Through the collection and compilation of experience and data from literature, a number of factors of importance to further developments of test procedures for examination of the leaching of organic compounds from soil and waste materials have been identified:

When organic compounds are present in contaminated soil and waste materials as free phases, the leachability of these organic compounds will be controlled by their water solubility. If no free phase of an organic compound is present, or the free phase is totally dissolved, the leaching will be controlled by desorption from the solid particles of the soil/waste material.

Dissolution from free and residual phases of organic compounds in soil is a relative fast process (minutes to hours). The course of the release of organic compounds from a free phase may be predicted if the composition of the free phase is known. This would, however, require additional information on the properties of the compounds and the soil. Such information is often difficult to procure.

Desorption of organic compounds from soil is often a slow process (days to weeks). Desorption rates are difficult to predict, probably because they depend both on the organic compounds and the soils in question. Slow desorption rates are seen in particular for hydrophobic compounds. There are indications that "young" soil types in general are characterised by exhibiting relatively fast desorption rates and small hysteresis effects, whereas the opposite is true for geologically older soil types. Except for determinations of the O/C ratio, which increases with the age of the soil, no easy or well-established methods for the determination of the age of soils exist. It should be noted that top -soils in some cases may consist of or contain geologically old materials.

A general impression of the expected sorption capacity of a soil may be based on a determination of its content of natural organic carbon. Furthermore, the above mentioned ratio between oxygen and carbon (O/C) is considered an indicator of the age of the solid organic carbon present in the soil. Low values of O/C indicate that the organic material is old and that low desorption rates may consequently be expected.

In relation to testing of the leaching of organic compounds it is of key importance that their release from soils in many cases will be controlled by desorption kinetics. This has an impact on the required contact time between the soil and the leaching agent in the test. Considerations should be given to the determination of realistic contact times under field conditions for relevant physical scenarios. It seems obvious to attempt to develop test systems, which ensure that equilibrium-like conditions between the organic compounds in the solid phase and the leaching agent, respectively, are achieved. In those cases where long time periods are required to achieve equilibrium-like conditions, this would not necessarily be a suitable criterion for establishing the contact time. The reason for this is partly the impracticability of long contact times, partly the fact that equilibrium-like conditions are not always attained in the field.

Most of the above mentioned factors, which influence and are of importance to the leaching of organic compounds from soil/contaminated soil, may be expected to be of similar importance to the leaching of organic compounds from (largely) inorganic waste materials. However, for many such waste materials the organic contaminants constitute a more or less integral part of the materials themselves, whereas for soils they normally are associated only with the surface or the pores of the particles. In contrast to soil, a number of waste materials may react chemically with water when they are moistened or percolated, which e.g. may give rise to more extreme pH values in the water phase than usually seen for soils.

Based on the in formation collected on test methodology and the experimental work conducted in this study, a number of factors of importance to the development of a batch leaching test for organic compounds have been identified:

A certain minimum of pre-treatment of a soil or waste sample is necessary to ensure sufficient homogenisation of the sample. Such pretreatment procedures should be gentle and should in many cases be carried out within a closed system to minimise loss by evaporation of volatile organic components. This means either that sieving and crushing of the sample is not possible, or that a certain loss by evaporation must be accepted.

The liquid to solid ratio (L/S) is of key importance to the performance of a leaching test and the interpretation of the results. The leaching experiments performed in this study were carried out at L/S = 2 l/kg, which in many cases approaches the lowest practicable L/S ratio at which a batch leaching test can be executed. The practical difficulties with suspended particles in the system combined with the need for relatively large volumes of eluate for chemical analysis have meant that the use of the low L/S value has not been optimal from all perspectives. During the performance of the batch tests, the headspace (the volume of air) in the test bottles has been minimised in order to reduce the loss of volatile organic compounds and to reduce the amount of oxygen in the test systems

The separation of the eluate from the solid phase has turned out to a very critical part of the leaching test procedure. Compared to the application of a similar procedure to the testing of the leaching of trace elements and inorganic salts, the separation procedure for organic components is made much more difficult by the fact that organic compounds are far more prone to evaporation and to sorption to filter materials etc. than trace elements and salts. In addition, there is a general requirement for larger volumes of eluate for chemical analysis of organic compounds than for analysis of trace elements and salts.

In order to minimise the losses (or gains) of the organic compounds being studied, equipment primarily made of glass and stainless steel, and to a lesser extent teflon, has been used in the leaching experiments carried out in this study. Filter materials made of glass resulted in losses, but apart from this, the test system performed as intended. The composition of the leaching agent, the temperature and the mode of agitation appear to have be non-critical in the leaching tests carried out on the selected leaching tests.

For some of the relevant groups of organic compounds a reasonable amount of background information exists on which further development of procedures for the testing of the leaching properties may be based. This is true for oil components, chlorinated solvents and polycyclic aromatic hydrocarbons (PAH), whereas less information is available on components such as MTBE and DEHP. The potential problems associated with small particles in soil samples and the eluate resulting from the leaching test has been (and is being) examined in relation to the leaching of PAHs. Due to the high affinity of PAHs for solid particles, the presence of small particles/colloids in the test system is expected to be potentially important primarily to the leaching of PAHs. However, once the final particle cut-off has been decided, it should be examined whether the presence of particles influences the test results for other organic compounds of high priority. For practical reasons, i.e. to save time and reduce the difficulties involved in the handling of larger volumes of eluate, particle cut-offs corresponding e.g. to 0,7 m m have been used in this study instead of the 0,45 m m cut-off, which is most commonly used for trace elements and inorganic salts.

Based on simplified calculations of the distribution between the phases in a combined air/water/soil system it is proposes to classify those organic compounds for which ecotoxicological criteria in soil and water exist according to the feasibility and relevance of testing their leaching from soil and waste materials. It is proposed that they should be divided into three groups:

The first group consists of consists of non-volatile compounds which sorb strongly on soil and have low solubilities in water. Most PAHs and diesel oil components belong to this group. It seems obvious that a methodology for determination of the leaching of organic compounds from soil and waste materials should include these compounds, which on the one hand will be present in and move with the soil and on the other hand to a certain extent will be leachable. The most critical factors that may influence the leaching procedure and results and need consideration are likely to be the separation of the eluate from the solid phase, including the material which may be sorbed on colloids, and the contact time. A preliminary proposal for a protocol for testing of the leaching of non-volatile organic compounds from contaminated soil has been produced. The proposed protocol does not specify a definite contact time (1 to 5 days are suggested) and it does not address the question of colloidal transport.

The second group includes volatile compounds with relatively high water solubilities. The chlorinated solvents and BTEX are among the compounds in this group. Many of these compounds have low criteria values in groundwater and it would therefore be advantageous to be able to test their leachabilities. Some of them have far higher diffusion coefficients in air than in water and will therefore migrate faster through air that through water. In the context of utilisation of soil and waste materials for building and construction purposes, these compounds are not likely to cause a general (indoor) air problem, since covering is required and may be assumed hinder any substantial transport by air. In relation to testing of the leaching of these compounds, the potential loss of material through volatilisation is likely to pose the major problem. By minimising the headspace in the test system this may be taken care of in a batch test since only a minor part of the total amount be present as a gas, but the volatility will be difficult to control when handling the solid sample and the eluate before and after the test itself. Especially the separation of the solid phase and the eluate will be critical in relation to loss of material and hence the feasibility of the test.

The third group consists of compounds, which have very high water solubilities or are water miscible, and which do not sorb very strongly on soil. This group includes e.g. MTBE. The phenols have high water solubilities and also belong to this group, although they sorb stronger on soil than e.g. MTBE. Other compounds in this group will only to a very limited extent be associated with the solid phases, and it will therefore in general be difficult or irrelevant to attempt to use leaching tests to describe their behaviour. To the extent it seems relevant, the leachability of compounds belonging to this third group may also be tested by the method mentioned above. Colloids are not likely to play any significant role in the transport of these compounds due to their weak sorbing properties.

It will be necessary to develop appropriate methodologies for testing the leaching properties of all the organic priority compounds. This includes further develop the proposed test protocol. A number of the critical factors identified in this study and their influence on the test results must be coped with and accounted for. High priority should be given to the development or adjustment of methods for chemical analysis of the organic compounds with detection limits comparable to those applicable to analysis of groundwater. Application of most of the existing analytical methods leads to requirements for relatively large volumes of eluate, which again leads to a need for inconveniently large test systems (or multiple systems) causing the separation of the eluate and the solid phase to become difficult and time consuming.

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