The well may have been opened to clean it, and a disinfection treatment was not made on the system upon completion of the work.
When a new pump or new pipe is installed, bacteria from the soil or contamination from other sources may get into the water system.
Surface water may have flooded over the well cap or cover or otherwise seeped into the water system.
Coliform bacteria are microbes found in the digestive systems of warm-blooded animals, in soil, on plants, and in surface water. These microbes typically do not make you sick; however, because microbes that do cause disease are hard to test for in the water, "total coliforms" are tested instead. If the total coliform count is high, then it is very possible that harmful germs like viruses, bacteria, and parasites might also be found in the water.
Fecal coliform bacteria are a specific kind of total coliform. The feces (or stool) and digestive systems of humans and warm-blooded animals contain millions of fecal coliforms. E. coli is part of the fecal coliform group and may be tested for by itself. Fecal coliforms and E. coli are usually harmless. However, a positive test may mean that feces and harmful germs have found their way into your water system. These harmful germs can cause diarrhea, dysentery, and hepatitis.
If you are applying for a VA/FHA/HUD Loan they require additional testing of Nitrate, Nitrite and Lead.
Nitrate is naturally found in many types of food. However, high levels of nitrate in drinking water can make people sick. Nitrate in your well water can come from animal waste, private septic systems, wastewater, flooded sewers, polluted storm water runoff, fertilizers, agricultural runoff, and decaying plants. The presence of nitrate in well water also depends on the geology of the land around your well. A nitrate test is recommended for all wells. If the nitrate level in your water is higher than the EPA standards, you should look for other sources of water or ways to treat your water.
At a minimum, check your well every spring to make sure there are no mechanical problems; test it once each year for total coliform bacteria and E. Coli. If you suspect other contaminants, you should test for those as well. However, spend time identifying potential problems as these tests can be expensive. The best way to start is to consult a local expert, such as the local health department, about local contaminants of concern.
1) Mix 2 quarts of bleach in 10 gallons of water. Pour the solution into the well. Using a hose, recirculate the water back into the well for at least an hour, then close the tap.
2)Run all inside and outside cold water taps until chlorine odor is detected, then shut them all off. If you have a water treatment system, switch to bypass before running the faucets.
3)Mix 2 more quarts of bleach in 10 gallons of water and pour this chlorine solution into the well. Replace the well cap. Allow the well to stand idle for at least 8 hours and preferably 12 to 24 hours.
4)Using a hose on an outside spigot, pump the well to waste, away from grass, shrubbery, streams, ponds, and septic tanks until the odor of chlorine disappears.
5)Retest for bacteria 7 to 10 days after disinfecting.