Your well gets its water from an underground water source called groundwater. Groundwater originates from surface water and precipitation, including rain and melting snow, which has infiltrated the earth, filling the cracks and open spaces in the rocks and soil. The saturated layers of ground below the water table that store and allow movement of groundwater - i.e., enough to supply a well - are called aquifers (See diagram to the right).
Groundwater can become contaminated by sources above and under that ground, for example, from leaking fuel storage tanks or malfunctioning septic systems. Poorly constructed, older or deteriorating wells can act as a direct pipeline for surface pollutants to contaminate the aquifer. Unused and unmaintained wells are of special concern if they haven't been safely plugged and sealed.
Depending on the type and amount of soil or rock the water passes through on its way to the aquifer, your groundwater may be filtered and very clean before it reaches your well. But once an aquifer is contaminated, it can take a very long time to recover, if ever. An aquifer is not a running stream or pool of water, but saturated layers of sediment that filter, store and allow the passage of water to your well.
The deeper your well is, the deeper the aquifer from which the water in your well comes from. Dug wells get water from a shallow aquifer and drilled wells get water from a deeper aquifer. Water in a shallow aquifer has not been passing through the ground for as long period of time so it will not be as filtered as water in a deeper aquifer. Water in a deeper aquifer will be pass through, or be filtered by, the ground for a longer period of time and is considered to have a lower chance of containing harmful bacteria. Most harmful bacteria and coliforms are usually filtered out from surface water when it reaches a depth of greater than 3 meters although you need to test the water regardless of the depth of your well.
It is impossible to determine the exact direction of groundwater flow based on surface features alone. That is why there are acceptable distances between your well and possible sources of contaminants, even if your well is located upslope. The danger of groundwater contamination is greatest when the contaminant source is close to your well, regardless of the slope. On rare occasions contaminants have been known to spread over several kilometres.
The speed of groundwater is also extremely difficult to be certain of. Some surface water will flow directly into ponds and lakes and some will evaporate before it sinks below the surface. So there may be varying time spans for your groundwater source to replenish itself. This is important because it means that changes in the movement of water, weather patterns and other factors can affect the speed at which your well fills up. Other factors are the soil types surrounding the well and the type of well itself.