Disodium molybdate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

25.5 mg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

4.89 mg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

46.6 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

45 500 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

5 080 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

21.2 mg/kg soil dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

 

This section of the Report provides a concise summary of the effects assessments that was conducted, and which resulted in the derivation of PNEC-value for the various environmental compartments. More detailed information can be found in the Background Document “Environmental effects assessment of molybdenum”.

 

Freshwater compartment

Twelve reliable long-term no-effects data points for the freshwater compartment were used for the construction of a Species Sensitivity Distribution (SSD) from which a median 5th percentile was derived. This value represents the HC5,50% with 5%-95%-confidence interval. The confidence interval is calculated using a Monte Carlo analysis on the log-normal distribution that was fitted through the 12 data points. The outcome of this analysis allows the derivation of the HC5,50% with 5%-95% confidence interval, and this value should be used for PNEC-derivation (i. e., PNECaquatic= HC5,50%/ Assessment Factor). A detailed overview of the data and calculations is provided in the Background Document “Environmental effects assessment of molybdenum”.

The HC5,50% (± 95%CL) that was associated with this distribution was 35.7 mg Mo/L (95%CL: 18.6 – 52.9 mg Mo/L).  The assessment factor that is applied on this HC5,50% for calculating the PNEC for molybdenum in the freshwater aquatic environment depends on the uncertainty analysis of the dataset. For uncertainty considerations the London Workshop 2001 recommended, for the freshwater compartment, to apply an additional assessment factor on the 50% confidence value of the 5th percentile value (thus PNECaquatic = HC5,50%/AF), with an AF between 1 and 5, to be judged on a case-by-case basis. Based on the available chronic NOEC data, the following points were considered when determining the size of the assessment factor:

 

1) The overall quality of the database and the end-points covered, e.g., if all the data are generated from “true” chronic studies using relevant endpoints; representativity of the physico-chemistry of the test media;

2) The diversity and representativeness of the taxonomic groups covered by the database;

3) Statistical uncertainties around the 5th percentile estimate, e.g., reflected in the goodness-of-fit or the size of confidence interval around the 5th percentile;

4) Comparisons between field and mesocosm studies and the 5th percentile and mesocosm/field studies to evaluate the laboratory to field extrapolation;

5) Comparison of the HC5,50% with unbounded NOEC-values.

 

 

The Mo-data properly reflects the variability in physico-chemical conditions encountered in European surface waters (calculated from measured water chemistry in EU surface waters). The Mo-database also fulfills the recommendations of 10-15 different NOEC values. Data for twelve different species are used for calculating the species sensitivity distribution, and covers the eight different taxonomic groups that should be included in the effects database (as defined by The London Workshop (2001)).

The HC5 value is derived as the 5th percentile of the SSD. Different types of SSD curve fitting functions and goodness of fit approaches were investigated when estimating the HC5. The choice of SSD curve fitting and goodness of fit approaches impacts the derivation of the HC5 to some degree, but the impact was no greater than a factor of 1.15 and thus not of large significance. The log-normal distribution turned out to be the best fitting curve, and resulted in the most conservative HC5-value compared to other distributions that resulted in a significant fit (e.g., Log-Gamma, Log-PearsonV, Log-inverse Gaussian,…). According to the guidelines presented in the TGD (EC, 2003) and RIP 3.2, Chapter 10 (ECHA, 2008) the 50 % confidence interval (or median confidence interval) is considered in deriving the PNEC. This percentile was calculated by conducting an analysis according to the methodology presented by Aldenberg and Jaworska (2000). The resulting HC5,50% with 95% CL was 35.7 mg Mo/L (95% CL: 18.6 – 52.9 mg Mo/L).

 

No mesocosm data were available that allow the derivation of threshold concentrations of molybdenum in freshwaters at the field scale. However, the lack of such data does not imply that field testing might generate lower NOEC values. Stubblefield (2010) and Regoli et al (2012) reported on a 120 day bioaccumulation study with the most sensitive aquatic species of Mo-SSD (rainbow trout O. mykiss). Fish were exposed to 11.1 mg Mo/L (measured value) for a 60 day period, followed by a 60 day depuration period. No significant effects on growth or survival were noted among the different exposure concentration (control, 0.88 mg/L, 11.1 mg Mo/L) throughout the 120 day test period. Mo-levels: fish of all three exposure concentration groups remained healthy with no significant signs of stress or abnormalities. The size and growth of the fish throughout the study showed similar growth patterns amongst each concentration. This finding indicates that application of an assessment factor that would result in a PNEC below a concentration level of 11.1 mg Mo/L could be considered as overly conservative.

 

The evaluation of literature data revealed some good-quality studies that resulted in unbounded NOECs (i.e., highest test concentration did not reveal any significant effect), and therefore these studies were not suited for the derivation of a HC5/PNEC-value. However these studies have scientific value, and therefore the reported unbounded NOECs are compared with the HC5,50% of 38.2 mg Mo/L. McConnell (1977) reported an unbounded NOEC of >17 mg/L (nominal value) for the rainbow trout Oncorhynchus mykiss. This value is a factor of 2.25 below the HC5,50% of 38.2 mg Mo/L. Ennevor (1993), on the other hand, observed no significant adverse effects at the highest test concentration of 19.5 mg Mo/L, i.e., a factor of 1.96 below the HC5,50%, and this for the trout Oncorhynchus kisutch. An unbounded 50d-NOEC of >8 mg Mo/L (nominal value) has been reported by Adhikari and Mohanty (2012) for the brown trout Cirrhinus mrigala. Although this value is slightly below the final PNEC that was determined for Mo (see further), this non-guideline study does confirm that no adverse long-term effects are expected in the low mg-range. It is concluded that the results of this study do not warrant a revision of the HC5,50% assessment factor that has been determined in 2010. Finally, Kennedy et al (2019) reported that exposure of juvenile O.mykiss for 21 days to a nominal concentration of 500 mg Mo/L had no significant effect on mortality, ventilation rate, post exposure exercise-induced mortality or Cu-binding to the gill.

 

These findings support the conclusion that the HC5,50% that is calculated with this dataset is sufficiently protective of the aquatic environment. This value of 35.7 mg Mo/L is below the lowest NOEC is the dataset (43.2 mg Mo/L; Oncorhynchus mykiss), and therefore protects all species that are included in the species sensitivity distribution.

There are, however, some arguments that may promote the application of an additional assessment factor in the HC5,50% for the determination of the PNECaquatic. The major deficiency is the lack of a mesocosm study with molybdenum, therefore creating some uncertainty with regard to the lab-to-field translation of Mo-related effects.

 

 

Based on all of these arguments – and taking sufficient conservatism into account - an Assessment Factor of 3 on the HC5,50% is put forward for the derivation of an aquatic PNEC, resulting in a PNECaquatic of 11.9 mg Mo/L. The use of an AF of 3 results in a PNECaquatic that is below the reported reliable unbounded NOECs and the 95% CL that is associated with the HC5,50%. An assessment factor of 3 is sufficient to cover the uncertainty with regard to the lab-to-field translation of Mo-related effects.

 

 

Marine water compartment

Eleven reliable long-term no-effects data points for the marine compartment were used for the construction of a Species Sensitivity Distribution (SSD) from which a median 5th percentile was derived. This value represents the HC5,50% with 5%-95%-confidence interval. The confidence interval is calculated using a Monte Carlo analysis on the log-normal distribution that was fitted through the 11 data points. The outcome of this analysis allows the derivation of the HC5,50%with 5%-95% confidence interval, and this value should be used for PNEC-derivation (i. e., PNECaquatic= HC5,50%/ Assessment Factor). A detailed overview of the data and calculations is provided in the Background Document “Environmental effects assessment of molybdenum”.

 

Using the RIVM software package ETX, a Log-Normal Distribution was plotted through this data set. The HC5,50% (± 95%CL) that was associated with this distribution was 6.85 mg Mo/L (95%CL: 0.92 – 22.1 mg Mo/L), respectively. Application of an assessment factor between 1 and 5 on this HC5,50% will result in the final PNECmarine for molybdenum in the marine environment. The value of this assessment factor depends on the uncertainty analysis that is conducted on the chronic marine toxicity dataset.

 

An uncertainty analysis - comparable to the one that was done for the freshwater compartment - was conducted, and the outcome of this assessment was the basis for assigning the assessment factor (AF) of 1-5 that should be applied on the HC5,50% for the determination of the PNECmarine. 

 

The uncertainty analysis revealed that the effects database for molybdenum fulfils all requirements that were stipulated at the London Workshop (2001) for the application of the statistical extrapolation method (SSD-method) when deriving a PNECmarine. High quality (Klimisch 1) no-effects data are available for eleven different species and eight different trophic levels, each representing a trophic level that that were relevant for the marine environment and had additional ecologic and/or economic value: fish (two species), a calanoid copepod, an opossum shrimp (mysid), echinoderms (two species), molluscs (two species), a diatom, a microalga, and a macroalga. From an ecological point of view all species, live stages, exposure periods and evaluated endpoints (growth, reproduction, developmental malformations) are relevant for a proper assessment of the marine aquatic environment. All tests were conducted according to standard guidelines that have been published for these organisms, or that were acceptable for regulatory purposes. Test media are relevant for the marine environment, covering different salinities that are relevant for the marine environment (20 -35 ppt).

It should be noted, however, that no mesocosm studies with molybdenum is available for the marine environment, therefore creating some uncertainty with regard to the lab-to-field translation of Mo-related effects. However, the lack of such data does not imply that field testing might generate lower NOEC values.

 

Based on all of these arguments – and taking sufficient conservatism into account - an Assessment Factor of 3 on the HC5,50% is put forward for the derivation of an aquatic PNEC, resulting in a PNECmarine of 2.28 mg Mo/L. An assessment factor of 3 is considered sufficient to cover the uncertainty with regard to the lab-to-field translation of Mo-related effects.

 

The marine effects database also contains a number of unbounded NOEC for taxonomic groups and/or species that are not represented in the species sensitivity distribution (the microalga I.galbana, the marine snail N.dorsatus, the barnacle A.amphitrite and the hermit crab C.variabilis (Trenfield et al, 2015,2016; van Dam et al, 2016, 2018). No effects were noted at the highest evaluated exposure concentrations (7.0-9.5 mg Mo/L). These unbounded NOECs are a factor of 3 to 4 higher than the derived PNECmarine of 2.28 mg/L; the latter value is therefore also considered to be sufficiently protective for these four additional species.

 

 

Sediment compartment

According to REACH Guidance (RIP 3.2.2: Guidance on information requirements and chemical safety assessment Part B: Hazard Assessment), the Predicted No Effect Level for sediment (PNECsediment) can be derived in two ways. The first method uses the results of tests with sediment living organisms. When such data are not available, the equilibrium partitioning method (EPM) is applied, using toxicity) for aquatic (pelagic) organisms as starting point.

The estimation of the PNECsediment with equilibrium partitioning method is based on the assumption that the sensitivity of pelagic and sediment living organisms is comparable, but that in sediment the availability of the substance is reduced due to sorption to the (organic matter of the) sediment. This implies the use of partitioning calculations, assuming that equilibrium is obtained. It should be noted that EPM considers only uptake via the water phase. For highly adsorbing chemicals, however, the uptake via other exposure pathways like ingestion or direct contact with sediment becomes more important, depending on the organism used for testing. Uptake via the gut is likely to play an increasingly important role for compounds with a log Kow greater than 5 or with a correspondingly high adsorption or binding behaviour. In such cases, the equilibrium partitioning method can only be used in a modified way. In order to allow for uptake of substances via ingestion of sediment, an additional factor of 10 is applied to the PEC/PNEC ratio for such substances. Based on its physicochemical properties, however, molybdenum cannot be categorized as such a compound, and there is no need to apply this additional safety factor of 10 on a PNEC that is derived with the EPM.

 It should be taken into account that EPM may result both in an overestimation or underestimation of the toxicity to benthic organisms (Di Toro et al. 2005). Therefore, this method can only be used as rough screening for assessing the level of risk to sediment dwelling organisms; based on the outcome of the EPM it is decided whether sediment toxicity tests with benthic organisms are required. If the outcome of the EPM-approach leads to a PEC/PNEC ratio of >1, data improvement is necessary either by refining the exposure assessment or by performing tests with benthic organisms using spiked sediment to support a refined risk assessment for the sediment compartment.

 

Freshwater sediment

For molybdenum, no reliable acute or chronic toxicity data for the freshwater sediment compartment were identified on open literature or in the grey literature. It was therefore decided to derive a EPM-based PNECsediment using the PNECaquatic of 11.9 mg Mo/L as a starting point.

This value, in combination with the KD, sediment of 1,778 L/kg (Log KD: 3.25) that was estimated with the data generated in the FOREGS monitoring campaign, resulted in a PNECsediment of 21.2 g/kg dry wt – based on a PNECaquatic that already includes a safety factor of 3 – is more than three orders of magnitude above EU-country-specific reasonable worst-case baseline levels of molybdenum in freshwater sediments (data from FOREGS- monitoring campaign). EU-country-specific 90th percentiles were situated between 0.50 and 12.4 mg/kg dry wt. A comparable difference was noted when the regional RWC-ambient PECs for molybdenum in freshwater sediment were considered. These values ranged from 2.7 to 28.8 mg Mo/kg dry wt.

Modelled concentration levels (Tier 1, including worst-case default values for missing parameters) in freshwater sediments in the proximity of different Mo-related industry sectors (steels, powder production, catalysts, corrosion inhibitors) are situated between 11.5 and 358 mg Mo/kg dry wt, i. e., two or more orders of magnitude below the calculated PNECsediment.

Liber et al (2011) reported unbounded 10d-NOECs of 3589 ± 2152 mg Mo/kg dw and 3742 ± 2210 mg Mo/kg dw for C. dilutus and H. azteca, respectively. These values confirm the low toxicity of molybdenum for sediment organisms, as theoretically derived with the equilibrium partitioning method.

 

Marine sediment

For molybdenum, no reliable acute or chronic toxicity data for the marine sediment compartment were identified in open literature or in the grey literature. It was therefore decided to derive a EPM-based PNECsediment,marine using the PNECmarine of 2.28 mg Mo/L as a starting point. No specific marine KD for suspended particulate material or sediment had been identified. However, based on the derived RWC-ambient PEC for marine sediment (14.1 mg Mo/kg dry wt) and the RWC-ambient PEC for marine water (13.6 µg Mo/L), and assuming equilibrium between both values, a KD for marine sediment can be determined: with these numbers, a value of 1,037 L/kg was derived. This value is comparable to the typical value of 1,778 L/kg that was previously identified for the freshwater compartment.

The calculated PNECsediment,marine of 2.37 g/kg dry wt – based on a PNECmarine that already includes a safety factor of 3 – is more than two orders of magnitude above EU-country-specific reasonable worst-case baseline levels of molybdenum in marine sediments (difference of a factor of 168).

 

Conclusion

According to Annex X long-term toxicity tests for sediment organisms shall be proposed if the result of the Chemical Safety Assessment (CSA) indicates the need to investigate further the effects of the substance and/or relevant degradation products on sediment organisms. The need to conduct testing may be triggered when PEC/PNEC >1 based on Equilibrium Partitioning Method (EPM); for molybdenum, the PEC << EPM-based PNEC, and therefore no experimental test data are required. Note that the PNEC includes already an assessment factor of 3, and that the difference between PEC and EPM-based PNEC is more than one order of magnitude.

Based on the equilibrium partitioning method (EPM), a freshwater PNECsediment of 21.2 g Mo/kg dry wt is derived (input data: PNECaquatic of 11.9 mg/L; KD of 1778 L/kg). For the marine compartment a PNECsediment,marine of 2.37 g Mo/kg dry wt is derived according to the EPM (input data: PNECaquatic,marine of 2.28 mg Mo/L; KD of 1,037 L/kg).

 

 

 

Soil compartment

The derivation of a realistic PNEC for the soil compartment requires that important concepts such as leaching/ageing and bioavailability processes are taken into account. A more detailed description of these concepts for molybdenum is provided in the Background Document “Environmental effects assessment of molybdenum” 

Comparison of toxicity of Mo in freshly spiked and 11 months aged soils show that long-term equilibration of Mo in soils generally decreases its toxicity to soil organisms. The generated toxicity data, however, did not allow any conclusions on the potential effect of soil properties on the L/A factor. Eighteen-month fixation factors in soil were also determined for molybdenum, and ranged between 1.0 and 2.8 with a median fixation factor (E-value) of 1.4 (valid for both measured and predicted values). A generic L/A factor of 2.0 is proposed for molybdenum. This factor is the 44th and 32nd percentile of the individual L/A values based on EC10 and EC50, respectively. The L/A factor of 2.0 is about equal to the product of the median factor found for chemical fixation of Mo in several soils (factor 1.4) and the median factor for the effects of leaching on the toxicity thresholds for other metals (factor 1.3-1.5). Both these leaching and ageing processes are included in the L/A factor.

 

 

The evaluation of the bioavailability of molybdenum required the analysis of single linear and multivariate regressions between soil toxicity thresholds and various soil properties. The key soil properties that were identified to govern molybdenum toxicity in soils are the pH and the clay content for plants and the clay content for invertebrates and micro-organisms. Multiple linear regressions, using stepwise addition, did not consistently improve the regressions for invertebrates and microbial processes. For plants, multiple regressions with pH and log clay content significantly improved the regressions compared to single linear regressions. Toxicity of Mo to soil organisms generally decreases (i.e. increasing EC50 values) with decreasing pH and increasing clay content, organic matter content and iron oxide content.

Because of the large R2 values and good predictions of EC50 values based on pH and clay content for all 5 plant species, these multiple regression models were selected for normalisation of the plant data. For invertebrates a consistent correlation with the clay contents was observed and slopes of the regression equations were very consistent. It should be noted that only 3 bounded EC50 values were observed for Folsomia candida, and no correlation analysis with soil properties could therefore be performed for this species. It was decided to use the smallest slope observed for the other invertebrate assays (0.72, for Enchytraeus crypticus) for normalization of Folsomia candida EC10 values for soil-specific PNEC derivation (conservative approach).

For the microbial processes studies, only models for substrate induced nitrification and respiration could be developed. Although only 4 bounded EC50 values were available for glucose induced respiration, it was decided that still a regression analysis could be performed because the soils represented a sufficient range in soil properties (pH: 5.2 – 7.3; org C: 0.9 – 2.8%; clay: 2 – 12%; CEC: 4.1 – 14.3 cmolc/kg; Feox: 1-2.2 g/kg). For both processes, a consistent correlation with the clay content was observed. Only 1 bounded EC50 value was observed for the plant residue mineralisation (PRM) assay and therefore, no correlation analysis with soil properties could be performed. It is proposed to use the slope for the substrate induced respiration assay for normalization of EC10 values for PRM for soil-specific PNEC derivation because both assays are related to carbon mineralisation process. Moreover this slope (0.73) is the smallest observed for microbial endpoints and therefore this is a conservative approach.

 

The size of available the ecotoxicity database for the effect of Mo to soil organisms is sufficiently large in order to justify the use of the statistical extrapolation method is – as specified by the Guidance document on information requirements and chemical safety assessment Chapter R.10.3.1.3– preferred for PNEC derivation. The derived PNEC is based on the 50% confidence value of the 5th percentile value of the effect NOEC/EC10 data (HC5,50%) and an additional assessment factor taking into account the uncertainty on the HC5,50% (thus PNEC = HC5,50%/AF).

 

Generic, non-normalised HC5,50%

The non-normalised terrestrial HC5,50% was derived based on either all 86 individual reliable NOEC/EC10 values or the 11 species mean NOEC/EC10 values for the most sensitive endpoint. Two different approaches were used. The first approach did not consider either the normalisation or the generic L/A factor of 2. The second approach did not consider normalisation but included the application of a generic L/A factor of 2.

Using statistical extrapolation and the best fitting distribution results in a HC5,50% of 8.8 mg/kg (8.4-9.2 mg Mo/kg) based on all NOEC/EC10 data without consideration of both the normalisation and the application of a generic L/A factor of 2. Taking into account all NOEC/EC10 data and a generic L/A factor of 2, still without consideration of the normalisation results in a HC5,50% of 16.6 mg Mo/kg (15.8-17.5 mg/kg). Based on the Anderson-Darling goodness-of-fit statistics, the log-normal distribution was rejected for the distributions based on all individual reliable observations.

 

Normalised HC5,50% for soils: implementation of bioavailability

Normalisation of the individual NOEC or EC10 data towards specific soil properties reduces the within species-variation in NOEC or EC10 values. Taking into account the effect of soil properties allows one to normalise all individual EC10 or NOEC values and to calculate soil-specific HC5 values. The normalisation approach is applied for six reference soil scenarios for metal risk assessment (Ecoregions) that represent typical European soil conditions. These include agricultural and natural soils that exhibit wide ranges of textures (sandy, loamy, clay and peaty soils), pH (3.0 and 7.5), clay content (7 and 46 %), and eCEC (2.4 and 36 cmolc/kg dwt).

The HC5-values for these reference soil scenarios vary between 12.2 and 271.1 mg Mo/kg dw.

 

Uncertainty analysis

The use of the statistical extrapolation method is –as specified by the Guidance document on information requirements and chemical safety assessment Chapter R.10.3.1.3– preferred for PNEC derivation rather than the use of an assessment factor on the lowest NOEC. Based on uncertainty considerations the London Workshop recommended to apply an additional assessment factor on the 50% confidence value of the 5th percentile value (thus PNEC = 5th percentile value/AF), with an AF between 1 and 5, to be judged on a case-by-case basis. The following points were considered when determining the size of the assessment factor:

1) The overall quality of the database and the end-points covered, e.g., if all the data are generated from “true” chronic studies

2) The diversity and representativeness of the taxonomic groups covered by the database

3) Statistical uncertainties around the 5th percentile estimate, e.g., reflected in the goodness-of-fit or the size of confidence interval around the 5th percentile

4) Evaluation of NOEC values below the HC5,50%

5) Comparisons between field/microcosm studies and the 5th percentile to evaluate the laboratory to field extrapolation.

 

 

The Mo-database covered ecologically relevant endpoints (yield based on root elongation and shoot yield for the terrestrial plants; reproduction for the invertebrates; N- and C-transformation for microbial processes), and the reliable EC10/NOEC data were extracted from tests performed in natural and artificial soils, covering a wide range of the soil characteristics in Europe. Data are either from tests focusing on sensitive life stages (e.g. root elongation, reproduction) or from “chronic exposure” (e.g. growth, reproduction). The overall quality of the database was therefore considered to be optimal. The Mo-database also largely fulfils the requirement of 10-15 different EC10/NOEC and covers sufficient different species and/or processes: the database is composed of plant, invertebrate and microbial data and all major taxonomic groups are included. 

Different distributions have been evaluated for different soil types or scenarios. Both statistical (e.g. Kolmogorov-Smirnov, Andersen-Darling tests) and visual (e.g. Q-Q plots) goodness-of-fit techniques were used in order to select the most appropriate distribution function for the compiled chronic data set. The final distribution function was selected on the basis of the Anderson-Darling goodness-of-fit test as this test highlights differences between the tail of the distribution (lower tail is the region of interest) and the input data. The log-normal distribution has been selected for derivation of the HC5,50%. A comparison of the normalized HC5,50% values with the species/process mean normalized NOEC/EC10 values for several EU soil scenarios shows that generally no species/process mean NOEC/EC10 values fall below the HC5,50% derived by either the log-normal or best fitting distribution. Only for acidic soils (pH < 4.5), the normalized NOEC for Eisenia andrei reproduction can be slightly lower than the HC5,50% according to the log-normal distribution. Finally, the available field studies indicate the HC5,50% to be protective under field conditions.

Based on the above uncertainty analysis, and in particular the availability of normalisation models and field validation, it was concluded that the available database and models allow for the derivation of an HC5,50% that is protective for the terrestrial environment. The application of an AF = 1 is therefore proposed on HC5,50% values that are derived with the statistical extrapolation method. PNEC values for the generic soil types with physico-chemical properties within the 10th-90th percentile of EU soil conditions range between 4.9 and 353 mg Mo/kg (statistical extrapolation method with the log-normal distribution).

 

Derivation of a reasonable worst-case generic PNEC, based on the GEMAS dataset

The information of the GEMAS data set which contains the physicochemical characterisation of more than 4000 soil samples (arable and grazing ) allowed the derivation of a site-specific PNEC for each sample, and with these data a represenatative distribution of PNEC values in EU soils can be constructed.

All the individual data were pooled for grassland and arable soil samples, and a 10th percentile was derived for the two soil types. The values of 10.5 and 9.9 mg Mo/kg dw were determined for arable and grassland, respectively. These values provide robust and ecological relevant PNECs to be retained for risk characterisation purposes. The lowest value is considered as a relevant PNECsoil for molybdenum.

Conclusion on classification

As the lowest short-term and long-term ecotoxicity reference value is higher than 1 mg Mo/L, environmental classification is not required.