Page 337 - Soil and water contamination, 2nd edition

324 Soil and Water Contamination

17.7 EFFECTS OF RETARDATION
Retardation occurs when substances are subject to sorption onto mineral or organic
aquifer materials and are therefore transported more slowly than the average groundwater
flow velocity. Examples of such substances are cations or many organic pollutants. Net
adsorption or desorption with the aquifer materials occurs when the dissolved phase is not in

equilibrium with the solid phase of the aquifer. This is the case if the geochemical properties
of the aquifer material vary along the groundwater flow line or when the composition of the
percolating water changes in time: for example, near the infiltration front of agriculturally
polluted water or at the edge of contaminant plumes. Since sorption processes are generally
fast compared to groundwater flow, local equilibrium will usually be attained between the
solid and dissolved phases.
Figure 17.14 shows an example of the effects of retardation on the transport of
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agriculturally derived K . The figure depicts the depth profiles of K and Cl concentrations
measured in a multi-level well (A2) located in a sandy aquifer in the Salland area underneath
agricultural land (Vissers et al., 1999). This well is approximately 1 km from well A1 whose
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Cl profile was presented in Figure 17.8. Manure spreading has clearly influenced the K and
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Cl profiles: the upper groundwater is polluted by both K and Cl . Some of the K could
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be potentially derived from desorption from the exchange sites . The K at the adsorption
complex would then be exchanged when the TDS concentration of the infiltrating
agriculturally polluted water increases. However, Griffioen (2001) has shown that by
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far the most K in the Salland area is directly derived from agricultural inputs of manure .
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Comparison of the K and Cl profiles reveals that the infiltration depth of anthropogenic
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Cl is more than 20 m (compare the Cl concentrations in Figure 17.8), whereas the K has
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not been transported deeper than 15 m below the surface. This illustrates that K has been
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retarded compared to Cl , which behaves almost conservatively.
As noted above, retardation also affects the advective and dispersive transport in solute
plumes from point source s. DeSimone et al. (1997) studied the early development of a

plume of wastewater effluent in a phreatic aquifer on Cape Cod , Massachusetts, USA. The
aquifer was composed of alternating layers of fine and coarse-grained sands. This plume
originated from a surface discharge of septage (i.e. waste from septic tanks ) from infiltration
beds, which started in February 1990 and consisted mainly of major dissolved constituents
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(Cl , Ca , Mg , Na , K , HCO ) and nitrate . It spread mainly in the 1.5–18 m thick
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Concentration (mg/l)
0 20 40 60 80 100 120 140 160 180
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Depth (m below surface) 15
10
20
25
30 Cl -
K +
35 6642 6642 6642
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Figure 17.14 K and Cl concentration depth profiles in a well (A2) in Salland , the Netherlands, in 1996. Data
from Vissers (2006).

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