There can be many things that cause well contamination but one of several issues are usually the culprit:

I.  Degraded sanitary seals at the well head.  Most fairly modern wells are constructed of a PVC or steel casing that extends approx a foot above grade.  The well is closed off with a metal or heavy gauge plastic cap that has a rubber gasket (the sanitary seal) with inlets for wires and the water distribution pipe.  Over time this rubber seal  can degrade and not provide a water tight barrier.  Any cracks that form around this seal can allow debris and insects to enter the well.  This can cause water contamination. 

II.  Cracked well casings.  Cracks in well casings are most commonly seen at ground level.  These cracks  come from the well casing being hit by something (lawmowers, cars, etc.).  Less commonly, the casing can crack below the ground level, this requires examination by a well drilling contractor to detect.  Once  a crack forms in the casing this allows foreign material to enter the well water and cause contamination  to occur.

III.  Older wells that draw from groundwater.  Many older wells were not drilled to a sufficient depth to reach the drinking water  aquifer and draw water from the shallower ground water supply.  These wells are often subject to contamination from  nearby septic systems, runoff waters, etc..  The only true solution for this  situation is to drill a deeper well into a drinking  water aquifer.  This situation is usually limited to older wells, new drinking water  wells are not normally constructed this way.

If your well is tested and found to have Coliform bacteria present there are some steps you can take to correct the problem.  You should follow these guidelines:

I. The first thing you should do is inspect the well and look for any obvious problems which would allow foreign material to get inside the well (cracked casing, loose or missing sanitary seals, etc.).  If any problems are found you should first make all repairs needed to correct the issue.  

II.  Once any problems are corrected (or if nothing was found upon inspection) then the well should be sanitized.  There is a fairly simple procedure prepared by SCDHEC for this purpose.  This procedure involves adding chlorine to the well in the form of liquid bleach, circulating this water inside of the well and throughout all associated piping.  Click here for a copy of the procedure.  NOTE:  The procedure outlined by DHEC says to use one gallon of bleach, for deeper wells you should use several gallons to ensure that a sufficient amount of chlorine is introduced into the system for proper sanitizing to occur.

III. Once the well has been chlorinated a second bacteria sample is collected after all bleach has been flushed back out.  The majority of the time this procedure corrects the problem and samples test Coliform bacteria free.  However, if samples are still found to be Coliform bacteria positive at this point it is recommended that a well driller be contacted to diagnose the problem as some source of external contamination is most likely occurring.  

Useful documents regarding drinking and/or well water sampling and analysis are located below:

Total Coliform Sample Collection
Well Disinfection Instructions
Drinking Water from Household Wells – EPA