If the line is galvanized, you should plan on replacing it. Likely a fairly old install on the well. Don't want to panic you but you may want to get water tested ... old lines like that have been known to leech lead into water supply depending on water ph. With high demand of refilling pool, it will clear out many sediments. Problem may arise with lower water flows.
Should be able to locate with metal detector depending on how deep it is buried and should lead you right to the well. Can you locate where the old "cess pool" is so you have another data point?
Whoa now. First, there is no lead in galvanized iron pipe. The galvanized coating is zinc and this pipe is not a source of lead contamination.
https://www.dcwater.com/waterquality/plumbing/faq.cfm
Don't worry about lead on a residential system unless you have or had actual lead pipes in your system.
Now, copper is still often used outside in the dirt. It comes in rolls and some cities require it. Most have switched to polyethylene now which is a wonderful product for your underground runs. Unlike copper, it can't corrode. Do not use PVC, it is undesirable. It gets brittle and fractures. Also do not use galvanized iron outside of the well.
For wells it is still VERY common to use galvanized iron for the downhole pipe. We still use it. 20 foot sticks of 1.25" pipe with threaded couplers. Shallower wells, less than 250 feet deep or so, we use the same polyethylene flexible pipe as we use in the dirt for the downhole pipe. PVC in inferior for this application as well.
So even though the pipe from the well to the house is copper, there may be galvanized pipe in the well. All is modern and could have been installed last week.