How can oil, not under pressure, contaminate water that is near the surface? Again is the oil under pressure or not? Oil wells are hundreds if not thousands of feet deep. Are you saying the well fills up with water? The casing fail? Is that the problem?
Are you saying the well fills up with water?
It will always be full of fluid. As the wells age, most of the flow becomes water (salty, contaminated water from the reservoir the oil/gas was in). Thatnwalty water adds to corrosion problems.
The hydrostatic pressure at these depths may not produce flowing oil, but the fluid will usually always rise to the level of the water table.