Do know that if you use a well as a drinking water source, it is recommended you test your well annually for nitrates and E-coli. E-coli is an indicator that your well may have other dangerous microorganisms and should be treated for them. Levels of contamination are more critical if infants and pregnant women are involved. If infants or pregnant women will be consuming the well water, it should be tested more often.
Do maintain your point-of-use water treatment system according to manufacturers recommendations. For most systems, lack of adequate maintenance results in useless or even dangerous system operation.
Don't assume if you tested clean one year you will be clean again next year.
Don't assume that a test done by a salesman is accurate.
Don't assume that because your drinking water comes from a well that it is free of contamination.
Don't assume that if your septic system is completely functional, your water will be clear of contaminates. Ground water is fed not only from existing aquifers but also from seepage from above. Your neighbor's leach field may affect your ground water; your waste water disposal system may affect their well.