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Figure 4.1 shows the rate of exceedance over the time period from 2007 to 2017 of the three groups insecticides, herbicides and fungicides. This assessment is based on the number of monitoring sites, because the effects of these three groups hamper three different aquatic organism groups. For this reason, the exceedance per year and sampling site is crucial for the assessment.
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This paragraph is not clear to me. What is the observation unit? The single sampling event (in a specific site) or each individual site with all samplings "aggregated" by year?
I find this assessment a bit lacking in information and ignoring potential spatial (and perhaps temporal, pending on the comment above) correlation. For example, if multiple sampling points are present in the same watershed (or even on the same water body) they will be likely to present correlated results. More in general, areas with more sampling sites will weight more on the final result of the index.
In addition, with the current approach, an exceedance of 1% of the EQS is counted as equal to a 10-fold exceedance.
One alternative way for [partially] accounting for most of these issues is the STE (Spatial, Temporal and Extent of PNEC exceedances) approach promoted by the JRC (Carvalho et al. 2015).