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Fig. 17.6 Relationship
between total hardness and
alkalinity in water at three
different times, from sites in
southern Sweden (Jacks and
Knutsson 1982 )
The pathway of groundwater chemistry change due to anthropogenic acidifi-
cation was depicted by Knutsson ( 1994 ) in four distinct stages: (a) Seasonal
decreases in groundwater pH, alkalinity, cation concentration, and increases in
sulfate concentration, are caused by storms and snowmelt events, expressed by
''acid surges.'' This pattern is shown in Fig. 17.7 where seasonal depressions in pH
values are observed; (b) groundwater chemistry is characterized by stable pH and
long-term increases in calcium, magnesium, sulfate and nitrate concentrations; (c)
groundwater acidification is expressed by long-term reduction in alkalinity and/or
pH. This process may occur in noncalcareous sandy aquifers under local (point
source) high acid deposition from industrial or municipal origins; and (d)
groundwater acidification occurs when alkalinity decreases to zero, pH drops to
*5, and the concentrations of aluminum, nitrate, and sulfate are relatively high
(Fig. 17.8 ).
In a field study of a noncalcareous sandy aquifer in western Denmark, Kjoller
et al. ( 2004 ) found that acid rain lowered groundwater pH to 4.4, with an Al 3+
concentration of about 0.2 mM in the upper 3-4 m of the saturated zone. The
downward migration rate of the acidification front was 3.5-5.0 cm/year. As a
result, the pH in deeper aquifer water rose to 5.2-6.5, following increases in
desorbed Ca 2+ and Mg 2+ that occurred by exchange with Al 3+ originating from the
acid rain. This pathway of pseudo-irreversible groundwater acidification may lead
to complementary changes in vadose zone chemistry, by affecting the distribution
of trace metals that are desorbed into groundwater in the acidification zone, and
subsequently adsorbed on the solid phase of the aquifer.
Ward et al. ( 2010 ) show that the upper layer of groundwater in the Gnagara
Mound, Western Australia, is acidifying as a result of acid deposition from air
pollution onto the land surface. Profiles of pH indicate that shallow groundwater is
acidified to a depth of between 4 and 10 m. In this region, marine-derived salts
control the major ion water chemistry, as reflected by high Na +
and Cl -
levels as
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