Vulnerability and Resilience of Estuaries to Contamination by Antibiotics and Antibiotic-Resistant Bacteria: a Challenge for the Next Decade - Vulnerability of Coastal Ecosystems and Adaptation - page 70

to 76% [GAR 05]. In the mouth of the estuary, where the
microbiological quality of the water influences that of the mussels
from the coastal zone, there is a permanent contamination by fecal
bacteria, mainly due to the input of, the Risle tributary [TOU 07] 3
since 1995.
2.3.2. The fate of contamination by antibiotics, from the sources of
contamination to the estuary
The concentrations of antibiotics in water reported in the
bibliography are dependent on the analytical methods and the methods
of sampling. However, the maximal values (100 µg L -1 ) are observed
on the level of the principal sources of contamination, such as effluent
from hospitals or emissions from pharmaceutical factories, whereas
the values that are ten times weaker (10 µg L -1 ) are detected
immediately downstream from the discharged effluent of WWTPs
(Table 2.1). Antibiotics, from raw effluent of the WWTPs, are
degradated or eliminated in activated sludge. To that can be added
photodegradation by ultraviolet rays (254 nm). In the surface waters,
the lowest concentrations (10 ng.L -1 ) observed can be explained by
both the dilution in the receiving water and the degradation of the
antibiotics or their metabolites, depending on the physico-chemical
properties to each molecule ([KUM 09, TAM 08] and Table 2.1). For
example, β-lactams are rarely detected since the β-lactam cycle is
rapidly hydrolyzed. In the euphotic zone of the water column,
photodegradation by sunlight is the major degradation pathway
[LAN 09, TON 11]. However, this reaction may be partially
3 The chemical and microbiological contamination of the waters of the Seine and its
estuary have been studied within the framework of two multidisciplinary research
programs: the “PIREN-Seine” program (www.piren-seine.fr), which concerns the
study of the hydrographic network from the headwaters of the basin to Poses (the
physical limit between the river and estuarial sections), and the “Seine-Aval” (Seine-
Downstream) program (http://seine-aval.crihan.fr/web/) which covers the geographic
zone from Poses to the mouth of the river.