Influence of substrate water saturation on pesticide dissipation in constructed wetlands
Résumé
Constructed wetlands are an effective and practical
option for removing pesticide pollution from runoff or subsur-
face drainage water. The objective of this study was to assess
the efficiencies of a ditch with a bundle of straw placed in its
centre and a vegetated pond installed in grass cover bands at
downstream of a drained plot. The dissipation rates of three
herbicides and three fungicides were monitored on four sub-
strates commonly found in constructed wetlands (two soils,
sediment and straw). The influence of water content was de-
termined in a sequence of three steps (flooded-unsaturated-
flooded) over 120 days. The pesticide dissipation rates ob-
served during the 120 days of incubation ranged from 1.4 to
100 %. Isoproturon and 2,4-MCPA (MCPA) showed the
highest dissipation rates, which ranged from 61.0 to 100 %
of the applied quantities during the 120 days of incubation. In
contrast, boscalid and tebuconazole showed the lowest dissi-
pation rates, which ranged from 1.4 to 43.9 % of the applied
quantities during the 120 days of incubation. The estimated
DT
50
values ranged from 20.5 days to more than 1 year and
were influenced by the substrate water content. The soil and
straw substrates had the lowest DT
50
values during the unsat-
urated conditions, whereas the sediments had the lowest DT
50
values during the flooded conditions. These results could
be explained by an adaptation of microbial communities to
their environmental conditions. Thus, the most favourable
conditions of dissipation for soils and straw are observable
when the drainage ceases (spring and summer). However,
favourable conditions occur all year for the sediments, except
when the constructed wetlands are dry. The results suggest
that the dissipation of pesticides in constructed wetlands con-
tributes to the long-term effectiveness of these buffer zones for
reducing water pollution.