How do I assess complex contaminated sites that stump me?

You can assess most sites now, but the complex, contaminated ones still stump you, the ones where the problem hides underground, spreads in ways you cannot see, and refuses to fit the usual patterns. That difficulty is not a gap in your worth. Contaminated sites are genuinely among the hardest work in the field. The hard sites overwhelm because you want to understand the whole subsurface at once, and you cannot, no one can. The way through is to treat it like an investigation: gather what you know, form a reasonable picture of where the contamination likely is and how it moves, test that picture, and revise it as new data comes in. You are building understanding in steps, not receiving it whole. With the next tough site, write out your working theory of the contamination and what evidence would confirm or break it. Let the data correct you. And when it reaches past your experience, bring in someone who has worked that kind of site. The complex ones are solved by patient, honest investigation, not by being expected to know the unseeable at a glance.