Page 347 - Soil and water contamination, 2nd edition

334 Soil and Water Contamination

pH
4.5 4.9 5.3 5.7 6.1 6.5
-2.0 -2.0
Depth below surface (m) -3.0 Depth below surface (m) -3.0
-2.5
-2.5
-3.5
-3.5
-4.0
-4.0
-4.5
-5.0
-5.0 -4.5
0 20 40 60 80 100 0 1 2 3 4 5 6
mmol l -1 mmol l -1
pH DIC
HCO 3 Fe(II) x2 Mg K Br x300
-2.0 -2.0
Depth below surface (m) -3.0 Depth below surface (m) -3.0
-2.5
-2.5
-3.5
-3.5
-4.0
-4.0
-4.5
-5.0
-5.0 -4.5
0 3 6 9 12 15 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
mmol l -1 mmol l -1 ; atm
Mn(II) x100 Ca Na NH 4 SO 4 pCH 4 pCO 2 6642 6642 6642
Figure 17.23 Hydrochemical depth profiles for a well (M2) in the top of the Banisveld landfill leachate plume , the
Netherlands, in 2001. Adapted from Van Breukelen and Griffioen (2004).

organic micro-pollutants at the fringe of this landfill leachate plume (Van Breukelen and
Griffioen, 2004).
After the source of a leachate plume has been depleted or eradicated, the original redox
potential of the aquifer will gradually recover. This natural attenuation will obviously also
be reflected in the concentration of redox-sensitive species. Repert et al. (2006) conducted a
study on the long-term natural attenuation of carbon and nitrogen in a groundwater plume
in the Cape Cod phreatic aquifer (the same site as depicted in Figure 17.15) after removal of
the pollutant source. This source had consisted of treated wastewater that was nevertheless
rich in nitrate and dissolved organic carbon (DOC) and had infiltrated into infiltration
beds, but emissions of the wastewater were stopped in December. Figure 17.24 shows the
downgradient development contaminant plume for DOC, nitrate and ammonium during 8.5
-
years after removal of the source of contamination. Along the transect, the DOC, NO , and
3
+
NH concentrations decreased with time but remained higher than the natural background
4
concentrations. The highest DOC concentrations were typically in the uppermost zone directly
below the infiltration beds, although pulses of DOC between 200 and 300 μM C travelled
through the plume transect. Mobilisation of DOC from sediment organic matter in the
infiltration beds remained a DOC source even after the cessation of wastewater emissions.
-
The NO concentrations in groundwater decreased considerably during the study period:
3
from values of more than 1000 μM in the oxic/suboxic zone prior to December 1995 and
shortly thereafter to about 230 μM in 2004. The anoxic, nitrate-free zone at the centre of the
plume persisted for the entire period of sample collection and even grew bigger directly after
the oxygenated wastewater source had been removed. In 2002 the nitrate-free zone started to
shrink, in response to uncontaminated groundwater entering the plume area. However, there
-
were still relatively high concentrations of NO (150-200 μM) in the shallow groundwater,
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10/1/2013 6:47:09 PM
Soil and Water.indd 346
Soil and Water.indd 346 10/1/2013 6:47:09 PM