What to sample for can vary based on well depth, known naturally occurring contaminants, and local concerns.  Including the list of things we recommend, you should contact your local health agency and ask them what they suggest. It might also be helpful to contact your local cooperative extension office or driller to ask if they know of any concerns in your area. In some states, one or more state agencies might have information on water quality that will be helpful.  Below are two examples. In Massachusetts, you can type in your address, and the system will let you know if you are at risk for Arsenic or Uranium (Figure 1), and in Rhode Island, the middle of the state has a beryllium issue (Figure 2). Investigate, so you aren’t surprised later.  We have had more than one well owner contact us, upset that they didn’t know there was an issue in their area, like arsenic, until after they purchased their home.

    

Fig 1. From the Massachusetts DEP website

 
 
From the Rhode Island Dept of Health (large pink splotch in the middle of the state)

We recommend sampling for nitrate and coliform bacteria annually. These constituents are common and provide an indicator that there is likely a pathway into your well from or near the surface. In some cases they can indicate contamination from your septic system or from livestock, which should not happen if your well is properly located and constructed.  The point of sampling annually for these two relatively inexpensive constituents is they will point out an immediate problem that should be dealt with. If you do have coliform bacteria in your well, you should not drink it until disinfected, and you should also test for E. coli as a follow up. Contact your local or state health agency, and they can give you follow up recommendations.

We also suggest sampling for the following constituents once every 3 -5 years.  Again, we recommend you contact your local health agency for advice and additional information.  The goals we all share are safeguarding public health and source water protection. Sampling for the constituents below will give a water quality professional a lot of valuable information about your well water, including its corrosiveness and other information needed to make treatment decisions.
 
Arsenic Fluoride
Iron Sulfate
Lead Hardness
Manganese pH
Chloride Turbidity
Total Dissolved Solids Copper (if copper piping)
Zinc and Cadmium (if galvanized piping in home or well)