If during a sampling event in a well that the water changes from a lower turbidity to a visually higher turbidity, what would be the proper action to take if purge volume is numerically sufficient?

In that situation if you are actually seeing an increase in turbidity during purging the first recommendation is to develop the well further, if possible. It may benefit from some additional rehabilitative development in order to remove fines. We have developed a well for 8 to 12 hours in order to get all the fines up. However, if it’s a low yield formation or adequately developed, as long as the other parameters are stable (specifically dissolved oxygen, EC and turbidity were below a threshold of about 20 to 25 NTU) take the sample. If the turbidity is excessive, above 50 NTU or greater, you have an increased chance of seeing some difference between what would be a filtered and unfiltered concentration. A good reference to look is the 1993 EPA workshop document that has the Gibbons and Sarah paper in it. They document the number of statistical hits they saw between 0 and 25 NTU, 25 and 50 and then 50 and above. But if turbidity is below 20 to 25 NTU, and other parameters are stable, it’s going to be a representative sample.