Page 342 - Soil and water contamination, 2nd edition

Patterns in groundwater 329

in the Salland area (Vissers et al., 1999). The upper 12 m of the groundwater is polluted by
-
agricultural nitrate. With increasing depth, NO is the first to disappear (at approximately
3
2-
2+
14 m below the surface), rapidly followed by an increase of Fe , and disappearance of SO
4
2+
at greater depth. Note the lower concentrations of Fe in the sulphate reduction zone,
caused by the insolubility of iron sulphides.
If nitrate -polluted water enters an aquifer that contains sufficient amounts of pyrite , the
pyrite can act as an electron donor in the denitrification of nitrate. The disappearance of
nitrate is then accompanied by an increase in sulphate concentrations. Figure 17.21 shows
an example of this process in the Oostrum aquifer in the north of Limburg Province, the
Netherlands. This aquifer contains 0.1–0.9 percent weight of pyrite and is nearly calcite free
(< 0.1 percent weight) (Broers and Buijs, 1997). At approximately 10 m below the water
table , nitrate is reduced by oxidation of pyrite. Below the denitrification front the sulphate
concentration increases. In addition, the concentrations of the trace elements As, Co, and
Ni also increase below the denitrification front. These trace elements can be derived from
the pyrite, in which they are present as substitutions for Fe or S in the crystal lattice (e.g.
Co and Ni for Fe, As for S). However, pyrite oxidation alone is not sufficient to explain the
increase in trace element concentrations. Broers and Buijs (1997) found that desorption of
these elements from the adsorption complex is an additional mechanism for the increase of
trace element concentrations in the groundwater. This is because the oxidation of pyrite by
nitrate produces not only sulphate, but also acid:

FeS + 3 NO + 2 H O Fe ( OH ) + 1. 5 N + H + + 2SO 2 (17.1)
2 3 2 3 2 4
+
In this case of calcite -poor sediments, the H ions cause desorption of the trace elements
from the sediment. In the case of carbonate -bearing aquifer sediments, the acid released
would have been buffered by the dissolution of the carbonate minerals, thereby increasing
the hardness of the groundwater. This illustrates that after denitrification by pyrite
2-
oxidation, agriculturally polluted groundwater can still be identified by the enhanced SO
4
concentration, possibly accompanied by increased hardness or trace metal concentrations.
Redox zoning also develops downgradient of landfills, due to the decomposition of
organic-rich materials in the leachate . Compared with pristine groundwater, leachate
+
contains high concentrations of the following redox species: DOC, Fe(II) , CH , and NH .
4 4
In this case, the redox zones appear in the reverse order, as the decomposition of dissolved
organic material creates anoxic conditions in groundwater just below the landfill . The
anoxic plume enters the oxic aquifer , where the redox potential increases progressively due
to dispersion and depletion of the dissolved organic matter . Figure 17.22 illustrates the redox

zoning downgradient of a landfill site near Grindsted , Denmark: methanogenic (methane -
2-
producing) conditions close to the landfill, through SO , Fe(III), Mn(IV) , and NO -
4 3
reducing conditions, to oxic conditions farthest away from the landfill. Recent investigations
have shown that redox zones may overlap and the redox processes sometimes occur
simultaneously, although one process usually dominates in terms of actual rates (Christensen
et al., 2001).
2+
From Figure 17.22 it is clear that in the leachate plume downgradient from the landfill, Fe
2+
and Mn are mobilised through reduction of Fe(III) and Mn(IV), respectively. In the sulphate
reduction zone close to the landfill, however, the Fe(II) concentrations are lower due to the
precipitation of pyrite. The mobilised Fe and Mn species will migrate downgradient until the
2+
2+
redox potential has risen to a critical level. At this point, the Fe and Mn will be re-oxidised
and will precipitate as oxyhydroxides.
Thus, redox reactions occur in particular at the fringe of a landfill leachate plume , which
is typically some decimetres to a few metres thick. These reactions involve the oxidation of
+
dissolved organic carbon (DOC), CH , Fe(II) , Mn(II) , and NH from leachate and reduction
4 4

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