Chromium trioxide

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to soil microorganisms

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

See Read-across statement in section 13 for justification.

Reason / purpose for cross-reference:

read-across source

Dose descriptor:

EC50

Remarks:

extrapolation, see below

Effect conc.:

5.9 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

not specified

Basis for effect:

other: see table below

Remarks on result:

other: see table below

Details on results:

The test results ranged from 1.0 mg/kg dw to 3,332 mg/kg dw (both values being for arylsulphatase).

For this risk assessment, data were selected from this survey, taking values where a NOEC was obtained directly or where the LOEC related to an effect level of 20% or less (and using LOEC/2 as the NOEC). A total of 37 values were obtained, and a further selection was made giving preference to longer exposure times in the same studies, resulting in a final data set of 30 values. The statistical extrapolation method has been used to derive an HC5-50% value of 5.9 mg/kg.

The values used in this risk assessment were selected from those presented in Table 4.4 of Appendix IV in the Crommentuijn review, applying the following criteria. Values for the NOEC or EC10 which were reported directly were used as NOEC values. Where an EC value for an effect between 10 and 20% was reported, a NOEC of half the EC value was taken. Effect levels greater than 20% were not used. Where results from different exposure periods were reported for the same study, the result from the longest available exposure matching the above criteria was taken. In one case, a NOEC and an EC10 value were presented for the same study and duration; in this case the geometric mean of the two values was used. The basic data are presented in Table VII.1. This includes the original values where the effect was between 10% and 20% (ie before division by two), and the values for different durations.

Toxicity of chromium (III) to soil processes (after Crommentuijn et al, 1997)

Process	Soil type	pH	% OM	%clay	Temp °C	Exposure	Endpoint	Result (mg/kg dw)	Reference
Arylsulphatase	sand	7.7	1.6	2	20	18 m	EC10	2.1	Hanstra and Doelman, 1991
	Sandy loam	5.1	5.7	9	20	6 w	EC10	(46)
						18 m	EC10	1.0
	Silty loam	7.4	2.4	19	20	18 m	EC10	83
	clay	6.8	3.2	60	20	6 w	EC10	(43)
						18 m	EC10	276
	Sandy peat	3.0	12.8	5	20	6 w	EC10	(3338)
						18 m	EC10	2730
Nitrification*		7.2	2	17	30	21 d	NOEC	100	Denneman and Van Gestel, 1990
N-mineralisation		5.8	4.4	23	30	20 d	EC20	260^a
		6.6	5	45	30	20 d	EC15	260^a
		7.8	6.4	30	30	20 d	EC13	260^a
Phosphatase (acid)	Webster	5.8	4.4	23	37	1.5 h	NOEC	130	Juma and Tabatabai, 1977
Phosphatase (alkaline)	Okoboji	7.4	9.3	34	37	1.5 h	EC14	130^a	Doelman and Haanstra, 1989
Phosphatase	sand	7.7	1.6	2	20	6 w	EC10	(1092)
						18 m	EC10	723
	Sandy loam	6	5.7	9	20	6 w	EC10	(2782)
						18 m	EC10	858
	Silty loam	7.4	2.4	19	20	6 w	EC10	(728)
						18 m	EC10	280
	clay	7.5	3.2	60	20	6 w	EC10	(52)
						18 m	EC10	2153
	Sandy peat	4.4	12.8	5	20	6 w	EC10	380
Respiration	Sandy loam	5.1	5.7	9	20	8 w	EC10	(5)	Denneman and Van Gestel, 1990
						10 m	NOEC	(148)
						10 w	EC10	(7)
						43 w	EC10	6
	Silty loam	7.4	2.6	19	20	21 m	EC10	86^b
						21 m	NOEC	182^b
	Sandy peat	4.3	12.8	5	20	19 m	EC10	71
	clay	6.8	3.2	60	20	19 m	NOEC	400
Urease	sand	7.7	1.6	2	20	6 w	EC10	(1880)	Doelman and Haanstra, 1989
						18 m	EC10	390
	Silty loam	7.4	2.4	19	20	6 w	EC10	(2050)
						18 m	EC10	890
	Clay	7.5	3.2	60	20	18 m	EC10	350
	Sandy peat	4.4	12.8	5	20	6 w	EC10	360
	Harps	7.8	6.4	30	37	2 h	NOEC	26	Denneman and Van Gestel, 1990
	Luton	6.8	7.4	30	37	2 h	EC17	260^a
	Okoboji	7.4	9.3	34	37	2 h	EC19	26^a

Notes: All results basedon measured concentrations and on added amount of chromium. All involved addition of chromium (III) chloride except for * where chromium (III) sulphate was used.
( ): values in parentheses not used in the statistical extrapolation since a value from a longer exposure is available.
^a: NOEC determined as ECx/2 as x=20 for use in the extrapolation.
^b: geometric mean of the two values used in the extrapolation.

Validity criteria fulfilled:

not specified

Executive summary:

In the environment, it is likely that chromium (VI) will be reduced to chromium (III) in soil, and it is also likely that such conversion would have taken place in many of the toxicity tests. Crommentuijn et al. (1997) reviewed the toxicity of chromium (III) to soil processes. The results of 51 determinations were reported, covering arylsulphatase, nitrification, N-mineralisation, phosphatase, respiration and urease. The test results ranged from 1.0 mg/kg dw to 3,332 mg/kg dw (both values being for arylsulphatase). All studies used soluble chromium (III) compounds, largely chromic (III) chloride. For this risk assessment, data were selected from this survey, taking values where a NOEC was obtained directly or where the LOEC related to an effect level of 20% or less (and using LOEC/2 as the NOEC). A total of 37 values were obtained, and a further selection was made giving preference to longer exposure times in the same studies, resulting in a final data set of 30 values. The statistical extrapolation method has been used to derive an HC5-50% value of 5.9 mg/kg

Reference 2

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

supporting study

Study period:

Studies were performed over the period 1981 to 1997

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

The EU RAR summaries the results from three studies on Cr(VI) substances and a review of Cr(III) data. Each study may have minor limitations, nevertheless, collectively these studies provide an adequate assessment of effects on microbial-mediated soil processes.

Qualifier:

no guideline followed

Principles of method if other than guideline:

Several studies in the review, providing a weight-of-evidence to address this endpoint.

GLP compliance:

no

Analytical monitoring:

yes

Details on sampling:

None provided

Vehicle:

not specified

Details on preparation and application of test substrate:

None provided

Test organisms (inoculum):

other: native microbial communities in soil

Remarks:

Range of exposure periods from several studies (see results section)

Test temperature:

Range (see results section)

Moisture:

Not reported in the summaries

Details on test conditions:

See results section

Nominal and measured concentrations:

See results section

Reference substance (positive control):

no

Details on results:

Ross et al. (1981) investigated the effects of both chromium (VI) (as potassium dichromate) and chromium (III) (as chromic chloride) on the microbial activity of soil. Two soils were used, a loam (pH 6.4) and a fine sandy loam (pH 5.9). In the experiments, 1 kg dry weight of soil and either chromium (VI) (concentration 10 or 100 mg Cr (VI)/kg dry soil) or chromium (III) (concentration 100 mg Cr (III)/kg dry soil) was added. The soils were wetted to give water contents of 25% (loam) or 20% (sandy loam), and then the soils were incubated for 22 days in the dark at 25°C. At intervals, the amount of CO2 evolved from the soils was determined, along with the amount of chromium (VI) present in the soil. Microbial activity in the control experiments was found to be higher than all chromium-treated soil. The differences between the three chromium treatments in the amount of CO2 evolved/day were small. Chromium (VI) was found to be rapidly reduced to chromium (III) in the experiments (about 75% of the chromium (VI) added in the 100 mg/kg treatment was not extractable after 3 days incubation). A small amount of chromium (VI) was also found in the chromium (III) experiments after 3 days in the loam soil, but this had disappeared by 13 days.

Ueda et al. (1987) investigated the effects of chromium (VI) (as sodium chromate) and organic amendments on the composition and activity (as measured by CO2 evolution) of microbial flora in soil. In the study, chromium (VI) was added at concentrations of 10, 20, 50 and 100 mg Cr (VI)/kg dry soil to alluvial soil (total carbon content 1.1%), along with an organic amendment (dried rice straw and/or fresh cow manure), and incubated at 28°C in the dark for 20 days. Separate experiments were also carried out to look at the reduction of chromium (VI) to chromium (III) in the test system. The chromium (VI) added to the soil was found to be rapidly reduced to chromium (III), and the reduction was fastest in the soil amended with rice straw and cow manure.
Only traces of extractable chromium (VI) were found in the soil after 14 days. The chromium (VI), at an initial concentration of 10 mg Cr (VI)/kg dry soil did not suppress the evolution of CO2 from the soil system, whereas a slight suppression in CO2 evolution was seen at a concentration of 20 mg Cr (VI)/kg dry soil, with a marked decrease occurring at 50 and 100 mg Cr (VI)/kg dry soil. When the composition of micro-organisms present in the soil was investigated, it was found that although the total number of micro-organisms present were approximately the same in soil exposed to 100 mg Cr (VI)/kg dry soil as compared with the control soil, the population of fungi had increased and the population of actinomycetes had decreased in the exposed soil compared with the control soil.

The effects of chromium (VI) (as sodium chromate) and chromium (III) (as chromium chloride) on soil nitrification and ammonification have been studied by Ueda et al. (1988). The soil used in the study was a loam with a total carbon content of 1.1% and a pH of 6.2-6.5. Ammonium sulphate was added to the soil at 250 mg/kg to act as the nitrogen source in the nitrification experiments and urea was added to the soil at 250 mg/kg in the ammonification experiments. The chromium compounds were added to the soil at concentrations of 10, 100 and 1,000 mg/kg dry weight (equivalent to 3.2, 32.1 and 321mg Cr (VI)/kg soil) and the soil was incubated in the dark at 28-30°C for 4 weeks. Chromium (VI) was found to inhibit nitrification at all three concentrations. At the lowest concentration, a slight reduction in nitrification was seen over the first 2 weeks exposure, but after this period the soil recovered to control levels. Almost complete inhibition of nitrification occurred at the two highest exposure concentrations, and this persisted
throughout the 4 week experiment at the highest dose. Chromium (III) was found to be much less toxic with partial inhibition of nitrification occurring only at the highest exposure concentration (~330 mg Cr (III)/kg soil). Chromium (VI) was much less toxic to ammonification, with only partial inhibition of ammonification occurring over the first 3 days at the highest concentration tests (321 mg Cr (VI)/kg soil). Overall, the LOEC from this study is around 3.2 mg Cr (VI)/kg soil. A similar transient inhibition of soil nitrification was reported by James and Bartlett (1984) using potassium dichromate at a concentration of 100 µM (~10.4 mg/l) in soil suspensions.

The rapid transformation of chromium (VI) to chromium (III) noted in some of the above studies indicates that data on chromium (III) toxicity to soil processes is probably more relevant to the assessment.

Crommentuijn et al. (1997) reviewed the toxicity of chromium (III) to soil processes. The results of 51 determinations were reported, covering arylsulphatase, nitrification, N-mineralisation, phosphatase, respiration and urease. The test results ranged from 1.0 mg/kg dw to 3,332 mg/kg dw (both values being for arylsulphatase). All studies used soluble chromium (III) compounds, largely chromic (III) chloride. For this risk assessment, data were selected from this survey, taking values where a NOEC was obtained directly or where the LOEC related to an effect level of 20% or less (and using LOEC/2 as the NOEC). A total of 37 values were obtained, and a further selection was made giving preference to longer exposure times in the same studies, resulting in a final data set of 30 values. The statistical extrapolation method has been used to derive an HC5-50% value of 5.9 mg/kg (see table below).

Data on the toxicity of chromium (III) to soil processes have been taken from the review by Crommentuijn et al (1997). The values used in this risk assessment were selected from those presented in Table 4.4 of Appendix IV in the Crommentuijn review, applying the following criteria. Values for the NOEC or EC10 which were reported directly were used as NOEC values. Where an EC value for an effect between 10 and 20% was reported, a NOEC of half the EC value was taken. Effect levels greater than 20% were not used. Where results from different exposure periods were reported for the same study, the result from the longest available exposure matching the above criteria was taken. In one case, a NOEC and an EC10 value were presented for the same study and duration; in this case the geometric mean of the two values was used. The basic data are presented in Table VII.1. This includes the original values where the effect was between 10% and 20% (ie before division by two), and the values for different durations.

The selected values were used to determine a PNEC value for soil processes using the statistical extrapolation method. The log NOEC values were fitted to a normal distribution. The Kolmogorov-Smirnov test did not reject the hypothesis, that the log NOEC values came from a normal distribution. A plot of the observed and expected cumulative frequencies is included as Figure VII.1. The result of the statistical extrapolation calculation is a HC5-50% value of 5.9 mg/kg. For comparison, the HC5-95% value is 2.1 mg/kg.

The data cover a range of processes: arylsulphatase, nitrification, N-mineralisation, phosphatase, respiration and urease. A plot of the distribution of the different processes is presented in Figure VII.2. This shows that there are no processes clearly more sensitive than others. The two lowest values relate to arylsulphatase, but the two highest values are also for the same process, and there are other values for this process in the data set as well. Only the two lowest values are below the HC5 - 50% value derived. In this case it is considered that an assessment factor of 1 is sufficient, hence the PNEC for soil processes with chromium (III) is 5.9 mg/kg.

Validity criteria fulfilled:

yes

Conclusions:

From an EU review of studies to assess the impact of chromium (VI) and (III) on microbially-mediated soil processes a statistical assessment was carried out. A substantial amount of information is available for the toxicity of chromium (VI) to terrestrial organisms. In the environment, it is likely that chromium (VI) will be reduced to chromium (III) in soil, and it is also likely that such conversion would have taken place in many of the toxicity tests.

Executive summary:

From an EU review of studies to assess the impact of chromium (VI) and (III) on microbially-mediated soil processes a statistical assessment was carried out. A substantial amount of information is available for the toxicity of chromium (VI) to terrestrial organisms. In the environment, it is likely that chromium (VI) will be reduced to chromium (III) in soil, and it is also likely that such conversion would have taken place in many of the toxicity tests.

Description of key information

From an EU review of studies to assess the impact of chromium (VI) and (III) on microbially-mediated soil processes a statistical assessment was carried out. A substantial amount of information is available for the toxicity of chromium (VI) to terrestrial organisms. In the environment, it is likely that chromium (VI) will be reduced to chromium (III) in soil, and it is also likely that such conversion would have taken place in many of the toxicity tests.

Key value for chemical safety assessment

Short-term EC50 for soil microorganisms:

5.9 mg/kg soil dw

Additional information

Crommentuijn et al. (1997) reviewed the toxicity of chromium (III) to soil processes. The results of 51 determinations were reported, covering arylsulphatase, nitrification, N-mineralisation, phosphatase, respiration and urease. The test results ranged from 1.0 mg/kg dw to 3,332 mg/kg dw (both values being for arylsulphatase). All studies used soluble chromium (III) compounds, largely chromic (III) chloride. For this risk assessment, data were selected from this survey, taking values where a NOEC was obtained directly or where the LOEC related to an effect level of 20% or less (and using LOEC/2 as the NOEC). A total of 37 values were obtained, and a further selection was made giving preference to longer exposure times in the same studies, resulting in a final data set of 30 values. The statistical extrapolation method has been used to derive an HC5-50% value of 5.9 mg/kg. In this case it is considered that an assessment factor of 1 is sufficient, hence the PNEC for soil processes with chromium (III) is 5.9 mg/kg.