Ill. Admin. Code tit. 35, § 604.235
a) Each community water supply using groundwater must collect and analyze one sample per well per month for total coliform bacteria. The analysis must be performed by a certified laboratory.
c) When maintenance or equipment replacement on a well occurs that does not require a construction or operating permit under 35 Ill. Adm. Code 602, one sample from the well must be submitted to a certified laboratory for analysis for total coliform bacteria.