It depends on the source. I sample water – that's what I do – and I drill
wells (at least supervise), I map contamination and design water treatment
systems. I have one area where the geology just stinks but there is no public
water supply and everyone is on wells. The main problems (beside a gasoline
spill that i deal with separately), are the metals in the water and the low pH.
Now one of the biggest problems is iron – up to 30 parts per million (ppm or 30
mg/l). Its not particularly toxic but its awful – makes sinks and other bath
fixtures red and stained and it discolors clothes. It tastes awful in the water
so we treat it and remove as much as we can. The other two problems are not as
easily addressed – arsenic and copper. You see, there is gold in this formation,
and you always get an association of arsenic and copper – while the iron is
annoying, the arsenic and copper are in the range of 1-0.05 mg/l range. Copper
is regulated at 1.0 mg/l and arsenic at 0.01 micrograms per liter. You can't
taste them, smell them nor do they stain fixtures or effect the taste of coffee
– but they are highly toxic – especially the arsenic for people and the copper
for fish and pets – but also certain people that get depression. It I raise the
pH too high to get rid of the iron, I can mobilize the arsenic and copper
because they behave more amphoteric than the iron. (amphoteric means they can
easily go into solution at both a low pH and a high ph) – The point is that they are all metals – you treat them about the same to get
rid of them but not all metals behave in the same way – some are annoying and
some are toxic even at low concentrations. All the heavy metals that are
radioactive emitters as well as metals – are toxic just because of their metal
behavior and their molecular weight – add in radiation – and you have a super
toxic soup. That being said – the ones dissolved in water that are oxides,
chlorides, or other salts can somewhat be removed and precipitated into a highly
toxic sludge by simple pH adjustment and CaCO3 or KMg3 filters (not all by any
means)-however tritium (H3) (heavy hydrogen) cannot. Hydrogen is a metal – even
though we think of it as a gas – its to the left on the periodic table. Its a
really bad one in that it can be easily incorporated into almost all life forms
killing cells along the way. The problem with the treatment is that you create a
waste that is just as toxic – or more so – than you started with because its
more concentrated – now what do you do with it. Its not refined metal – so it
can't reach critical mass – but its still hot as hell and you need a disposal
site. We just should not be creating this stuff in the first place.
Daisy207 
