Bore water is rich in minerals and may therefore taste and even smell different.
If you are worried about contamination, you can always have a sample checked out by a local laboratory (or give your Council/Landcare group/Waterauthority a call and ask for advise).
Horses are very smart in regard to water quality and they would not drink it if it would be harmful to them. However, water is essential for life, so if they don't have a choice they will have to drink what is available to them.
My horses are on bore water as well and are healthy. However I have heard of horses that have developed some sort of calculi because of the rich mineral content of their drinking water.