If you think it might ever be useful again- then cover it, but if you want to fill it you should do some research…

A quick google search-

There are very good reasons for well owners to make sure inactive or abandoned wells on their property are properly decommissioned:

-Improperly abandoned wells threaten drinking water supplies by providing open conduits into aquifers.

-If a drinking water well is being replaced because of water quality problems in the original well, the abandoned well is a direct threat to the new water supply if it is not properly sealed.

-Improperly abandoned wells can create a liability problem at the time of property resale or if the well causes contamination in neighboring wells.

-Shallow dug wells create a physical hazard simply because of their large diameter and the potential for animals or people to fall into them. Typically, the older fieldstone-lined wells are the most dangerous because many were finished flush to the ground surface and were covered with wooden covers, which are now decayed or non-existent.