How can a representative sample be obtained from a highly stratified water table?

Even though you could have highly stratified contamination, for example in the case of petroleum hydrocarbons, where most of the contaminant mass might be in the upper part of the water column, by turning on the pump and causing water to flow into the well, we are going to get a flow weighted average concentration across that screen, regardless of where the contaminant is located. It's really important to make a distinction here, that if you are trying to identify the maximum contaminant concentration, then it's not pump placement that would be critical but actually screen placement. When you build a monitoring well and give it a specific screen length, a 1.5-meter, 3-meter, or a 5-meter screen, whatever size it is you have to accept that whatever passes the screen is what you are going to sample. You can’t put in a 10-meter screen and just take samples from the top, middle and bottom to see what your concentrations are, they will not be representative.