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3.4Environmental characterisation of alternative materials

3.4.1Leaching

Leaching is the process by which constituents in a construction material, are released into the environment through contact with water. Understanding the rate and extent to which constituents of interest may be released is central to defining (i) potential environmental impacts through water-borne mechanisms including soil, groundwater and surface water contamination, (ii) effectiveness of certain treatment processes for materials, (iii) degradation of structural performance of certain materials in the environment. The specific rates and extents of constituent release are a function of (i) the chemical and physical properties of the material under consideration, (ii) the chemistry of the constituent(s) of interest, (iii) characteristics of the local environment in which the material is placed, including chemical properties (e.g., pH, oxidation-reduction potential, presence of reacting constituents such as carbon dioxide) and the nature of water interaction (e.g., frequency, amount, interfacial contact area).

Fundamental understanding of leaching processes is achieved through study and research on material testing, geochemistry, constituent mass transfer, and development and verification of mathematical models to estimate long-term behaviour and characterise risks under varied environmental conditions. Extensive research and evolution in understanding fundamental aspects of leaching processes and impact evaluation has been carried out over the past two decades. This research, which has been carried out using the same consistent fundamental leaching test methods provides a sound foundation for evaluating different scenarios and associated impact assessment. Recognising the risks and environmental damage caused by uncontrolled materials use, national and regional regulatory organisations have developed widely variable, and often disparate, test methods and regulatory control frameworks to characterise leaching and make decisions about acceptable and unacceptable use of materials. These regulations, which began evolving in the early 1980’s, were based on the best understanding at the time, but are largely inadequate in the context of current understanding and needs. Many circumstances that have resulted in misapplication of procedures, erroneous decisions based on inadequate science, and resulting misuse of economic resources, damage to the environment or human health impacts have been documented based on these shortcomings.

Efforts have been undertaken in the European Union (van der Sloot - NL, Hjelmar - DK) and the United States (Kosson) to develop a more robust and scientifically sound, while practical, framework for characterisation of soils, sludges, wastes and constructions materials subject to environmental leaching and decision-making based on assessment of potential impacts. Recognition of the benefit is evolving at the regulatory level (US EPA, VROM -NL, EU DG ENV) on an overarching framework and methodological details for implementation (Kosson et al, 2002) The framework is a tiered approach, allowing the user to select the level of testing and evaluation required based on the degree of conservatism required, prior information available, and balancing costs of testing against benefits from more detailed information (e.g., reduced management costs or alternative management options). Use of this approach is beginning to emerge in both the European Union and the United States. A central challenge is converting the depth and breadth of knowledge currently available to decisions on selection of characterisation methods, assessment models and decisions on acceptability, and implementation of these choices based on specific needs of a diverse set of users. A database of characterisation leaching data is a means to facilitate cross-field comparison of data. A further key aspect in such an approach is to identify the aspects besides leaching that need to be covered to be able to make a full evaluation of material behaviour in a given scenario. This is best done in an expert system, that allows tying the relevant aspects together to come to a decision on a time dependent source term description for the material in its application.

3.4.2Previous leaching studies on road base and embankment

A number of previous European studies on road construction materials are listed in Table 3.1. Few of these address the full range of conditions: laboratory testing – lysimeter studies - field verification studies. The ultimate goal of testing is that a statement can be made that the material considered for use can be accepted for a given application and a given exposure scenario. This implies that a main point in developing test methods is to emphasize a link as close as possible between testing and field or at least a translation from one to the other. A variety of factors playing role in determining the release from different materials in different settings are chemical factors such as pH (carbonation), redox (oxidation), organic matter content, salt load, etc and physical parameters such as permeability, infiltration rate, porosity, hydrology, temperature, etc.
In Table 3.2, a schematic overview of: two typical road applications is given with the main design features and influencing factors. This information was developed in the framework of a Dutch research programme associated with standardisation needs (van der Sloot et al., 2003).
In Table 3.3 relevant processes are split out by material type. From the work on a wide range of materials the message emerges that it is better to focus on mechanisms in test development than on materials (van der Sloot et al 2003). There are to many of the latter and many behave in very similar ways from a leaching behaviour point of view. At the end of this chapter material specific properties are given that illustrate for a given material class that leaching characteristics are quite similar. This can be used as basis to select appropriate compliance type tests.

Table 3.1: Studies of materials at different scales of testing

Material Category	Laboratory			Lysimeter	Field	References
	pH dependence	Column test	Tank leach test
MSWI Bottom ash	X				X	Hjelmar et al. (1991), Hjelmar (1996)
	X	X		X		Fallman (1997)
	X	X			X	CROW (1996)
					X	Johnson et al. (1999)
	X	X				IAWG (1997), Meima (1997) Comans et al. (1991)
				X		Mulder (1991)
		X		X		Anthonissen et al. (1990)
				X	X	Hjelmar and Hansen (1993)
	X	X			X
	X	X			X	Hjelmar et al. (2003a, 2003b),
		X			X	RWS/TAUW (Pers. Comm.2003).
Coal fly ash	X	X		X	X	Hjelmar et al. (1991), Hjelmar (1990)
				X	X	Hjelmar and Hansen (1993)
		X			X	Schreurs et al, (1997)
Steel slag	X	X	X	X	X	Van der Sloot et al. (1995), Comans et al (1991)
	X	X		X	X	Fallman (1997)
		X		X tank		Mulder en Gerritsen (1990)
Construction debris /crushed concrete	X	X				Van der Sloot (2000)
				X tank		Mammoet semi-praktijk (1985-90), CROW (1996)
	X	X			X	Hjelmar in Reid et al. (2001) (ALT-MAT)
				X		Mostbauer en Lechner (2000)
Soil		X X				Keijzer et al. (1992) Zevenbergen et al. (1997)
	X					Comans en Geelhoed (1997)
	X	X				Van der Sloot et al (2000a), Van der Sloot et al. (2001c)
		X				Hjelmar (2002)

Table 3.2: Schematic overview of scenarios and main characteristics for the different material types
Materials	Scenario’s Short description of situation, hydrology and external influences
MSWI-bottom ash Phosphorus slag Steel slag Coal fly ash stabilisation Construction debris	Base layers of ashes and slag. A layer of 0,5 m mildly compacted under a road cover and above groundwater level. Under the road cover very limited percolation. Neutralisation and oxidation possible by exposure to external CO2 and O2 from the atmosphere, and by CO2 resulting from biodegradation in the soil.
Mildly contaminated soil	Embankments Material in a layer of 5 m, above groundwater level Percolation 300 mm/j. Exposure to the atmosphere. Influences by biological activity.

Table 3.3: Description of relevant processes by scenario and by material type
Material	Scenarios Short description of process affecting release
MSWI-bottom ash Phosphorus slag Steel slag Coal fly ash stabilisation Construction debris	Base layers of ashes and slag. Changes in pH by. CO2 from degradation (Meima, 1997) and from CO2 from the atmosphere (Mostbauer and Lechner, 2000). Reduction of metallic components by reacting with base Influence of limited exposure by the road cover or isolation measures.
Mildly contaminated soil	Embankments Changes in pH, uptake by plants Changes in Eh leading to other release behaviour (Keijzer et al, 1992)

3.4.3Suitable leaching test methods

World wide many leaching tests are used (Environment Canada, 1990). Several of these methods have been used in the studies listed in the previous sections. Many of the different leaching tests largely address the same aspects of leaching under slightly different conditions. Others differ significantly in the concept behind the method, such as the sequential chemical extraction procedure (Tessier et al, 1979). In recent years, attempts have been made to harmonise leaching tests (van der Sloot et al, 1997). Recently a framework has been developed (Kosson et al, 2002), which combines different test results in an encompassing evaluation that allows conclusions on a range of aspects relevant to long term environmental impact of alternative materials. A few of the most promising tests in this context are described below.
pH dependence leaching test

Based on a comparison of methods and test data the pH dependence test (PrEN 14429, 2003; pH stat, 2004) has been identified before as a method that allows mutual comparison of several different test methods (van der Sloot et al., 1997). Here the properties of the method to quantify the acid neutralisation capacity and to use the data for geo-chemical speciation modelling are most relevant. Fresh MSWI bottom ash is known to exhibit changing chemical properties over time, of which the most important is pH, due to the uptake of CO₂ from the atmosphere. In turn, pH strongly affects leaching properties. The element specific leaching curve (metals, oxy-anions, major and minor elements and DOC) obtained with the pH dependence test can be seen as a material characteristic, a “geo-chemical fingerprint” of the material under study.

Percolation test

For the percolation behaviour of (granular) materials the presentation of release and concentration as a function of the liquid to solid (L/S) ratio is the most suitable form of data presentation (van der Sloot et al , 1997), as it allows comparison with data from larger scale experiments (e.g. lysimeter) and field data ( van der Sloot et al, 2003). The latter does require an estimate on the amount of liquid that has passed through the material. The release behaviour as obtained in a percolation type test can be used to derive parameters for long- term prediction. A relatively simple approach for granular materials is the CSTR (Continuously Stirred Tank Reactor, a first order decay function) model, which provides a first crude estimate of release:

C_L/S = C₀ * e^{- κ L/S}

In the above CSTR equation, C₀ is the initial concentration (as obtained in the lowest L/S fraction) and κ is a first-order release factor. Obviously, more mechanistic models provide more accurate predictions (Dijkstra et al, 2002; Dijkstra et al, 2003). This can be seen as a hierarchy in the use of models: sophisticated when needed and simple when sufficient discrimination can be obtained. Both column leaching tests and the more simple batch leaching tests may be classified as percolation tests.
Tank leaching test

In this test the release is related to the surface area of the specimen to be tested. In Europe the Dutch standard tank leaching test (NEN 7345, 1996) has been widely used. The specimen is subjected to leaching in a closed tank. The leachant is renewed after regular time intervals roughly related to the square root of time and generally up to around 64 days at a specified leachant to product volume ratio (L/V). The results are expressed in mg/m². This test is a procedure to evaluate the release from monolithic material by predominantly diffusion control (e.g. exposure of structures to external influences). This method can be applied as long as the product maintains its integrity. To assess the behaviour after disintegration or demolition of monolithic forms, the information obtained in the pH dependence leaching test is very relevant, as in this situation the pH is likely to change to more neutral conditions. Dynamic leaching tests for monolithic materials are now subject of standardisation in CEN TC 292 (DMLT, 2004), both for basic characterisation and compliance purposes. The tests under development are intended to be able to describe both the situation where the components on the surface of the material are in equilibrium with the same components dissolved in the water phase and a more dynamic situation where the release of components from the monolith is controlled by diffusion.

Selection of test method(s) for a particular scenario

The main character of each of these methods is that they each cover an important part of the story. For practical purposes material are divided in granular materials (percolation dominated) and monolithic (surface area related release mechanism). The pH dependence test provides the insight in the chemical controlling factors, while the time dependent aspects are covered by either percolation or tank test. This implies that in more than 80 % of the cases two tests per material will suffice to allow an evaluation of release under a wide range of exposure conditions. More sophistication can be added when needed. The database/expert system to be discussed in more detail below will provide much more potential to take relevant parameters into account. In Table 3.4 typical scenarios are linked with relevant test methods.

It should be noted that one of the standard leaching tests listed in Appendix 1, EN 1744-3: 2002 “Test for chemical properties of aggregates – Part 3: Preparation of eluates by leaching of aggregates” should be avoided when testing materials with particle size larger than 4 mm in relation to impact assessment or quality control in road construction. The problem is that the test does not take into account that the release of components from larger particles may be dominated by diffusion processes rather than equilibrium conditions. This may lead to meaningless results and erroneous conclusions as regards the potential environmental impact from leaching of contaminants from a coarse, granular material (van der Sloot and Mulder, 2002).

Table 3.4: Test methods related to the application of materials
	Application	Exposure conditions	Materials	Tests
1	Pavement	Monolithic material	Asphalt concrete	Tank leach test
2	Road base	Stabilised to form a monolithic layer - oxygen and carbon dioxide uptake (CO2 from the air and from biodegradation in soil) through unsaturated zone	Cement stabilised material	Tank leach test
3	Road sub base	Granular material; road cover, oxygen and carbon dioxide uptake (CO2 from the air and from biodegradation in soil) through unsaturated zone	MSWI bottom ash; Recycled construction debris; slag	Percolation test and pH dependence leaching test
4	Embankment	Granular material; limited oxygen and carbon dioxide uptake	MSWI bottom ash; Recycled construction debris; slag	Percolation test
5	Sound barrier	Granular material; limited oxygen and carbon dioxide uptake	MSWI bottom ash; Recycled construction debris; slag; soil	Percolation test (in case of clay possibly a batch test)

3.4.4Observations from leaching studies

Key observations as obtained from different studies on leaching in relation to road construction with alternative materials are:

Different leaching tests used in the regulatory frameworks now existing in Europe can be linked to one another through the pH dependence leaching test.

Tests based on the material imposed pH may provide the wrong information for a given scenario as depending on the external influences the material may carbonate and thus change in pH and thus in leachability. Such changes may lead to order of magnitude changes in leachability.

Understanding chemical speciation is important to assess long term behaviour as solubility control drives most of the release of critical components.

Redox conditions are rather poorly understood today, whereas several materials now used in road base feature such properties, which like a high pH is not likely to be maintained when exposed to atmospheric conditions.

The role of dissolved organic carbon (DOC) in metal mobilisation is much better understood now. In many matrices DOC proves to play an important role not only for inorganic contaminants, but also for organic contaminants.

The leaching of inorganic contaminants is fairly well understood, and most of the leaching tests, which have been developed and standardised, are addressing inorganic components or very soluble organic components. The leaching of specific organic contaminants is much less understood, and no leaching methods for organic contaminants have yet been standardised. During recent years substantial R&D efforts have been spent on this issue, and progress is being made towards a better understanding of organic leaching and standardisation of test methods for leaching of organic components.

The hydrology of a scenario is crucial for understanding the potential impact from an alternative road construction material on soil and groundwater. Percolation and diffusion are relevant release mechanisms in this context.