A properly maintained septic system poses no threat to ground water. However, a failing system can be harmful as wastewater can include many types of contaminants.
There are regulations that require the septic system to be certain distances from water wells, streams, lakes, and houses. These are horizontal separation distances.
In order to remove contaminants effectively, the absorption field must also be adequately separated from the ground water. This is the vertical distance.
Both distances are specified by local regulations, visit the Central District Health Departments information links below:
http://cdhd.idaho.gov/EH/water/
http://cdhd.idaho.gov/EH/water/septic.htm
http://cdhd.idaho.gov/EH/water/septicsearch.htm