From the Mayor's Desk | Report on the Survey of Private Wells

Approximately 2/3 of our Village is served by municipal water sourced from Lake Michigan and purchased through the Village of Northbrook. Not all residents who have access to municipal water choose to connect. Residents who do not have access (or who have not chosen to connect) have private wells that supply their homes with water. A number of years ago, the Village embarked on a program to monitor the depth at which water was available to private wells. This program arose during a drought season and during a time of water restrictions on outside usage. This program was voluntary and the volunteers agreed to make the depth results available anonymously. The volunteer wells are located in various areas of the Village; they are not concentrated in any one area. This year a water quality test was added. Individual results were given to the volunteers. 

We have received results of the recent well testing. As might be expected changes in water levels have varied, but on balance, have increased over the past year. The depth to water level in the wells ranged from 44 feet to 86 feet. If we look back one year, the water levels ranged from 52 to 91 feet. 
Well water quality test results for about 1/3 of the volunteer wells indicated bacteria levels requiring follow up action by the owners, that is, further inspection and possibly disinfection by a private well service company. 

Private well owners should be aware that while the water is "free", it is neither guaranteed nor is the delivery through their well pumping mechanisms without cost. Private wells should be periodically inspected by the owners for mechanical condition as well as any possible contamination. Municipal water is monitored to meet Illinois Environmental Protection Agency's regulations which limit the amount of certain contaminants in water provided by public water systems. 

The following link provides information, including information on testing, provided by the Illinois Environmental Protection Agency (IEPA) on private wells: 
https://epa.illinois.gov/topics/drinking-water/private-well-users.html