The surveys, mailed to people selected at random, ask where they get their drinking water and whether they have experienced nausea, vomiting, fever or other symptoms in the last six months.
MacRae said the county has known for years that there is a relatively large amount of bacteria in most of the county's groundwater, which can be traced to its porous limestone geology.
The porousness allows contaminants to travel easily, posing a threat to wells, springs and other private sources from which about 40,000 to 45,000 county residents get their drinking water, MacRae said.
Contaminated water carries micro-organisms that often accompany harmful bacteria, but their presence does not necessarily mean people will get sick, MacRae said.
He pointed to a 1989 study that showed that 25 percent of 151 tested wells showed signs of possible contamination that could lead to gastrointestinal upset. In 11 percent of the wells, results indicated an even greater possibility that harmful bacteria were present.
All of the wells previously had been tested and found to be free of contaminants, MacRae said. Because of the county's geology, contaminants can move rather easily into an untreated water supply, he said.
"Quite as quickly, it can pass. That's sort of been a problem for us," he said. "It's not as if you can single out an individual well and say, 'This is harmful.'"
The county already has taken steps to reduce the chances of people getting sick from well water, MacRae said. Those include more stringent rules for the construction and siting of wells and septic systems.
Information from the survey will allow county health officials to compare the frequency of illness among people drinking from wells or public water systems. But MacRae cautioned that many other factors determine that as well.
He said, however, the survey would provide an initial frame of reference.
"I would like to hope it would be just the beginning," he said.