All groundwater originates from precipitation. The problem is that there is no way of telling what they carry on the way down, at least not until people test them for contaminants. The belief that all groundwater is a potable, secure trove of nature is simply erroneous, given that all groundwater has to pass layers of literal dirt before finding itself in subterranean storage.
If anything, finding potable water underground is much more difficult than finding oil — at least oil did not go through any dirt.
When people drill wells into rock formations, at least those that are permeable and porous enough to hold significant amounts of water, there is a sense of mystery when it comes to the nature of the water they end up pumping up towards the surface.
Whilst it is true that porous rock can filter out large particles such as leaves, bugs and rubbish, it cannot prevent substances mixed in with the water from entering and contaminating the reservoir.
Professionals from Carlyle Drilling note that among the numerous naturally occurring chemicals that may seep into underground wells, the most notorious is a chemical named hydrogen sulphide. Simply put, it turns a pure underground water well into a poisonous, corrosive, flammable and explosive hole. Hydrogen sulphide even has a nifty method of informing drillers of its presence, as this dense, colourless gas smells like rotten eggs.
Another substance groundwater may contain is petroleum, which, if found in large quantity, may offset the disappointment in finding contaminated groundwater. However, it is more likely for a leaking gas station storage tank to cause this contamination than for a water and oil well to form side-by-side.
Groundwater is a precious resource, especially in locations where potable water is hard to come by. Efforts to preserve the purity of these wells must be maintained for the sake of the communities depending on these underground gifts from nature.