In this case, the obtained results are in disagreement with
other studies presented, where the highest concentration values
were measured during low rainfall periods and rainfall assumed to
dilute the stream [7]. The Llobregat River is well-known for having a high human pressure, which should be taken into account
in order to argue the behavior of the analyzed compounds. Specifically, one of the most important sources of EC could be identified as
WWTPs discharges [20]. During intense rainfall and consequential
hydraulic overload it is a common practice that part of the collected
sewage is by-passing the secondary treatment in wastewater treatment plants or directly entering the environment via combined
sewer overflows, in each case foregoing biological treatment [21].
This may well have happened during the sampling event M2 and
the hypothesis is reinforced by the ammonia concentration profile
determined in the river, which increased from 0.07 to 0.67 mg/L
when the river flow rose in M2. Moreover, since some of the EC are
typically associated with municipal wastewater and are present
at concentration higher than most other EC, they may potentially
serve as “indicators” of wastewater contamination [7]. For example,
acetaminophen, which was proposed as raw wastewater indicator
[22] among the non-steroidal anti-inflammatory drugs (NSAIDs),
showed concentrations of 91, 82 and 85 ng/L for the samplings
M1,M3 and M4, respectively, and of 543 ng/L for the M2, when
the highest river flow took place. Similarly, caffeine, that was also
considered as indicator in tracking wastewater contamination in
surface water by Buerge, et al. [23] also showed its maximum concentration of 879 ng/L in the M2 sampling.