Trichloroethylene

Biodegradation in water and sediment: simulation tests

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

biodegradation in water: sediment simulation testing

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Published in peer reviewed literature, limitations in design and/or reporting, but adequate for assessment

Principles of method if other than guideline:

The depletion of trichloroethylene was studied in microcosms containing water and natural sediment ranging in composition from highly organic to a calcareous sedimentary rock. The depletion of trichloroethylene was followed by test material analysis.

GLP compliance:

not specified

Oxygen conditions:

anaerobic

Inoculum or test system:

natural sediment

Details on source and properties of sediment:

Organic muck was collected from the Everglades, a graminoid wetland that is the recharge basin for the Biscayne Aquifer in Southern Florida. The VB sand was obtained from a site that was previously contaminated by a trichloroethylene spill in Vero Beach (FL, USA). The rock was obtained from the Everglades near the muck collection site.

Details on inoculum:

Inoculum present in natural sediments

Duration of test (contact time):

6 mo

Initial conc.:

5 mg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

test mat. analysis

Details on study design:

Each microcosm was spiked to contain 5 mg/L of trichloroethylene with a solution of trichloroethylene in nitrogen-purged distilled water. The microcosms were spiked 2 weeks after construction to allow equilibration and oxygen depletion to occur inside the test and control bottles and thus simulate original conditions of the sample site. All microcosms and controls were allowed to incubate in the dark at 25 C for measured time periods up to 6 months. Replicate were constructed for each sampling period.

Reference substance:

other: none

Key result

% Degr.:

100

Parameter:

test mat. analysis

Sampling time:

6 mo

Key result

Compartment:

entire system

DT50:

73.1 d

Type:

(pseudo-)first order (= half-life)

Temp.:

25 °C

Remarks on result:

other: average for muck

Key result

Compartment:

entire system

DT50:

42.5 d

Type:

(pseudo-)first order (= half-life)

Temp.:

25 °C

Remarks on result:

other: averge for sand

Key result

Compartment:

entire system

DT50:

90.3 d

Type:

(pseudo-)first order (= half-life)

Temp.:

25 °C

Remarks on result:

other: rock

Other kinetic parameters:

first order rate constant

Transformation products:

yes

No.:

#1

Evaporation of parent compound:

not specified

Volatile metabolites:

not specified

Residues:

not specified

The depletion (i.e. the removal mechanism was not determined) of trichloroethylene in microcosms containing water and three types of natural sediment (muck, sand and rock) was studied. The organic carbon contents for the muck, sand and rock were 25%, 2% and < 1% respectively and the sand had already been exposed to trichloroethylene through a major spill from a storage tank. The muck and sand had similar depletion rates and the depletion in the rock microcosm was slower. The results suggest that depletion depends on the organic content of the sediment and the microbial biomass.

Reference 2

Endpoint:

biodegradation in water: sediment simulation testing

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Remarks:

Non-GLP near guideline study, published in peer reviewed literature, notable restrictions in design and/or reporting but otherwise adequate for assessment

Principles of method if other than guideline:

Trichloroethylene was transformed in microcosms composed of aquifer materials. Microcosms consisted of natural uncontaminated sediments and groundwater bacteria. Biotransformation products were purge-trapped and analysed by GC equipped with electrolytic conductivity detectors.

GLP compliance:

no

Radiolabelling:

no

Oxygen conditions:

anaerobic

Inoculum or test system:

other: groundwater

Details on source and properties of surface water:

Distilled water was used

Details on source and properties of sediment:

Natural sediments were collected from uncontaminated sites in the Everglades

Details on inoculum:

Bacterial cultures from groundwater taken from a trichloroethylene spill site. These cultures were composed of mixed populations of aquatic or soil microorganisms.

Duration of test (contact time):

> 7 - < 8 wk

Initial conc.:

> 4 - < 5 µg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

test mat. analysis

Details on study design:

Sediments were thoroughly mixed and passed through a 6.35 mm sieve. Sediment microcosms were prepared by adding an equivalent of 4 g dry weight of muck or mud or 6 g dry weight of marl to 50-mL septum bottles and completely filling the bottles with distilled water.
Initial time (T0) microcosms were sampled immediately; all other microcosms were incubated at 25 C in the dark until analysis.
Control microcosms consisted of sediment and culture without trichloroethylene. Abiotic controls consisted of sediment and culture, but were autoclaved before spiking with trichloroethylene.
After spiking, the microcosms were sealed with PTFE-lined septa and shaken for 30 s to mix the contents.
Sampling was done at three periods: T0 (immediately after spiking), T1 (after 2-3 weeks) and T3 (after 7-8 weeks).

Key result

Remarks on result:

not measured/tested

Transformation products:

yes

No.:

#1

No.:

#2

Details on transformation products:

A maximum concentration of 167±6.4 µg/L of cis-1,2-dichloroethylene (#1) was observed after 2-3 weeks of incubation. The maximum concentration of trans 1,2-dichloroethylene (#2) was 4.26 µg/L after 7-8 weeks of incubation.

Evaporation of parent compound:

no

Volatile metabolites:

yes

Residues:

not measured

Reductive dechlorination was observed to be the first step in the biotransformation of trichloroethylene. Further dechlorination or other chemical transformations occurred because the products of reductive dechlorination reduced after attaining maximum concentrations.

Reductive dechlorination produced both isomers of 1,2 -dichloroethylene. The cis-isomer was always produced in greater concentration than the trans-isomer.

Reference 3

Endpoint:

biodegradation in water: simulation testing on ultimate degradation in surface water

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Published in peer reviewed literature, notable limitations in design and/or reporting, but adequate for assessment

Principles of method if other than guideline:

Degradation was examined in seawater under aerobic conditions

GLP compliance:

not specified

Oxygen conditions:

aerobic

Inoculum or test system:

natural water: marine

Key result

Remarks on result:

other: detailed information not available, no biodegradation observed

Transformation products:

not specified

No degradation of trichloroethylene in seawater was observed under aerobic conditions

Reference 4

Endpoint:

biodegradation in water: sediment simulation testing

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Published in peer reviewed literature, notable limitations in design and/or reporting, but adequate for assessment

Principles of method if other than guideline:

Sub-surface sediments were incubated with trichloroethylene (50 mg/l), and with either methanol or propane for 10-30 days.

GLP compliance:

not specified

Radiolabelling:

not specified

Oxygen conditions:

not specified

Inoculum or test system:

other: Sub-surface sediment

Duration of test (contact time):

30 d

Initial conc.:

50 mg/L

Based on:

test mat.

% Degr.:

25

Sampling time:

10 d

Remarks on result:

other: incubated in subsurface sediment with methanol

% Degr.:

95

Sampling time:

30 d

Remarks on result:

other: incubated in subsurface sediment with methanol

% Degr.:

0

Sampling time:

10 d

Remarks on result:

other: incubated in subsurface sediment with propane

% Degr.:

45

Sampling time:

30 d

Remarks on result:

other: incubated in subsurface sediment with propane

Transformation products:

not specified

After 10 days incubation with methanol, 25% trichloroethylene was degraded and after 30 days 95% was degraded. After 10 days incubation with propane, 0% was degraded, and after 30 days 45% was degraded. Results are given in Table 1

 

Table 1 Anaerobic degradation of trichloroethylene

Test conditions	Test results	Reference
Trichloroethylene (50 mg/l) incubated in subsurface sediment (10g) with methanol	25% degradation after 10 days, 95% degradation after 30 days	Phelps et al, 1988
Trichloroethylene (50 mg/l) incubated in subsurface sediment (10g) with propane	0% degradation after 10 days, 45% degradation after 30 days	Phelps et al, 1988

Description of key information

Within a weight of evidence approach it was concluded that TCE was not degraded in marine water under aerobic conditions.

The biotransformation by freshwater sediment microbiota seem to occur more readily under anaerobic conditions under specific conditions like co-substates.

Additional information

A variety of information is available for TCE investigating the biodegradation in sediments under certain conditions. It can be deduced from available data that degradation is more pronounced under anaerobic conditions and in the presence of co-substates.

For example, Trichloroethylene can be degraded under aerobic conditions by a process of co-oxidation when other suitable co-substrates (methane, propane, toluene, phenol) are also present to support growth of the microorganisms and induce the formation of enzymes which due to their broad substrate specificity, can also degrade trichloroethylene.

Furthermore, there is extensive work which shows that under anaerobic conditions, trichloroethylene degrades by a process of reductive dehalogenation, resulting in the formation of lower chlorinated homologs as reaction products. Although metabolites are known to occur, the terminal product of reductive dehalogenation is ethylene.

From review of literature additional information is available supporting the conclusions above and provide more information on degradation process under specific conditions.

However, for assessment as a worst case EUSES degradation estimates calculated based on input parameter “not biodegradable” were used.

This is agreement with EU risk assessment (2004) and recently published EPA draft risk assessment (2020).