When using a portable pump that cannot get 5 feet from the bottom due to length, how critical is pump placement in the well?

As long as the pump intake is set within the saturated portion of the screen, it should be acceptable to pump from any position for low flow purging and sampling. You don’t need to be in the middle of the center or the bottom. Now, of course the problem in some situations becomes a very low water level. If you only have 3 to 5 feet of water in the well, and the pump is on the longer side, in essence there is not sufficient submersion to operate the pump properly. So that becomes more of a physical issue. There are pumps (bladder pumps as well as electric pumps) that are as short as 12 inches in length that can be used in those applications. Generally speaking, pump placement is not critical as long as you are within saturated thickness and can maintain submersion of the pump for operation, you should be able to get a representative sample.