Chromium iron oxide

Sediment toxicity

Administrative data

Endpoint:

sediment toxicity: long-term

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

other:

Justification for type of information:

JUSTIFICATION FOR DATA WAIVING
According to Column 2 of Information Requirement 9.5.1, Annex X, Commission Regulation (EU) 1907/2006 ” Long-term toxicity testing shall be proposed by the registrant if the results of the chemical safety assessment indicates the need to investigate further the effects of the substance and/or relevant degradation products on sediment organisms. The choice of the appropriate test(s) depends on the results of the chemical safety assessment.”

According to Section 8.4.2 of ECHA Guidance on IR & CSA, Part B: Hazard assessment (Version 2.1; ECHA, 2011), “For substances which are classified as harmful, toxic or very toxic to aquatic life (i.e. H412, H411, H410 and H400), an aquatic PNEC can be derived. In these circumstances there are unclassified hazards to the sediment and soil compartments because toxicity to aquatic organisms is used as an indicator of concern for sediment and soil organisms, and a screening risk characterisation is undertaken using the equilibration partitioning method (EPM) to derive PNECs for sediment and soil. Hence quantitative exposure assessment, i.e. derivation of PECs, is mandatory for the water, sediment and soil environmental compartments.

Substances with the only environmental classification as ‘May cause long lasting harmful effects to aquatic life’ (i.e. H413) have been established as persistent in the aquatic environment and potentially bioaccumulative on the basis of test or other data. There are also potential hazards for these substances for the sediment and soil compartments, because these substances are potentially bioaccumulative in all organisms and are also potentially persistent in sediment and soil. Hence exposure assessment is mandatory for the water, sediment and soil environmental compartments, which may be quantitative or qualitative as appropriate. PBT and vPvB substances have been established as persistent and bioaccumulative (and the former also as toxic) in the environment as a whole. Hence qualitative exposure assessment is mandatory for the water, sediment and soil environmental compartments…

If there are ecotoxicity data showing effects in aquatic organisms, but the substance is not classified as dangerous for the aquatic environment, an aquatic PNEC can nevertheless be derived thus indicating a hazard to the aquatic environment. In these circumstances there are also unclassified hazards to the sediment and soil compartments because toxicity to aquatic organisms is used as an indicator of concern for sediment and soil organisms and a screening risk characterisation is undertaken using the equilibration partitioning method (EPM) to derive PNECs for sediment and soil. Hence quantitative exposure assessment, i.e. derivation of PECs, is mandatory for the water, sediment and soil environmental compartments.”

Chromium iron oxide can be considered environmentally and biologically inert due to the characteristics of the synthetic process (calcination at a high temperature of approximately 1000°C), rendering the substance to be of a unique, stable crystalline structure in which all atoms are tightly bound and not prone to dissolution in environmental and physiological media. This assumption is supported by available transformation/dissolution data (Pardo Martinez, 2010) that indicate a very low release of pigment components. Transformation/dissolution of chromium iron oxide (24-screening test according to Oecd Series 29, loading of 100 mg/L, pH 6 and 8) resulted in metal concentrations that are below the respective LODs for iron and chromium (< 0.5 µg/L). Dissolved metal concentrations remained also below the respective LOD after 7 days with 1 mg/L (and also 100 mg/L) and after 28 days with 1 mg/L at pH 6. Thus, the rate and extent to which chromium iron oxide produces soluble (bio)available ionic and other chromium- and iron-bearing species in environmental media is limited. Hence, the pigment can be considered as environmentally and biologically inert during short- and long-term exposure. The poor solubility of chromium iron oxide is expected to determine its behaviour and fate in the environment, and subsequently its potential for ecotoxicity.

Proprietary studies are not available for chromium iron oxide. The poorly soluble substance chromium iron oxide is evaluated by comparing the dissolved metal ion levels resulting from the transformation/dissolution test after 7 and 28 days at a loading rate of 1 mg/L with the lowest acute and chronic ecotoxicity reference values (ERVs) as determined for the (soluble) metal ions. The ERVs are based on the lowest EC50/LC50 or NOEC/EC10 values for algae, invertebrates and fish. Acute and chronic ERVs were obtained from the Metals classification tool (MeClas) database as follows: For trivalent chromium and iron ions, the acute and chronic ERVs are above 1 mg/L, respectively, and a concern for short-term (acute) and long-term (chronic) toxicity was not identified (no classification). According to ECHA Guidance on the Application of the CLP Criteria (Version 5.0, July 2017), “Where the acute ERV for the metal ions of concern is greater than 1 mg/l the metals need not be considered further in the classification scheme for acute hazard.” Further, ” Where the chronic ERV for the metal ions of concern corrected for the molecular weight of the compound (further called as chronic ERV compound) is greater than 1 mg/l, the metal compounds need not to be considered further in the classification scheme for long-term hazard.” Due to the lack of an acute and chronic aquatic hazard potential for soluble trivalent chromium and iron ions and the fact that dissolved chromium and iron concentrations were below the LOD of 0.5 µg/L after 7 and 28 days at pH 6 in the T/D test, respectively, it can be concluded that the substance chromium iron oxide is not sufficiently soluble to cause short- or long-term toxicity at the level of the acute or chronic ERVs (expressed as EC50/LC50 or NOEC/EC10, respectively).

In accordance with Figure IV.4 “Classification strategy for determining acute aquatic hazard for metal compounds” and Figure IV.5 „Classification strategy for determining long-term aquatic hazard for metal compounds “of ECHA Guidance on the Application of the CLP Criteria (Version 5.0, July 2017) and section 4.1.2.10.2. of Regulation (EC) No 1272/2008, the substance chromium iron oxide is poorly soluble and does not meet classification criteria for acute (short-term) and chronic (long-term) aquatic hazard.

Chromium iron oxide is not classified as dangerous for the aquatic environment, an aquatic PNEC cannot be derived, thus not indicating a hazard to the aquatic environment. In these circumstances there are also not unclassified hazards to the sediment compartment because toxicity to aquatic organisms is used as an indicator of concern for sediment organisms and a screening risk characterisation (using the equilibration partitioning method to derive a PNEC for sediment) cannot be undertaken. Thus, chromium iron oxide does not have a “non-classified hazard” potential.

Chromium is naturally abundant in sediments and a significant fraction of the total chromium is associated with the solid phase (e.g., bound to particulate phase, precipitation of insoluble inorganic complexes). In sediment, trivalent chromium will be converted rapidly to insoluble Cr(III)-species and therefore has a low mobility in this compartment. Monitoring data for chromium background concentrations in stream water and sediments are provided by the FOREGS Geochemical Baseline Mapping Programme, which offers high quality, multi-purpose homogeneous environmental geochemical baseline data for Europe. A total of 734 stream sediment samples were processed in the FOREGS-program for the EU-27, UK and Norway to determine representative chromium stream sediment concentrations. Chromium sediment concentrations range from 2.0 to 1,748.0 mg Cr/kg with 5th, 50th and 95th percentiles of 6.0, 21.0 and 63.0 mg Cr/kg, respectively.

The mobility of iron in water is strongly controlled by the redox conditions and the abundance of complexing agents in the aqueous solution. Under most natural conditions in stream water, iron is relatively immobile and precipitates rapidly, mainly due to the very low solubility of iron (III) hydroxide in its various forms. Its solubility is strongly influenced by redox conditions (Salminen et al. 2005). Based on the FOREGS dataset, iron concentrations of sediments range from 0.6 to 192.4 g Fe/kg with 5th, 50th and 95th percentiles of 5.8, 20.1 and 45.3 g Fe/kg, respectively.

Taking into account the high quality and representativeness of the FOREGS data set, the 95th percentiles of 63.0 mg Cr/kg and 45.3 g Fe/kg sediment can be considered as representative background concentration of chromium in European stream sediments.

Regarding essentiality, “Chromium(III) is required by only some microorganisms for specific metabolic processes, such as glucose metabolism and enzyme stimulation. Chromium(III), in trace amounts, has been reported to be an essential component of animal nutrition and is most notably associated with glucose and fat metabolism (WHO, 2009).”

Iron is an essential trace element for living organisms including animals, plants and microorganisms. It serves as a cofactor in a variety of enzymes involved in the electron transport processes of various metabolic pathways, i.e., photosynthesis, respiration, and nitrogen fixation. It is a structural component of the metalloenzyme nitrogenase, which is the key enzyme in the nitrogen fixation process performed by different microorganisms. Due to its involvement in the synthesis and maintenance of chlorophyll, iron is a vital factor for the photosynthetic performance of plants. As an essential component of the haemoglobin of red blood cells, iron functions as a carrier of oxygen in the blood and muscles of animals (e.g. US EPA, 2003; Colombo et al. 2014).

Regarding the potential of bioaccumulation for chromium, the EU RA on chromates (ECB, 2005) concludes that “uptake of chromium (III) directly from water is likely to be very low due to the limited water solubility and strong adsorption to sediment under most conditions found in the environment…Transfer of chromium via the alga -> bivalve, and sediment -> bivalve food chains appears to be relatively low.” A similar conclusion is reached by WHO (2009) in its assessment of inorganic chromium (III) compounds. “Chromium (III) is required by only some microorganisms for specific metabolic processes, such as glucose metabolism and enzyme stimulation. Chromium (III), in trace amounts, has been reported to be an essential component of animal nutrition and is most notably associated with glucose and fat metabolism (WHO, 2009).” For chromium as essential element, it is thus assumed that internal levels are homeostatically regulated and that it does not bioaccumulate and biomagnify in aquatic food-chains. Thus, the potential for bioaccumulation of chromium (III) in aquatic environments is low based on its poor solubility in environmental media.

The uptake of iron as essential element into cells is actively regulated by a strict homeostatic control system. The active regulation of iron uptake in combination with internal detoxification mechanism indicates a low potential for iron bioaccumulation. This assumption is supported by results of Bustamante et al. (2000) indicating that iron concentrations of digestive glands of cephalopods living in natural and in iron-enriched habitats are similar. Winterbourn et al. (2000) further demonstrate that iron does not biomagnify but rather “biodilutes” up the aquatic food chain. Thus, the potential for bioaccumulation of iron in aquatic environments can be expected to be low.

Considering abundance, bioavailability, essentiality and low bioaccumulation, the potential of chromium (III) and iron ions for toxicity to sediment organisms can be expected to be low.

Chromium iron oxide is not classified as harmful, toxic or very toxic to aquatic life or may cause long lasting harmful effects to aquatic life. Chromium iron oxide is also not an unclassified hazard to the aquatic environment. Based on the poor solubility, bioavailability, lack of a potential for bioaccumulation and toxicity to aquatic organisms and considering ubiquitousness and bioavailability of chromium and iron in sediment as well as essentiality and low bioaccumulation of chromium and iron, chromium iron oxide is also not considered an unclassified hazard to the sediment compartment. Results of the chemical safety assessment do not indicate the need to investigate further the effects of chromium iron oxide on sediment organisms. Therefore, the study on the long-term toxicity to sediment organisms does not need to be conducted in accordance with Column 2 of Information Requirement 9.5.1., Annex X, Commission Regulation (EU) 1907/2006.

References:

Bustamante et al. (2000) Bioaccumulation of 12 trace elements in the tissues of the nautilus Nautilus macromphalus from New Caledonia. Marine Pollution Bulletin 40/8: 688-696.

Colombo et al. (2014) Review on iron availability in soil: interaction of Fe minerals, plants, and microbes. Journal of Soils and Sediments 14: 538–548.

ECB (2005) European Union Risk Assessment Report: Chromium trioxide, sodium chromate, sodium dichromate, ammonium dichromate and potassium dichromate. EUR 21508 EN.

US EPA (2003) Ecological Soil Screening Level for Iron, Interim Final, OSWER Directive 9285.7-69.
Salminen et al. (2005) Geochemical Atlas of Europe - Part 1: Background information, Methodology and Maps. EuroGeoSurveys.

WHO (2009) Concise International Chemical Assessment Document 76 (CICAD). Inorganic chromium(III) compounds (evaluation), International Programme of Chemical Safety (IPCS), WHO, Geneva.

Winterbourn et al. (2000) Aluminium and iron burdens of aquatic biota in New Zealand streams contaminated by acid mine drainage: effects of trophic level. The Science of The Total Environment 254, 45-54.

Cross-reference

Cross-reference 1

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

monitoring data

Type of information:

other: report

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Qualifier:

no guideline required

Principles of method if other than guideline:

Evaluation and summary of high quality environmental geochemical data for Europe, which is provided by the Forum of European Geological Surveys (FOREGS) and the European Geochemical Mapping of Agricultural and Grazing Land Soil (GEMAS), with respect to iron concentrations in stream water, stream sediment and topsoil, as well as in agricultural soil and grazing land.

GLP compliance:

no

Type of measurement:

other: Geochemical background and ambient iron concentrations in different environmental compartments across Europe

Media:

other: Natural stream water, stream sediment and topsoil, as well as agricultural and grazing land soils

Details on sampling:

FOREGS and GEMAS data for EU-27 countries plus UK and Norway were considered, i.e. data from non-EEA countries such as Albania, Bosnia and Switzerland were excluded from further analysis.

FOREGS:
- The FOREGS sampling grid was based on GTN grid cells developed for Global Geochemical Baseline mapping. This grid divides the entire land surface into 160 km x 160 km cells covering an area of 4,500,000 km2.
- Sampling methodology, preparation and analysis are described by Salminen et al. (2005).
- FOREGS data for EU-27 countries plus UK and Norway were considered, i.e. data from non-EEA countries such as Albania and Switzerland were excluded from further analysis.
- A total of 795 stream water samples and 832 sediment samples were processed in the FOREGS-program, including 737 paired samples, i.e. samples with the same coordinates for the sampling location of stream water and sediment.
- The FOREGS dataset reports iron concentrations for 825 topsoil samples. A topsoil sample was taken at each site from 0-25 cm (excluding material from the organic layer where present).
- High quality and consistency of the obtained data were ensured by using standardised sampling methods and by treating and analysing all samples in the same laboratory of each country.

GEMAS:
- Samples from 33 out of 38 European countries were analysed to develop a suitable harmonised geochemical data base for soils. The sampling started in the spring 2008 and the first four months of 2009.
- The whole GEMAS project area of 5,600,000 km2 was divided into a grid with 50 km x 50 km cells.
- To generate harmonised data sets, all project samples were processed by a central sample preparation facility in Slovakia.
- GEMAS data for EU-27 countries plus UK and Norway were considered, i.e. data from non-EEA countries such as Bosnia and Switzerland were excluded from further analysis.
- The GEMAS dataset reports iron concentrations of 1,867 samples from the regularly ploughed layer (Ap-horizon) of agricultural land (arable land; 0-20 cm) and of 1,781 samples from the top layer of grazing land (soil under permanent grass cover; 0-10 cm) sampled on a grid across Europe.

FOREGS DATABASE STREAM WATER/SEDIMENT:

- Sampled stream water and sediments cover a wide range of environmental conditions. Water parameters such as pH, hardness and organic carbon concentrations extend over several magnitudes. Iron water levels range from < 1.0 (< LOQ) to 4,820.0 µg/L with 5^th, 50^th and 95^th percentiles of 4.1, 65.4 and 1,120.0 µg/L, respectively.

- In the sediment, iron concentrations range from 0.6 to192.4 g/kg with 5^th, 50^th and 95^th percentiles of 5.8, 20.1 and 45.3 g/kg, respectively (Table 1).

- Taking into account the high quality and representativeness of the data set, the 95^th percentile of 1,120.0 µg/L can be regarded as representative background concentration for dissolved iron in European surface waters and the 95^th percentile of 45.3 g/kg as representative background concentration of iron in European stream sediments.

- Regarding the partitioning of iron in the water column, stream water/sediment partition coefficients range from 1,413 to 294,000,000 L/kg. Since FOREGS sampled on a grid aiming to equally represent geochemical baseline concentrations across Europe, a European median log K_p value of 5.48 is derived.

Table 1: Water parameters and iron concentrations of stream sediment and stream water and respective partitioning.

	Parameter	#	Unit	Min.	Max.	5^thP	50^thP	95^thP
water	pH ^1	729 ^2	-	9.80	4.50	8.50	7.70	6.10
water	Ca	737	mg/L	0.23	592.00	1.61	42.44	143.52
water	Cl	737	mg/L	0.14	4,560.00	0.49	8.97	68.37
water	HCO3	735 ^3	mg/L	0.69	1,804.42	5.35	128.02	371.20
water	K	737	mg/L	< 0.01	182.00	0.14	1.62	9.81
water	Mg	737	mg/L	0.05	230.00	0.46	6.15	38.11
water	Na	737	mg/L	0.23	4,030.00	1.00	6.66	48.28
water	NO3	737	mg/L	< 0.04	107.00	< 0.04	3.09	39.91
water	DOC	735 ^4	mg/L	< 0.50	57.94	0.60	4.80	23.10
water	SO4^2-	737	mg/L	< 0.30	2,420.00	1.18	16.80	164.01
water	Fe	737	µg/L	< 1.00	4,820.00	4.10	65.40	1,120.00
sediment	Fe	737	g/kg	0.60	192.40	5.78	20.10	45.28
Partitioning (Kp)	Fe (sed/water)	737	L/kg	1,413	294,000,000	14,317	303,282	6,805,391
Log Kp	Fe (sed/water)	737	-	3.15	8.47	4.16	5.48	6.83

¹ Statistics are based on H⁺ concentrations rather than pH

² Removal of 2 outliers < pH 4.3 and 6 negative values.

³ Removal of 2 outliers < 0.01.

⁴ Removal of 1 outlier > 70 mg/L and 1 negative values.

FOREGS DATABASE Background soil concentrations

- Sampled soils cover a wide range of environmental conditions. Soil parameters, including pH and TOC, cover several magnitudes.

- Baseline iron levels in topsoil range from 0.7 to 152.4 g/kg with5^th, 50^th and 95^th percentiles of 3.5, 19.6 and 44.3 g/kg, respectively (see Table 2).

- Taking into account the high quality and representativeness of the data set,the 95^th percentile of 44.3 g/kg can be regarded as representative background concentration of iron in topsoil of EU countries.

Table 2: Concentrations of iron in topsoil samples.

Parameter	Unit	#	Min.	Max.	5^thP	50^thP	95^thP
pH ^1	-	798	7.55	3.38	7.31	5.49	4.29
TOC	%	805	0.07	46.61	0.55	1.72	5.87
Fe	g/kg	825	0.70	152.40	3.52	19.60	44.30

¹ Statistics are based on H⁺ concentrations rather than pH.

GEMAS DATABASE AGRICULTURAL AND GRAZING LAND SOIL CONCENTRATIONS:

- Iron levels of agricultural soil range from 404.5 to 133,926.1 mg/kg with 5^th, 50^th and 95^th percentiles of 3,223.4, 17,165.1 and 36,998.8 mg/kg, respectively (see Table 3). In grazing land, soil concentrations of iron range from 510.3 to 94,759.1 mg/kg with 5^th, 50^th and 95^th percentiles of 3,448.0, 16,949.0 and 38,345.0 mg/kg, respectively (see Table 4).

Table 3: Agricultural soil concentrations.

Parameter	Unit	Method	#	Min.	Max.	5^thP	50^thP	95^thP
CEC	meq/100g	AAS	1,867	1.80	48.30	6.10	15.80	33.30
pH (CaCl2)	pH	pH-meter	1,867	3.32	7.98	4.14	5.71	7.45
TOC	%	IR	1,854	0.40	46.00	0.70	1.70	5.67
Iron	mg/kg	AR	1,867	404.50	133,926.10	3,223.35	17,165.05	36,998.83
Iron	mg/kg	XRF	1,867	909.00	154,715.00	5,595.00	24,620.00	48,499.00
Iron	mg/kg	MMI	1,867	0.50	2,200.00	7.00	35.00	249.00

Table 4: Grazing land soil concentrations.

Parameter	Unit	Method	#	Min.	Max.	5^thP	50^thP	95^thP
CEC	meq/100g	AAS	1,781	2.54	49.88	8.27	17.96	37.74
pH (CaCl2)	pH	pH-meter	1,780	3.26	8.06	4.03	5.38	7.45
TOC	%	IR	1,780	0.41	49.00	0.94	2.80	11.05
Iron	mg/kg	AR	1,781	510.31	94,759.13	3,447.99	16,948.95	38,344.67
Iron	mg/kg	XRF	1,781	979.00	114,422.00	5,665.00	24,129.00	49,727.00

Conclusions:

Representative background or ambient concentrations of iron in environmental compartments are tabulated below:

compartment, unit, concentration (50th P), concentration (95th P)
background stream water, µg/L Fe, 65.4, 1,120.0
background stream water sediment, g/kg Fe, 20.1, 45.3
background topsoil, g/kg Fe, 19.6, 44.3
agricultural soil, mg/kg Fe, 17,165.1, 36,998.8
grazing land soil, mg/kg Fe, 16,949.0, 38,344.7

Based on the FOREGS dataset, the 95th percentile of 1,120.0 µg/L can be regarded as representative background concentration for dissolved iron in European surface waters and the 95th percentile of 45.3 g/kg as representative background concentration of European stream sediments. Regarding the respective partitioning between sediment and water, a European median log Kp value of 5.48 is derived.

Based on the FOREGS dataset, the 95th percentile of 44.3 g/kg can be regarded as representative background concentration of iron in topsoil of EU countries. Representative iron concentrations (95th percentile) of agricultural and grazing land soil (i.e. ambient levels) amount to 36,998.8 and 38,344.7 mg/kg, respectively, according to the GEMAS dataset.

Cross-reference 2

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

monitoring data

Type of information:

other: report

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Qualifier:

no guideline required

Principles of method if other than guideline:

Evaluation and summary of high quality environmental geochemical data for Europe, which is provided by the Forum of European Geological Surveys (FOREGS) and the European Geochemical Mapping of Agricultural and Grazing Land Soil (GEMAS), with respect to chromium concentrations in stream water, stream sediment and topsoil, as well as in agricultural soil and grazing land.

GLP compliance:

no

Type of measurement:

other: Geochemical background and ambient chromium concentrations in different environmental compartments across Europe

Media:

other: Natural stream water, stream sediment and topsoil, as well as agricultural and grazing land soils

Details on sampling:

FOREGS and GEMAS data for EU-27 countries plus UK and Norway were considered, i.e. data from non-EEA countries such as Albania, Bosnia and Switzerland were excluded from further analysis.

FOREGS:
- The FOREGS sampling grid was based on GTN grid cells developed for Global Geochemical Baseline mapping. This grid divides the entire land surface into 160 km x 160 km cells covering an area of 4,500,000 km2.
- Sampling methodology, preparation and analysis are described by Salminen et al. (2005).
- FOREGS data for EU-27 countries plus UK and Norway were considered, i.e. data from non-EEA countries such as Albania and Switzerland were excluded from further analysis.
- A total of 795 stream water samples and 832 sediment samples were processed in the FOREGS-program, including 734 paired samples, i.e. samples with the same coordinates for the sampling location of stream water and sediment.
- The FOREGS dataset reports chromium concentrations for 825 topsoil samples. A topsoil sample was taken at each site from 0-25 cm (excluding material from the organic layer where present).
- High quality and consistency of the obtained data were ensured by using standardised sampling methods and by treating and analysing all samples in the same laboratory of each country.

GEMAS:
- Samples from 33 out of 38 European countries were analysed to develop a suitable harmonised geochemical data base for soils. The sampling started in the spring 2008 and the first four months of 2009.
- The whole GEMAS project area of 5,600,000 km2 was divided into a grid with 50 km x 50 km cells.
- To generate harmonised data sets, all project samples were processed by a central sample preparation facility in Slovakia.
- GEMAS data for EU-27 countries plus UK and Norway were considered, i.e. data from non-EEA countries such as Bosnia and Switzerland were excluded from further analysis.
- The GEMAS dataset reports chromium concentrations of 1,867 samples from the regularly ploughed layer (Ap-horizon) of agricultural land (arable land; 0-20 cm) and of 1,781 samples from the top layer of grazing land (soil under permanent grass cover; 0-10 cm) sampled on a grid across Europe.

FOREGS DATABASE STREAM WATER/SEDIMENT:

- Sampled stream water and sediments cover a wide range of environmental conditions. Water parameters such as pH, hardness and organic carbon concentrations extend over several magnitudes. Chromium water levels range from < 0.01 (< LOQ) to 43.0 µg/L with 5^th, 50^th and 95^th percentiles of 0.06, 0.4 and 1.8 µg/L, respectively.

- In the sediment, chromium concentrations range from 2.0 to 1,748.0 mg/kg with 5^th, 50^th and 95^th percentiles of 6.0, 21.0 and 63.0 mg/kg, respectively (Table 1).

-Taking into account the high quality and representativeness of the data set, the 95^th percentile of 1.8 µg/L can be regarded as representative background concentration for dissolved chromium in European surface waters and the 95^th percentile of 63.0 mg/kg as representative background concentration of chromium in European stream sediments.

-Regarding the partitioning of chromium in the water column, stream water/sediment partition coefficients range from 1,080 to 4,800,000 L/kg. Since FOREGS sampled on a grid aiming to equally represent geochemical baseline concentrations across Europe,a European median log Kp value of 4.75 is derived.

Table 1: Water parameters and chromium concentrations of stream sediment and stream water and respective partitioning.

	Parameter	#	Unit	Min.	Max.	5^thP	50^thP	95^thP
water	pH ^1	726 ^2	-	9.80	4.50	8.50	7.70	6.10
water	Ca	734	mg/L	0.23	592.00	1.60	42.28	144.21
water	Cl	734	mg/L	0.14	4,560.00	0.49	8.95	69.08
water	HCO3	732^3	mg/L	0.69	1,804.42	5.34	128.01	370.00
water	K	734	mg/L	< 0.01	182.00	0.14	1.62	9.80
water	Mg	734	mg/L	0.05	230.00	0.46	6.15	36.81
water	Na	734	mg/L	0.23	4,030.00	1.00	6.66	48.30
water	NO3	734	mg/L	< 0.04	107.00	< 0.04	3.05	39.91
water	DOC	732 ^4	mg/L	< 0.50	57.94	0.60	4.84	23.11
water	SO4^2-	734	mg/L	< 0.30	2,420.00	1.18	16.79	164.56
water	Cr	734	µg/L	< 0.01	43.03	0.06	0.38	1.81
sediment	Cr	734	mg/kg	2.00	1,748.00	6.00	21.00	63.00
Partitioning (Kp)	Cr (sed/water)	734	L/kg	1,080	4,800,000	8,570	56,119	390,841
Log Kp	Cr (sed/water)	734	-	3.03	6.68	3.93	4.75	5.59

¹ Statistics are based on H⁺ concentrations rather than pH.

² Removal of 2 outliers < pH 4.3 and 6 negative values.

³ Removal of 2 outliers < 0.01.

⁴ Removal of 1 outlier > 70 mg/L and 1 negative values.

FOREGS DATABASE Background soil concentrations

- Sampled soils cover a wide range of environmental conditions. Soil parameters, including pH and TOC, cover several magnitudes.

- Baseline chromium levels in topsoil range from 1.0 to 2,340.0 mg/kg with5^th, 50^th and 95^th percentiles of 5.0, 22.0 and 69.8 mg/kg, respectively (see Table 2).

- Taking into account the high quality and representativeness of the data set, the 95^th percentile of 69.8 mg/kg can be regarded as representative background concentration of chromium in topsoil of EU countries.

Table 2: Concentrations of chromium in topsoil samples.

Parameter	Unit	#	Min.	Max.	5^thP	50^thP	95^thP
pH ^1	-	798	7.55	3.38	7.31	5.49	4.29
TOC	%	805	0.07	46.61	0.55	1.72	5.87
Cr	mg/kg	825	1.00	2,340.00	5.00	22.00	69.80

¹ Statistics are based on H⁺ concentrations rather than pH.

GEMAS DATABASE AGRICULTURAL AND GRAZING LAND SOIL CONCENTRATIONS:

- Chromium levels of agricultural soil range from 0.40 to 696.02 mg/kg with 5^th, 50^th and 95^th percentiles of 4.2, 19.9 and 67.2 mg/kg, respectively (see Table 3). In grazing land, soil concentrations of chromium range from 0.8 to 576.7 mg/kg with 5^th, 50^th and 95^th percentiles of 3.9, 19.8 and 67.8 mg/kg, respectively (see Table 4).

Table 3: Agricultural soil concentrations.

Parameter	Unit	Method	#	Min.	Max.	5^thP	50^thP	95^thP
CEC	meq/100g	AAS	1,867	1.80	48.30	6.10	15.80	33.30
pH (CaCl2)	pH	pH-meter	1,867	3.32	7.98	4.14	5.71	7.45
TOC	%	IR	1,854	0.40	46.00	0.70	1.70	5.67
Chromium	mg/kg	AR	1,867	0.40	696.02	4.16	19.87	67.16
Chromium	mg/kg	XRF	1,867	2.00	6,133.00	14.00	58.00	191.00
Chromium	mg/kg	MMI	1,867	< 0.001	1.30	0.01	0.07	0.26

Table 4: Grazing land soil concentrations.

Parameter	Unit	Method	#	Min.	Max.	5^thP	50^thP	95^thP
CEC	meq/100g	AAS	1,781	2.54	49.88	8.27	17.96	37.74
pH (CaCl2)	pH	pH-meter	1,780	3.26	8.06	4.03	5.38	7.45
TOC	%	IR	1,780	0.41	49.00	0.94	2.80	11.05
Chromium	mg/kg	AR	1,781	0.78	576.74	3.87	19.75	67.82
Chromium	mg/kg	XRF	1,781	2.00	6,594.00	13.00	59.00	187.00

Conclusions:

Representative background or ambient concentrations of chromium in environmental compartments are tabulated below:

compartment, unit, concentration (50th P), concentration (95th P)
background stream water, µg/L Cr, 0.4, 1.8
background stream water sediment, mg/kg Cr, 21.0, 63.0
background topsoil, mg/kg Cr, 22.0, 69.8
agricultural soil, mg/kg Cr, 19.9, 67.2
grazing land soil, mg/kg Cr, 19.8, 67.8

Based on the FOREGS dataset, the 95th percentile of 1.8 µg/L can be regarded as representative background concentration for dissolved chromium in European surface waters and the 95th percentile of 63.0 mg/kg as representative background concentration of European stream sediments. Regarding the respective partitioning between sediment and water, a European median log Kp value of 4.75 is derived.

Based on the FOREGS dataset, the 95th percentile of 69.8 mg/kg can be regarded as representative background concentration of chromium in topsoil of EU countries. Representative chromium concentrations (95th percentile) of agricultural and grazing land soil (i.e. ambient levels) amount to 67.2 and 67.8 mg/kg, respectively, according to the GEMAS dataset.