TR 092 – Soil and Sediment Risk Assessment of Organic Chemicals

Abstract

TR 092 : Soil and Sediment Risk Assessment of Organic Chemicals | January 2005

In an assessment of the risks of chemicals to organisms in both the soil and sediment compartments, the estimation of the likely concentration and of the potential for harm to the organisms needs to be properly understood. This report identifies a number of key variables that require better definition in the current TGD, especially as a risk assessment is refined and moves from the screening stage to the successive investigative phases.

1.

Assessment of input to the terrestrial environment – improving the estimates of emissions made using the TGD should be a priority when refining the risk assessment of a chemical.

a)

Current assessment of release of chemicals via sludge treatment – Once the release is better understood the next area for refinement is improving knowledge about how the chemical is released. In particular the release estimation to soil via sewage sludge has inaccuracies caused through the use of inappropriate distribution parameters, a lack of realism in the way that sludge is treated prior to land spreading and the extent to which farming land is used for spreading sludge.

b)

Within the TGD a major source of error or overestimation of the environmental concentration of a chemical in the terrestrial compartment relates to the amount of sludge spread. In particular, for a regional assessment the TGD should account for the different land-use types and the extent to which wastewater treatment plant sludge is spread.

c)

Other operating parameters regulated by the Sludge in Agriculture Directive that should be amended in the TGD are that the quantity of sludge spread will be 3 tonnes/hectare/year, less than the 5 tonnes/hectare/year for arable land, but more than the 1 tonne/hectare/year for grassland. Furthermore, the time limit included for harvesting crops should be changed for the same reason, to 6 weeks for grassland and 12 months for arable land. This compares to a no waiting period in the TGD.

2.

When assessing the fate of chemicals in the soil and sediment compartments, the partitioning behaviour and the degradation are two key parameters to consider:

a)

Partitioning behaviour

i)

Neither sorption nor desorption are instantaneous and can require a significant amount of time prior to attaining equilibrium. The amount desorbed will frequently change with increasing time, a process termed ‘ageing’. This process may have a significant effect on the actual bioavailable fraction of the environmental concentration.

ii)

The use of KD values estimated from Koc values needs to be addressed on a case-by-case basis and measured KDs should be used when this extrapolation is not justified, for example when the chemical is ionisable.

iii)

The measurement of the partitioning of the chemical, KD, extent and rate, previously estimated from Koc, should be performed with solids from the compartment of particular interest, e.g. sewage sludge, soil or sediment.

b)

Degradation

i)

The extrapolation from the aquatic ready biodegradation test to soil or sediment aerobic degradation may over- or underestimate the actual degradation rates. This is clearly an area for future research.

ii)

As a risk assessment moves to investigative and research phases, biodegradation in relevant compartments and in simulation studies should be investigated and the rates obtained incorporated in the risk assessment.

iii)

The assessment of degradation (biotic and abiotic) for the terrestrial and sediment compartments needs to be improved. Some of the work now being undertaken under the auspices of the ICCA Long-range Research Initiative (LRI) may be able to help provide alternative approaches to developing more realistic assessment of degradation rates in the environment. As this work develops the implications for testing and the defaults within the TGD should be examined.

iv)

Abiotic degradation should be addressed and factored into the risk assessment.

3.

Bioavailability is a key issue that affects how chemicals behave in the environment and amends their impact on organisms. The incorporation of bioavailability into the risk assessment process needs to be investigated.

a)

It is recommended that sensitivity analysis be used to assess the importance of removal constants from the solid matrix.

b)

If reduced bioavailability were known to occur for a specific chemical at a rate that would impact the assessment, it is suggested that data be generated and used within the risk assessment.

4.

Predicting the effect of chemicals to soil and sediment organisms is discussed in the report. The main issues addressed were a need for improved test protocols, identification of research topics to better understand the equilibrium partitioning theory (EPT) and a proposed improvement to the testing strategy for soil and sediment effects.

a)

Test protocols – the following factors need addressing: spiking methods, including for example sludge where appropriate, the substrate composition, preparation of the soil or sediment, conduct of the test including study length and culturing mechanisms, and testing at realistic concentrations. It is likely that greater standardisation of test methodologies will ensure greater consistency and the work being done by the OECD to develop terrestrial test methods is much welcomed.

b)

Equilibrium partitioning theory – The TGD makes a number of assumptions that run counter to the application of the equilibrium partitioning theory of Di Toro (Di Toro et al, 1991). This is an area where further research is needed to address these uncertainties and to provide clearer guidance of how the equilibrium theory may be applied and when the results obtained should be treated with caution. Some of this research is being addressed within the present Cefic Long-range Research Initiative (LRI).

c)

Soil and sediment testing strategy – There is a clear need for the development of models that can deal with ionisable substances, especially those that may cause a biological effect. The report describes a proposed sediment testing strategy that tries to use all available information, ensures the aquatic risk assessment has been fully refined, and that advocates testing should be stepwise, allowing alignment with equilibrium partitioning theory.