This is an interesting problem.  Alkalinity is the measure of how resistant
water is to pH change - it is not a measure of pH.  Water with a pH below
7.0 is said to be acid - above 7.0 it is called alkaline.  The use of the
alkaline only applies to the pH.  One method for measuring alkalinity is by
measuring the amount of acid added to a sample to bring about a decrease of
the pH of that sample to some target value.  The more acid it takes, the
higher the buffering capacity, or alkalinity of the water sample.

Surface water in much of middle America has been exposed to soils
originating from limestone and dolomite rock.  Both rocks contain high
levels of calcium and magnesium carbonates occurring as the minerals calcite
and dolomite, primarily.  Both of the minerals are soluble by acid in
rainwater; consequently, the acid is usually neutralized by the soil.  This
phenomenon explains why acid rain is not nearly the problem in the southeast
where coal is burned in order to generate electricity.  The soils in the
northern states and in Canada originate from granitic rocks and have very
little capacity to neutralize acid.  Even though the acid concentrations in
the rainfall there are much lower than in the southeast, the impact in the
lakes and streams is dramatic.

If the alkalinity of the water is known, then the amount of acid necessary
to change the pH should be calculable.  If the estimates of petroleum
deposits are accurate, the population growth rate remains unchanged and the
rate of consumption of fossil fuels per capita are stable, then the day when
the process of ocean acidification will reverse should be predictable.  When
the growing population exhausts the supply of petroleum for heat and
transportation, the waters should start healing.  If the doomsday prophets
are correct, that could be any day now.

A graphic in the Scientific American article predicts increasing acidity to
the year 3000.  I don't think the fossil fuels will last that long.  By that
time, we should be relying on relatively non-polluting products like
hydroelectricity, wind and solar power, hydrogen, etc.  If we rely on
nuclear reactions to provide energy, we may drastically reduce the demand
for energy from all sources.  THAT mass extinction will inconvenience our
hobby.