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selenite presence was weaker. This long-term study revealed that arsenic and
selenium persistence in groundwater differs as a function of speciation. Specifi-
cally, As(V), Se(IV), and Se(VI) remained stable for up to 12 months whereas
oxidation of As(III) was observed after three months.
17.4 Groundwater Contamination by Trace Organic
Compounds
Aquifer contamination by trace organic compounds of anthropogenic origin, which
can interact with inorganic trace elements, may lead to irreversible changes in
groundwater chemistry. In particular, metal ions can catalyze hydrolysis in a way
similar to acid catalysis. Organic hydrolyzable compounds susceptible to metal ion
catalysis include carboxylic acids, esters, amides, anilides, and phosphate-con-
taining esters. Metal ions and protons coordinate to the organic contaminant so that
electron density is shifted away from the site of nucleophilic attack, to facilitate
the reaction. Metal ion-induced catalysis generally occurs via complexation of the
reactant molecule.
Properties of metal ions, in general, and transition metal ions, in particular,
make them good catalysts for a broad range of organic and inorganic reactions in
the subsurface environment. These reactions are described in the literature (e.g.,
Huang
1999
; McBride
1994
; Smolen and Stone
1998
). Catalysis by surface-bound
metals on suspended particles is observed when the participating reactants are
adsorbed significantly and when rate constants for the reaction at the solid-liquid
interface exceed those in the surrounding liquid phase. Lewis acid properties of
metals, for example, are significant in mineral-catalyzed hydrolysis reactions.
Mineral-catalyzed degradation occurs mainly for the hydrolyzable organics having
a structure suitable for complexing with surface metal cations.
Organic contaminants can reach the soil-subsurface zone from a variety of
agricultural and urban-industrial practices. We selected from the literature-specific
examples of irreversible groundwater contamination following sewage effluent
disposal, pesticide application, and pharmaceutical discharge in the urban envi-
ronment. The extent of the problem is demonstrated by results of the first pan-
European reconnaissance survey of the occurrence of polar organic persistent
pollutants in European groundwater (Loos et al.
2010
). In total, 164 individual
groundwater samples from 23 European countries were analyzed for 59 selected
organic compounds and their degradates. The most relevant compounds in terms
of frequency of detection and maximum concentrations detected were DEET
(84 %; 454 ng/L), caffeine (83 %; 189 ng/L), PFOA (66 %; 39 ng/L), atrazine
(56 %; 253 ng/L), desethylatrazine (55 %; 487 ng/L), 1H-benzotriazole (53 %;
1032 ng/L), methylbenzotriazole (52 %; 516 ng/L), desethylterbutylazine (49 %;
266 ng/L), PFOS (48 %, 135 ng/L), simazine (43 %; 127 ng/L), carbamazepine
(42 %; 390 ng/L), nonylphenoxy acetic acid-NPE1C (42 %; 11 lg/L), bisphenol
