The kit that tests for coliform bacteria, E. coli and nitrates costs $22. Another costs $47 and tests for lead, pH levels and total dissolved solids, in addition to bacteria.
Census figures from 1990 showed about 30 percent of the 50,000 homes in the county had wells.
In some cases the source of contamination can be surprising, McCarty said.
"I visited a home in Adams County where they were having problems with sediment in the water," he said Tuesday. Inspecting a filter on the washing machine, he found it clogged by a black slime.
On closer inspection, he determined the muck was made up of earwig parts. McCarty traced the problem to a loose well cap that allowed the insects to nest and breed inside the casing.
For those with contaminated wells, the extension service will hold a water-testing clinic at 7 p.m. June 4 at its offices.
When a well tests positive for bacteria, McCarty said, "we usually encourage people to follow up with a shock chlorination procedure and retest." If that doesn't solve the problem, he said a disinfection unit can be installed.
Some units use chlorine to disinfect the water, but McCarty said ultraviolet light systems kill the bacteria, are easier to maintain "and have no impact on taste or odor."
Coliform and E. coli bacteria in water indicate contamination from sewage or manure, while nitrates often come from septic systems or heavily fertilized lawns and fields, according to the extension service. Bacteria and nitrate contamination can cause serious intestinal illnesses, particularly in infants and children.
High lead levels can cause brain, nerve and kidney damage and lead is more easily absorbed by children than adults. The source is usually from solder joints in copper pipes.
McCarty said wells should be tested at least once a year. Testing should also be done if repairs have been made to the well, or if there has been excavation work nearby.
For more information, contact county Agent George Hurd at 1-717-263-9226.