I'd choose dumping concentrated waste on a small area over dumping diluted waste over a large area.
Except when the dilution is so great that we're entering the domain of homeopathic 'concentrations'.
Yet, then still the idea of dumping over a huge are instead of keeping the smut contained in a small one appears to be wrong, the worst of the two choices.
But what concentrations are we talking about here?
When we use a few litres of water to wash our film, instead of many times that much, do we create a "huge plug of concentrated chemistry"?
Does having to treat those few litres contaminated with fix cause problems that having to clean many times that much of more dilute fix doesn't?
For me, the botom line is that, even though we may not think it, water is not 'free'. We all live in a 'desert', even when we don't.
It is taken from the environment in huge quantities, at a cost to that environment. And it needs to be treated before it is fit for use. And once again after we have flushed it down the drain.
The more we waste it, the larger the cost, both to the environment (and our health) and to our wallets.