Leave lawn clippings on the lawn, rather than removing them, thereby returning nutrients to the lawn without the need for as much fertilizer. Mow regularly to avoid too many clippings at once. Do not use spray devices that attach to the end of a hose to apply fertilizer or pesticides. These devices pose a risk of back flow of fertilizer into private well or public water supplies. Do not store any chemicals in a well pump house.
Accidental spills can reach the groundwater directly via the well hole.
Refuel all mowers, tillers, chain saws etc. Keep fresh animal manure covered if not in use. Store manure as far away from your well head as possible. The less excess flow, the less chance for leaching contaminants into local groundwater. Use low flow devices such as drip irrigation and soaker hoses.