A spring is water coming straight up through dirt and is usually surface water.  It is exposed to insects, animals, dirt, weather, runoff, etc.  Springs are almost always going to test positive for bacteria.  Depending on the way the spring is set up it can be chlorinated, but it is more likely to come back....

Springs are literally water coming up through the dirt – they are going to have bacteria. Depending on the set up and components, a spring fed system can be chlorinated to produce a clean test. However, bacteria can come back very quickly and you may want to test it more frequently than a standard well.....