If no-purge sampling, or passive sampling, is an acceptable sampling method, then why is well purging necessary? Why can’t we install a bladder pump in the well and sample right away, it seems like the same concept?

The answer is regulatory paradigms. For many decades, the expectation has been that wells need to be purged prior to sampling.  Then, in the last decade the no-purge, or passive sampling, approach was introduced and there have been a variety of devices produced to use with this method. But, there have not been a sufficient number of scientific studies to support this approach. More research is necessary to support the reasoning that water could be grabbed from the water column using a pump already installed in the well, by just turning that pump on, purging out the volume of the pump and tubing, and claiming that sample as sufficient and representative.