Sodium dichromate

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

other:

Remarks:

Non-guideline study similar to OECD 217 (Soil Microorganisms: Carbon Transformation Test), however with insufficient documentation and methodological shortcomings, i.e. no statistical analysis performed, insufficient characterization of the soils used (pH, organic carbon content, heavy metal content), no data on test temperature, no presentation of raw data. Only two Cr(VI) concentrations are tested.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 217 (Soil Microorganisms: Carbon Transformation Test)

Deviations:

yes

Remarks:

See "Remarks" (Rationale for reliability)

Principles of method if other than guideline:

The study investigated the effects of heavy metals including Cr(VI) on glucose-induced soil respiration in laboratory soil microcosm experiments.

GLP compliance:

not specified

Analytical monitoring:

not specified

Details on sampling:

Soil microcosms were destructively sampled on days 0, 4, 7, 10, 13, 16, 20, 25 and 31 for total CO2 and total biomass. 5 mL of headspace gas was removed from each microcosm using a syringe with subsequent injection into a gas chromatograph (HP series II 5890) for CO2 measurements.

Details on preparation and application of test substrate:

100 mL serum bottles containing 12 g soil d.w.. As a carbon source, 3 g/kg soil of either xylene or glucose were chosen. Glucose was added to the microcosms as sterilized solution. Subsequently, soils were mixed manually.
Concentrations of heavy metals were chosen to reduce the substrate-induced carbon mineralization by 50% and 90%, i.e. 0.4 and 4 mg/g of Cr(VI), respectively, as determined in previous experiments (no raw data presented)

Test organisms (inoculum):

soil

Total exposure duration:

31 d

Test temperature:

no data

Moisture:

60% of water holding capacity

Details on test conditions:

final moisture concentration of each microcosm was adjusted to a 60% water-holding capacity

Nominal and measured concentrations:

400 and 4000 mg/kg of Cr(VI) in experiments with glucose as a substrate.

Reference substance (positive control):

not specified

Duration:

28 d

Dose descriptor:

EC50

Effect conc.:

400 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

Cr(VI)

Basis for effect:

respiration rate

Remarks on result:

other:

Remarks:

Effect concentration derived from figure, no raw data presented, no statistics performed

Duration:

28 d

Dose descriptor:

other: EC90

Effect conc.:

4 000 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

Cr(VI)

Basis for effect:

respiration rate

Remarks on result:

other:

Remarks:

Effect concentration derived from figure, no raw data presented, no statistics performed

Details on results:

See below.

Results with reference substance (positive control):

No data presented.

Reported statistics and error estimates:

No data presented.

After 28 days of incubation, cumulative carbon mineralization of microcosms amounted for approx. 3000 µg CO2/g d.w. soil (value derived from figure) in setups with glucose only. At concentrations of 400 mg Cr(VI)/kg soil, respiration decreased to approx. 1500 µg CO2/g d.w. (value derived from figure), whereas a Cr(VI) concentration of 4 g Cr(VI)/kg soil decreased respiration to approx. 300 µg CO2/g d.w. (value derived from figure). The observed effect concentrations were derived in agreement with previous experiments, i.e. 28d-EC50 and 28d-EC90 values of approximately 400 mg Cr(VI)/kg soil and 4 g Cr(VI)/kg.

Validity criteria fulfilled:

no

Conclusions:

The study investigated the effects of Cr(VI) on glucose-induced soil respiration in a soil with so-called "mild chromium contamination" (total chromium concentration: 6 mg Cr/kg soil). Total background Cr concentrations of European topsoil range from <3 to 6230 mg/kg with a median of 60 mg/kg (Salminen et al., 2005: Geochemical Atlas of Europe). Thus, the chromium level of the applied soil is well below the European median of background chromium. Signs of metal-tolerant, adapted microbial populations were not observed. The study is not a guideline study. However incubation times, soil glucose amendments (3 g/kg soil d.w.) and analytical setup are considered relevant. A 28 d EC50 of approximately 400 mg Cr(VI)/kg soil can be derived for the inhibition of glucose-induced soil respiration. As the study lacks sufficient documentation, e.g. raw data are not presented, statistics on effect concentrations are not performed, soils are not characterised used is provided, the results should be considered with caution.

Reference 2

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

significant methodological deficiencies

Remarks:

Non-guideline study with significant methodological deficiencies and insufficient documentation, i.e. no test statistics were performed, no presentation of raw data, no pH monitoring, experiment performed at high temperature, effects of liming not addressed, no analytical monitoring of test substance in soil and results presented only graphically. In addition, it is unclear if the doses applied refer to the final heavy metal content or the dose of the applied metal salts and the data presented graphically does not match data referred to in the results part.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 216 (Soil Microorganisms: Nitrogen Transformation Test)

Deviations:

yes

Remarks:

See "Remarks" (Rationale for reliability incl. deficiencies)

Principles of method if other than guideline:

The study investigated the effects of chromates and other anionic heavy metal species on ammonification and nitrification processes in soil in a loamy soil (total carbon content 1.1%). Soil pH was adjusted to 6.2-6.6 with CaCO3.

GLP compliance:

no

Analytical monitoring:

no

Test temperature:

28 °C

Moisture:

60% of soil water holding capacity

Organic carbon content (% dry weight):

1.1

Nitrogen content (% dry weight):

0.13

Nominal and measured concentrations:

Hexavalent chromium was added to the soil as Na2CrO4 (Cr(VI)) at concentrations of 10, 100 and 1000 µg/g soil.

Reference substance (positive control):

no

Duration:

1 wk

Dose descriptor:

other: LOEC

Remarks:

LOEC as derived in the EU RAR on chromates (2005). However there are doubts regarding incubation time and the total applied Cr(VI) dose. Please refer to "Remarks" (Rationale for reliability incl. deficiencies)

Effect conc.:

3.2 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

nitrate formation rate

Details on results:

LOEC as derived in the EU RAR on chromates (2005). However there are doubts regarding incubation time and the total applied Cr(VI) dose. Please refer to "Remarks" (Rationale for reliability incl. deficiencies)

Reported statistics and error estimates:

No statistics performed.

In the paddy soil under investigation, nitrification was inhibited at all Cr(VI) concentrations. The reduction of nitrification which was induced by 10 mg/kg Cr(VI) however, did not persist longer than two weeks and aligned back to control levels on week 3. However, the incubation time of the data presented graphically does not match to the incubation times referred to in the results part (discussion of effects based on data obtained from 1-4 days vs. graph describing experiment performed within 1-4 weeks) and results should therefore be considered with caution.

 

Regarding the effect of chromates on ammonification, no effects were observed except for the 1000 mg/kg treatment.

Validity criteria fulfilled:

no

Conclusions:

The study investigated the effects of chromates and other anionic heavy metal species on ammonification and nitrification processes in a loamy soil. Effects on nitrification processes were observed at 10 mg/kg Cr(VI) applied to the soil. However, the study suffers significant methodological deficiencies and insufficient documentation, i.e. no test statistics were performed, no presentation of raw data, no pH monitoring, experiments were performed at high temperature, the effects of liming were not addressed, no analytical monitoring of test substance in soil was performed and results were presented only graphically. In addition, further uncertainties exist regarding incubation time and the total applied chromium dose and the study is therefore not considered reliable.

Reference 3

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

significant methodological deficiencies

Remarks:

Non-guideline study with significant methodological deficiencies and insufficient documentation, i.e. no test statistics were performed, no presentation of raw data, no pH monitoring, experiment performed at high temperature, use of alternative (uncharacterized) carbon sources, no analytical monitoring of test substance in soil, insufficient characterization of test substrate, results presented only graphically.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 217 (Soil Microorganisms: Carbon Transformation Test)

Deviations:

yes

Remarks:

See "Remarks (Rationale for Reliability"

Principles of method if other than guideline:

The study investigated the effects of chromate and organic amendments on the soil microbial flora by assessing CO2 evolution in a closed bottle system. In addition, the reduction potential of chromate to trivalent chromium in soil by organic matter was assessed colorimetrically and soil microorganisms were counted (dilution plate method)

GLP compliance:

no

Analytical monitoring:

no

Test temperature:

28 °C

Moisture:

50% of water holding capacity

Details on test conditions:

Soil characteristics: T-N: 0.13, T-C = 1.1%.
0.5 g of dried rice straw (T-N = 75%) and/or 2 g fresh cow manure (T-N, T-C = 2.3, 39%) were added to 55 g of soil.

Nominal and measured concentrations:

Treatments: 0, 10, 20, 50 and 100 mg/kg Cr(VI) (dry soil basis)

Reference substance (positive control):

no

Remarks:

not used

Duration:

20 d

Dose descriptor:

NOEC

Effect conc.:

10 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

respiration rate

Remarks on result:

other:

Remarks:

No statistics performed.

Reported statistics and error estimates:

No statistics performed.

- No inhibition of CO2 evolution at soil Cr(VI) levels of 10 mg/kg. At concentrations of 20 mg/kg, CO2 evolution, CO2 evolution decreased slightly. No statistical analysis was performed and no measures of variation are presented.

- After 20 days at soil chromium concentrations of 50 mg/kg and 100 mg/kg, a reduction in CO2 evolution of 33% and 44% was observed in soil with added rice straw, respectively.

- Chromate reduction: the presence of soil organic matter facilitated the reduction of Cr(VI) to Cr(III). After two weeks, only traces of extractable Cr(VI) were detected.

Validity criteria fulfilled:

no

Conclusions:

The non-guideline study investigated the effects of Cr(VI) and organic amendments on the soil microbial flora by assessing CO2 evolution in a closed bottle system. No inhibition of CO2 evolution at soil Cr(VI) levels of 10 mg/kg was observed after 20 days of incubation at 28 °C, whereas CO2 evolution decreased slightly at concentrations of 20 mg Cr(VI)/kg. In addition, Cr(VI) was found to be rapidly reduced to Cr(III) in the respective soil, and only trace amounts of extractable Cr(VI) were found after two weeks of incubation. However, due to significant methodological shortcomings and insufficient documentation, i.e. no test statistics performed, no presentation of raw data, no pH monitoring, use of alternative (uncharacterized) carbon sources and insufficient characterization of the test substrate, the results of this study can not be considered reliable and should therefore be considered with caution.

Description of key information

Reliable data on chromium (VI) toxicity to soil microorganisms are lacking. Available studies on inhibition of ammonification, nitrification and carbon transformation (Ueda et al. 1988a,b; Nakatsu et al. 2005) without sufficient documentation and/or with methodological short-comings are therefore applied in a weight-of-evidence approach as follows:

Ueda et al. (1988a) studied effects of sodium chromate (equivalent to 3.2, 32.1, 321 mg Cr (VI)/kg soil) on ammonification and nitrification processes in a loamy soil. Ammonification of urea was less sensitive, and effects were only observed at the highest test concentration. Nitrification was inhibited at the two highest test concentrations. A slight reduction in nitrification was observed at the lowest test concentration during the first 2 weeks of exposure, but nitrification returned to control levels afterwards. According to OECD TG 216, data obtained on day 28 are used to determine effect concentration. Thus, a 28-d NOEC of 3.2 mg Cr(VI)/kg soil may be derived for the toxicity to nitrifying bacteria.

Ueda et al. (1988b) studied the effect of sodium chromate (equivalent to 10, 20, 50 and 100 mg Cr (VI)/kg soil dw) on carbon transformation. A 20-d NOEC of 10 mg Cr(VI)/kg soil dw for the inhibition of the soil respiration rate was derived. A rapid reduction of Cr(VI) to Cr(III) was observed, and only trace amounts of extractable Cr(VI) were detected after two weeks of incubation.

The effects of potassium chromate on glucose-induced carbon transformation were examined by Nakatsu et al. (2005) in a non-guideline study similar to OECD TG 217 resulting in a 28 d EC50 value of 400 mg Cr(VI)/kg soil.

In sum, the lowest effect concentration for the toxicity of chromium (VI) to soil microorganisms is the 28 d NOEC of 3.2 mg Cr(VI)/kg soil for the inhibition of nitrification (Ueda et al. 1988a) and is thus used as key value. A similar conclusion was made in the EU RAR on chromates (ECB, 2005).

Key value for chemical safety assessment

Short-term EC50 for soil microorganisms:

3.2 mg/kg soil dw