5.4.1 Acidification from atmospheric inputs
Acidification of soilwater occurs if the rate of displacement of soil cations by H+
exceeds the rate of cation supply from weathering. Ion-exchange reactions in soils
(e.g. eqn. 4.18) help to buffer pH in the short term (see Section 4.8), but over
longer periods cation supply to soils from the underlying bedrock is very slow.
Rainwater is naturally acidic (see Box 3.7) and soilwaters are further acidified by
the production of H+from the decomposition of organic matter (see eqns
4.6–4.10). Thus, acidification can be a natural process, although acid rain (see
Section 3.9) has greatly increased the rate of these processes in many areas of the
world.
Acidification of freshwater is most marked in upland areas with high rainfall
(hence high acid flux), steep slopes (resulting in a short residence time for water
in the soil) and crystalline rocks (which weather, and supply cations, slowly).
Thus, while acid rain is a widespread phenomenon, acidified freshwaters are less
common and are controlled both by rates of atmospheric input and by rock types
(Fig. 5.7). All weathering processes, except sulphide oxidation (see Sections 4.4.2
& 5.4.2), consume hydrogen ions, driving pH toward neutrality. Hence, mature
rivers, which drain deeper, cation-rich lowland soils, have higher pH and lower
aluminium concentrations.
The effects of upland acidification of freshwaters can be dramatic. Between
1930 and 1975 the median pH of lakes in the Adirondack Mountains of north-
eastern USA decreased from 6.7 to 5.1, caused by progressively lower pH in rain-
water (Fig. 5.7). The acidified lakewater killed fish and other animals by several
mechanisms. The problem for fish is that the dissolved Al^3 +in the acidic water
precipitates as an insoluble Al(OH) 3 gel on the less acidic gill tissues, preventing
normal uptake of oxygen and suffocating the animal. Similar problems have
occurred in Scandinavia and Scotland. In addition to problems in freshwaters, the
loss of forests in high-altitude areas has been linked to acid leaching, which leads
to impoverishment of soils coupled with direct loss of cations from plant leaves.5.4.2 Acid mine drainage
Acidification of surface and groundwaters in mining regions is a worldwide
problem. In a mid-1980s survey, 10% of streams fed by groundwater springs in
the northern Appalachians (USA) were found to be acidified by mine drainage.
The acidity is caused by oxidation of sulphide minerals (see eqn. 4.4), common
in sedimentary mudrocks, mineral veins and coal deposits. When mineral and
coal deposits are mined, sulphide is left behind in the waste rock, which is piled
in heaps. These waste heaps have large surface areas exposed to the atmosphere
(Plate 5.1, facing p. 138), allowing extensive and rapid oxidation of the sulphide.
The problem is long-lived, and often intensifies once mining has finished, because
abandoned mines are rapidly flooded by groundwater once the pumps are
switched off.
While the oxidation reaction is a natural one (see Section 4.4.2), the mining
activities increase the scale and rate of the reaction (see eqn. 4.4). The resulting156 Chapter Five