Page 339 - Soil and water contamination, 2nd edition

326 Soil and Water Contamination

effluent-contaminated groundwater. This may have been gained from sediments through
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exchange with H , K , Ca , and from other cations in the plume , although exchangeable
sodium made up only less than 2.5 percent of the pre-discharge exchangeable cations
(DeSimone et al., 1997).
17.8 EFFECTS OF ACID –BASE REACTIONS

By nature, rainwater has a slightly acidic pH, ranging from 5.6 to 5.8 due to the presence
of carbonic acid . Normally, the acid rain water is neutralised as it passes through the vadose
zone . In areas with heavy rainfall and acidic soils with little buffering capacity, the pH of
infiltrating water largely reflects the rainwater pH values. Anthropogenic factors including
acid rain, waste disposal, and afforestation may, however, lower the pH of the infiltrating
water. While groundwater flow s through the aquifer , its acidity becomes partly or wholly
neutralised by interaction with the aquifer material. This acid buffering is accomplished in
two important ways: a) carbonate mineral dissolution and b) silicate mineral dissolution
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(Bricker and Rice, 1989). Both dissolution processes consume H ions and produce
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dissolved base cations (Ca , Mg , K , and Na ) and HCO ions, and thus an increase of the
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alkalinity . Furthermore, acidity can be neutralised through exchange with base cations on the
cation exchange complex.
Carbonate mineral dissolution is one of the key processes controlling groundwater
composition, particularly in aquifers made up exclusively of Ca and Mg carbonate rocks
such as limestone and dolomite . Not only carbonate rocks, but also sandy and sandstone
aquifers may contain sufficient carbonate minerals, especially calcite , to neutralise acidity .
The carbonates react readily and quickly with groundwater; this is directly manifested in a
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sharp increase of dissolved Ca and HCO concentrations at the calcite dissolution front.
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2+
For example, Figure 17.17 shows the Ca and HCO depth profiles measured in a multi-
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level well (A10) located in a sandy aquifer in the Salland area underneath a forest, not far
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from the Cl profile presented in Figure 17.8 (Vissers et al., 1999). Both profiles show a sharp
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increase in the Ca and HCO concentrations at about 6.5 metres below the surface. In the
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deeper part of the sandy aquifer, small amounts (up to 1 percent weight) of calcite are present
(Frapporti et al., 1995), whereas in the upper 6 metres of the aquifer all carbonates have
been removed by the infiltrating groundwater since the deposition of the sandy sediments
(in this case, late Pleistocene). In this decalcified zone above the calcite dissolution front,
3+
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the base saturation has declined over time as an increasing proportion of H and Al , rather

than base cations, has occupied the exchange sites , and the dissolved base cations have been
removed by the infiltrating water. It is especially this zone that is susceptible to accelerated
acidification by acid precipitation. In 1996, the pH of the groundwater in the decalcified
zone ranged between 4.0 and 4.2, whereas in the deeper part beneath the calcite dissolution
front, the pH was above 7. At 14 m below the soil surface a stream tube boundary is
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-1
2+
present, below which the Ca and HCO concentrations are approximately 150 mg l and
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-1
360 mg l , respectively. This water originates from agricultural land and is polluted due to
manure application. The ammonium in the manure is nitrified in the vadose zone , which
brings about an additional input of acid (HNO ) to groundwater (see Equation 6.2). This
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additional acid dissolves more calcite compared to the ‘pristine’ groundwater in the upper
stream tube.
In contrast to carbonate dissolution , silicate mineral dissolution is a very slow process.
This causes the changes in groundwater chemistry as a result of silicate weathering to be
less apparent. Nevertheless, silicate weathering is the major mechanism of acid buffering in
aquifers free from carbonate minerals (Appelo and Postma, 1996). In addition to buffering
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acidity and producing silicic acid (H SiO ) and HCO , silicate mineral weathering releases
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