A lot of good answers here. ...

A lot of good answers here. An older well should be checked for physical condition of the casing.  Is the reason for abandonment known? The well should then be pumped and surged at a minimum and scrubbed if possible. You want to be certain that water from the aquifer is tested, rather than water that has been sitting in the borehole or is left over after brushing. Depending on the site geology, water could be coming through cracks / karst from a great distance at a rapid rate, vs. sedimentary material with much lower travel time.

Check your pump literature to be sure the pump can be exposed to ~ 100 ppm chlorine. Whoever drilled the well may still have records. Disinfect using sodium hypochlorite solution or calcium hypochlorite dissolved in a bucket, surge, let stand 24+ hours, followed by flushing again to obtain zero chlorine residual before bacteriological testing. That is total coliform and e. coli. A heterotrophic plate count would provide additional info about general bact quality.

Please do the cleaning, flushing, disinfection, flushing and analyses first, both bacteriological and chemical analyses, before evaluating the need of any treatment schemes - it is possible that no treatment beyond startup cleaning is necessary. Disinfection is the only treatment generally done as a precaution; other treatments are not done unless a specific contaminant needs to be addressed.

Other notes, if near any flowing natural stream, even a dry arroyo, do not discharge chlorinated water onto the ground where it could possibly reach the stream.  If several days pass between cleaning etc. and drawing samples for analysis, pump to waste at least four times the volume of water standing in the bore column before sampling.