Charcoal filters will remove more than taste and oders. The manufactures will not say so from a liability stand point. So they deal with many of the chemicals you are concerned about. But will have a greatly reduced life span.

But say we are dealing with a witches brew like the contaminated waters around New Orleans after Katrena.

First I would rather go elsewhere for a better starting soure. But saying there is no other choice I would not bet my life on a single method. Rather I would use a multi tier approch.

First would be a gravity sand filter. I'm speaking of tricking water through a foot or better yet several feet of sand. This would be in layers of several grades or grain size of sand from couarse to fine. This would not really decontaminate really bad water but would remove many of the larger contaminant particals.

I would then use a distilation process. This could either be a heat source [Such as fire] driven still. Or it could be a solar powered method. With this the first and last portions would need to be discarded. Because of volitile liquid contamination. Those with a lower boiling point like petrolium products or those with a higher boiling point would be the first or last products cooked off. with the water in the middle portion. This is called fractional distilation. Usually spoken of when dealing with petrolium production. But the one thing that distilization will remove for sure are disolved solids and suppended solids such as the nasty parisites we are normally concerned about.

I would still not compleatly trust the result from a multiple contaminated water source. So I would then run it through a charcoal filter. Then to be sure I would run it through a reverse osmosis filter.

True all this isn't really man portable but more of a fixed site solution. But if you are dealing with chemical contamination it can be very complicated depending on what chemicals may be involved. But in a portable method charcoal filters would be the most likely to be portable and effective. Reverse osmosis gilters are on a par with charcoal but are less portable, less available and more expensive.

Intermediatly effective is distilation. The most portable would be a solar still such as shown in many survival books. A sheet of plastic, a hole and a catch container. But more perminant set ups can be nade of glass or ridged plastiic panes and metal back plate. But you need to monitor the different fractions.