7  Risk Assessment

This chapter describes how the ERAHUMED DSS assesses the ecotoxicological risks associated with pesticide exposure in the Albufera Natural Park. The ERAHUMED DSS follows a deterministic and a probabilistic approach.

The deterministic approach is based on the calculation of Risk Quotients (RQs) for each individual compound and for the mixture of substances. The RQ (acute) is calculated by dividing the calculated daily pesticide concentration in water by the Predicted No Effect Concentration (acute) provided by the user. The RQ (chronic) is calculated by dividing the 21-day pesticide rolling average concentration¹ in water by the Predicted No Effect Concentration (chronic) provided by the user. The RQ for the mixture is calculated as the sum of the individual RQs of the compounds that form the mixture. RQs larger than one are considered to pose an unacceptable risk for the aquatic ecosystem.

The probabilistic apporach is based on Species Sensitivity Distributions (SSD) (Posthuma, Suter II, and Traas 2002). This method enables the quantification of the Potentially Affected Fraction (PAF) of species at a given chemical concentration, offering a probabilistic estimate of ecosystem-level effects. SSDs are used to translate the pesticide concentrations computed in the Exposure module (Chapter 6) into daily values of PAF. Chemicals are grouped according to their TMoA classification (encoded by the tmoa_id chemical parameter, cf. Table 3.2), and the effects of different TMoAs are assumed to be independent. Two distinct SSDs are provided for each chemical:

• Acute risk is evaluated using daily concentrations.
• Chronic risk is evaluated using a 21-day rolling average of concentrations, reflecting longer-term exposure effects.

The SSDs are assumed to follow a log-normal distribution and are thus parametrized in terms of their mean and standard deviations (ssd_acute_mu/sigma and ssd_chronic_mu/sigma parameters in Table 3.2).

Probabilistic risk estimates are computed both at the level of individual chemicals and at the level of their associated TMoA. This dual perspective enables the identification of both compound-specific risks and mode-of-action-level cumulative effects. To obtain the overall risk, ERAHUMED combines the risks associated with each TMoA assuming concentration addition. Finally, the overall PAF is computed as follows, assuming effect addition:

\[ \text{msPAF} = 1 - \prod _i (1-\text{PAF}_i) \tag{7.1}\]

where \(\text{PAF}_i\) is the risk associated with TMoA \(i\). For further details, see (Martı́nez-Megı́as et al. 2024).

Martı́nez-Megı́as, Claudia, Alba Arenas-Sánchez, Diana Manjarrés-López, Sandra Pérez, Yolanda Soriano, Yolanda Picó, and Andreu Rico. 2024. “Pharmaceutical and Pesticide Mixtures in a Mediterranean Coastal Wetland: Comparison of Sampling Methods, Ecological Risks, and Removal by a Constructed Wetland.” Environmental Science and Pollution Research 31 (10): 14593–609.

Posthuma, Leo, Glenn W. Suter II, and Theo P. Traas. 2002. Species Sensitivity Distributions in Ecotoxicology. Boca Raton, FL: CRC Press.

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1. The average concentration \(\overline c (t)\) at day \(t\) is computed as \(\overline c(t) = \frac{1}{T} \sum _{k=0} ^{T-1} c(t-k)\), where \(c(t-k)\) are the regular daily concentrations, and \(T\) is the window size, held fixed at \(T = 21\) in our implementation.