Alkalinity is related to the amount of dissolved calcium, magnesium,
and other compounds in the water and as such, alkalinity tends to be
higher in "harder" water. Lime leaching out of concrete ponds is a
primary source of alkalinity but it is also slowly increased by
evaporation which concentrates the source compounds. Alkalinity is
naturally decreased over time through bacterial action which produces
acidic compounds that combine with and reduce the alkalinity
components.

And for us dum dums, one might say that it's stuff in the water
that buffers it from becoming acidic, even if there are acid
sources in the pond (which there are).

C//