Page 344 - Soil and water contamination, 2nd edition

Patterns in groundwater 331

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Figure 17.19 a) NO 3 and b) NH 4 concentration in shallow groundwater (5–17 m depth) in the Netherlands in
1991. The concentrations are visualised as the 95 percent confidence interval for the median value in grid cells of
4 km × 4 km (Pebesma and De Kwaadsteniet,1994); see Figure 17.5b for surficial sediments.
of Fe(II) and Mn(II) from potential oxidants outside the plume. Note that the type of cation
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exchanger present in the aquifer determines the extent of retardation of NH relative to Fe(II)
4
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and Mn(II). Illitic clay minerals bring about high retardation factor s for NH , whereas organic
4
matter brings about high retardation factors for divalent and trivalent metals (Van Breukelen
and Griffioen, 2004). Non-sorbing organic solutes (e.g. DOC and methane ) are transported
across the plume fringe without retardation, so they may be oxidised before the cations arrive.
Van Breukelen et al. (2004) showed that the flow of pH-neutral leachate through the slightly

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