Geoscience Reference
In-Depth Information
A (40 %; 2.3 lg/L), PFHxS (35 %; 19 ng/L), terbutylazine (34 %; 716 ng/L),
bentazone (32 %; 11 lg/L), propazine (32 %; 25 ng/L), PFHpA (30 %; 21 ng/L),
2,4-dinitrophenol (29 %; 122 ng/L), diuron (29 %; 279 ng/L), and sulfamethox-
azole (24 %; 38 ng/L).
In many cases, groundwater chemistry changes may occur by contamination
with mixtures of organic micropollutants, for example, consider the chemical
composition of a groundwater contaminant plume in Boston (USA) following
disposal of secondary sewage effluents (Barber et al.
1988
). About 50 organic
compounds were indentified in the contaminant plume, with concentrations ranging
between 10 ng/L and 500 lg/L (Table
17.4
). The prevailing organic micropollu-
tants found in the contaminated groundwater included di-, tri-tetrachloroethene, o-
and p-dichlorobenzene, C
1
to C
6
alkylbenzenes, and isomers of p-nonylphenol.
Barber et al. (
1988
) found that many trace compounds from chlorinated ben-
zenes, alkylbenzenes, and aliphatic hydrocarbons have persisted for more than
30 years, despite the biodegradation of labile organic compounds. Examination of
DCB (dichlorobenzene isomers), NP (nonyphenol isomers), and DTBB (2, 6-di-
tert-butylbenzoquinone) concentrations in the contaminated groundwater near the
disposal site was similar or even greater than their concentrations in the effluent,
indicating little degradation during infiltration through the vadose zone. The long-
term persistence of DCB and other potentially degradable compounds (e.g., aro-
matic and aliphatic hydrocarbons) in the downgradient zone of the aquifer can be
attributed to the dynamics of the groundwater system. In this case, carbon sources
were limited and the decrease in bacteria with distance from the disposal site led to
almost no biologically induced degradation of organic contaminants.
Pesticide contamination of groundwater has been the subject of extensive study
and monitoring since the second part of the last century. For example, pesticides
that were detected most frequently in European groundwater at concentrations
greater than 0.1 lg/L included chloridazon-desphenyl, NPE1C, bisphenol A,
benzotriazole N,N
0
-dimethylsulfamid, desethylatrazine, nonylphenol, chloridazon-
methyldesphenyl, methylbenzotriazole, carbamazepine, and bentazone (Loos et al.
2010
). In groundwater collected from the wells of seven agricultural areas from
1991 to 1998 in Portugal, detected herbicides included alachlor, atrazine, met-
olachlor, metribuzine, and simazine, reaching maximum concentration values of
13, 30, 56, 1.4, and 0.4 lg/L, respectively, (Cerejeira et al.
2003
). The most
frequently detected herbicides were atrazine (64 %), simazine (45 %), and ala-
chlor (25 %), with compounds found in more than 50 % of wells that sampled
shallow groundwater beneath agricultural and urban areas.
Cavaller et al. (
1991
) studied the persistence of chloroacetamide herbicides
(alachlor, metolschlor, propanil) and phenoxy herbicides (2, 4-D, dichlorprop) in
groundwater samples from three Arkansas (USA) locations. A trend of low deg-
radation rates for low concentrations (1 ppb) was consistent for all five compounds
and at all locations over an 18-month period. In contrast, pesticide degradation was
found to occur at various rates when the contaminants were applied at higher
concentration and kept at a higher temperature (Fig.
17.18
). The calculated half-
lives (t
1/2
) are given in Table
17.5
. Extrapolating over much longer periods of
