Early slug testing was conducted by quickly adding water to a well or quickly removing a bailer full of water from the well. When environmental monitoring wells are being tested, you don’t want to add (or remove) water from the well. This either alters the ambient water quality or generates potentially hazardous waste. Thus a “mechanical slug” is used to run a slug test. This is simply a 3 to 5 foot long section of PVC pipe filled with sand and capped on each end. A cord is attached to one end of the mechanical slug so it can be quickly lowered below the water level (slug-in test/falling head test) and then later quickly raised above the water level (slug-out test/rising head test). Again the change in water level and time are recorded. Quickly lowering or raising the mechanical slug in or out of the water may cause splashing in the well. For slug tests lasting several minutes or longer this is not a significant problem. However, for wells that recover from a slug test in less than a minute the splashing will interfere with a lot of the early time data so that a good determination of the changes in the water level can not be made. Because of these problems the pneumatic slug testing method was developed. In this method, the well head is sealed and air pressure is used to displace the water level.