I wasn't able to find a map, but one should be available. In general the Drain Commission takes care of water and sewage. Their website is http://www.allegancounty.org/government/dr/publicworks.asp
The County Health Department should have the information you need about septic tanks and wells.
On the website below is a link to the EPA environmental map for all documented ground contamination. I also suggest looking up the groundwater arsenic map for the County.
If you are concerned about living with a well/septic, lets just say there are pluses and negatives to all types of water and waste supplies. Some suggestions are available on this site.