  For example natural gas does not have an MCL; and
2. Your individual tolerance to some chemicals.  While some chemicals such as Benzene can be toxic to everyone in quantities as low as a few parts per billion, the toxicity of other additives depends upon the individual.  For example, some people are sensitive to Sodium due to conditions like high blood pressure.  Consequently, a tolerable level of sodium for them might be lower than for a person without a similar condition.  However, only you and your doctor can determine a safe level of exposure for you.  To see a more comprehensive evaluation of chemical toxicity you should visit the website of the U.S. Environmental Protection Agency Integrated Risk Assessment System (IRIS); and
3. The use to which the groundwater is put (e.g. will it be used for human consumption, livestock consumption, irrigation, washing or bathing etc…). 

The best way to determine if your groundwater is fit for its intended use is to have it analyzed by an accredited laboratory for all constituents of concern and to have that analysis evaluated by a qualified professional such as a toxicologist.  You can often obtain a list of accredited laboratories from your County Extension Agent, State Water Quality Agency or local Health Department.