of which have lasted up to 5 years, verify
that acid rain at pH 4 or higher is inefficient
in replacing base cations from podzols
and related acid soils (2-4, 18). As
soil pH increases above 5, base cations
are replaced more efficiently. Thus, the
less acid soils with low cation exchange
capacity developed on coarse siliceous
materials are actually more susceptible
to acidification (15). Little research has
been conducted on the impact of acid
rain on such soils (19).
Bicarbonate alkalinity is the principal
measure of the ability of lakes to resist
acidification. In highly siliceous watersheds
with acid soils where rates of
mineral weathering by H2CO3 are low,
inputs of HCO3- per unit drainage area
are also low. Lakes in such watersheds
with small ratios of drainage area to lake
area could be acidified by acid rain falling
directly on them. Little change in pH
will occur until 80 to 90 percent of the
bicarbonate has been neutralized (20).
One lake in Canada that occupies about
40 percent of an entire watershed has
apparently been directly acidified by
acid rain (21). However, this direct acidification
is usually considered a minor
factor relative to acid runoff