What's the name for the process where the joining of two underground streams increases the acidity, rather than averaging?

My memory and an online glossary are currently failing.

Someone will no doubt be along to correct me but I think you are refering to 'Mixed Water Corrosion' or something similar.

I think you might be correct, thanks.

From http://www.karstwaters.org/files/glossary.pdf
Mischungskorrosion. (German.)
Dissolution of calcite (and hence of
limestone) by ground water that is derived
from the mixing of two different waters
that were originally saturated with carbon
dioxide but had reached saturation under
differing carbon dioxide partial pressures.
The resultant mixture is undersaturated
and capable of further calcite dissolution,
because the relationship between calcite
solubility and carbon dioxide partial
pressure is non-linear[9].

mixture corrosion. See
Mischungskorrosion.