Drain field ground water problems are pretty common in this country. This is particularly true of shallow ground water. The people that get really sick from it are the young (because their immune systems are not fully developed), the old (because their immune systems are getting weak) and people that are already sick from other things (because their immune systems are already weak).  Healthy adults will build immunities to many of these bugs. Often visitors will get the stomach flu because they do not have these immunities built up. It’s the same thing in Mexico.  The first few months you are their you will get sick…but after a while you will build immunities and then you can tolerate it.

Drain field ground water problems can pollute your well. Shallow wells are 25′ or less, and are typically the most likely to be polluted by drain field ground water. Deep wells are usually cased down to 45′ as a standard then the well keeps going down past that. The drainfield should be at least 50′ away from a (cased) deep well and 100′ from a shallow well. Of course, it is also extremely important to properly maintain your septic system, which very few people do. Septic systems are designed to catch the large solids in the tank, then the wastewater or effluent is treated by the soil before it passes through, i.e. to ground water or your well. If the soil in the drainfield is failing and not properly treating the effluent, you could have problems.

Here is a picture which should help you better understand how well and septic system problems occur.

As you may know from researching this topic, failing septic systems are a major financial and environmental problem in this country. Expensive septic repairs can often run from $5,000 to $20,000 or more and a large number of systems are failing throughout the country. For news stories related to failing septic systems and tightening regulations you can go to: 
http://www.laundry-alternative.com/failing-septic-systems/regulations/