Goal, Scope and Background
The suitability of leaching tests for the assessment of soil water concentrations is a controversial issue. The aim of this paper is to review and critically discuss the applicability and comparability of leaching tests in the scope of groundwater risk assessment of inorganic contamination and to discuss soil water sampling methods as alternative.
Main Features
First, the theoretical background of leaching tests and the main leaching controlling parameters (pH, redox, ionic strength, reaction kinetics, surface, and speciation) are discussed. Experience with common batch leaching tests (the German DEV S4 test (S4), saturation soil extraction (SSE), ammonia nitrate extraction (ANE), and pHstatic tests) are compiled and an emphasis is set on the comparability of the results of batch leaching tests. Additionally, the comparability between batch tests and column tests is discussed and comparison studies are reviewed. As a feasible alternative, soil water sampling strategies (the suction cup method and centrifugation) are outlined.
Results and Conclusions
For an expedient application of leaching tests, it is crucial to identify the main release controlling parameters, which can be both site and contaminant specific. Possible controlling parameters are, for example, pH, redox conditions, specific surface area of the investigated material, liquid to solid-ratio, ionic strength, cation exchange capacity, DOC, speciation, temperature and contact time. It depends on the release process of the contaminant in the investigated material, which parameters are influencing the release predominantly. The following questions have to be answered: Is the release process availability controlled or solubility controlled and are there kinetic limitations? Suction cups are particularly useful for long-term monitoring. In contrast, centrifugation is a fast and simple method to sample soil water also at larger and heterogeneous sites. SSE or similar batch tests for coarse material are suitable if the soil water cannot be investigated directly. Contaminant concentrations of the SSE fit best to contaminant concentrations in soil water compared to other leaching tests. Concentrations measured with S4 tests are usually significantly lower than in real soil water due to the unrealistically high liquid to solid-ratio. The ANE is used for the evaluation of plant uptake. It does not represent realistic soil water concentrations. Cationic contaminants are usually significantly higher concentrated in ANE eluates. pHstatic tests provide an evaluation of the pH dependency of contaminant release and buffer capacity of the investigated material. It provides indications to release processes and often explains deviations in the results of different leaching tests. Previous practical experience and fundamental research studies show that a conversion of leaching test results, as is proposed by the (already withdrawn) DIN V 19735, is impossible. Correlations of results from different methods, regarding different kinds of materials and different sites, are not significant. This is due to different leaching processes.
Recommendation
. For short-term risk assessment, soil water should be sampled and investigated directly by suction cups (for monitoring purposes) or centrifugation (for inventory purposes). If this is not possible, the SSE or analogous batch tests for coarse material with a narrow L/S ratio should be applied. A suggestion could be a modified saturation soil extraction where a soil column is saturated and the eluate is removed by suction at the bottom of the column. With this method, an unsaturated column test could follow in the same vessel. In order to assess the long-term leaching behavior, total contents of the investigated material have to be taken into account additionally. Furthermore, it is essential to understand the dominant physical and chemical release processes and to figure out the main leaching controlling parameters. Therefore, the following methods are recommended: pHstatic tests at different pH values provide an insight to leaching processes and possible future leaching scenarios. Batch tests similar to the S4 procedure with different L/S ratios are useful to find out whether the release process is solubility or availability controlled. Additionally, this method allows the determination of maximum solubility and maximum availability, respectively. Furthermore, unsaturated column tests provide an insight into leaching processes and releasable amounts even though they require great experimental effort. Other leaching tests like S4 or ANE are not suitable for the estimation of soil water concentrations.
