W170: Chemistry and Bioavailability of Waste Constituents in Soils
Statement of Issues and Justification
STATEMENT OF THE PROBLEM:Disposal of residual waste products is a problem that requires practical scientific information to determine if the residual constituents can be safely reused without harming the environment or unfavorably impacting nutrient and trace element pathways. Land application of a variety of residual materials is known to be an effective means of recycling organic matter and plant nutrients, but must be done prudently to avoid degradation of the soil as a medium for plant growth. W-170 committee members are proposing to enhance ongoing research through the evaluation of biogeochemical cycling of plant nutrients, the movement of trace elements into the food chain, and the long-term bioavailability of trace elements in residuals and residual-amended soils. Research will continue to focus on information related to the EPA 503 rules in order to provide support for risk assessment of land-applied biosolids. Numerous long-term studies by W-170 members have been, and are currently being, conducted to address the hypothesis that sequestered metals will be released as the biosolid organic matrix is mineralized. Residual materials will be emphasized in the W-170 continuation project so that waste utilization is done in a manner that protects the sustainability of U.S. agriculture.
JUSTIFICATION:
The agronomic benefits from the use of various inorganic and organic residuals have long been recognized in agriculture, horticulture, and reclamation. These materials can provide nutrients, improve soil physical properties, and/or liming value. There is a considerable knowledge base regarding the beneficial reuse of manures and biosolids, but many other residual materials are also potentially recyclable. Some additional residual materials that have been amended to soils include: municipal solid waste (MSW) composts, yard wastes, cement kiln dusts, pharmaceutical biomass, brewery wastes, flue-gas desulfurization by-products, drinking water treatment residuals, wood ash, and food-processing wastes. With the costs of incineration and disposal in landfills increasing dramatically, the quantity and variety of residuals that are being considered for land application is also increasing. Several key issues have been examined in past W-170 research, such as: 1) determining the availability of plant nutrients in the residuals; 2) determining the bioavailability of trace elements in the residuals and soil-residual mixtures; and 3) determining the content and fate of other contaminants, e.g., pathogens, xenobiotics, and salts, in the residual materials. Efforts are underway to incorporate land application of residuals with assessment of soil quality (Sims and Pierzynski, 1998). The W-170 Committee and its predecessors (W-124 and NC-118) have been actively involved in research and regulatory aspects. As recently as 1998, a sub-committee of W-170 provided a critical peer review of an EPA risk assessment for the land application of cement kiln dust. The W-124 and W-170 committees were extensively involved in the development of the EPA 503 national sludge rule (USEPA, 1993), and continue to be involved in the refinements of that rule.
Despite the knowledge base that exists, new issues demand further research as more regulations are being written and new concerns arise. Several examples will illustrate this point. The EPA 503 national sludge rule provided limits for the concentrations of 10 trace elements (As, Cd, Cu, Cr, Hg, Mo, Ni, Se, Pb, and Zn) in biosolids, and limits on the annual and cumulative loadings of the trace elements to soils. The W-170 Committee continues to build the data base for elements such as Cr, Mo and As to address critical gaps in our knowledge related to these trace elements. Further, subsequent to the publication of the rule and the pathway analysis used in the risk assessment, the protectiveness of certain aspects of the rule was called into question (McBride, 1995; Schmidt, 1997). The major issue of concern was the long-term bioavailability of trace elements in biosolids-amended soils. The W-170 Committee continues to address this issue by utilizing data from several long-term biosolids studies and by working toward the development of new techniques for assessing trace element bioavailability in soils. Yet another issue revolves around the chemistry and bioavailability of P in waste residuals and residual-amended soils, which is increasing in importance as national or state regulations are being proposed or enacted that will limit application rates for residuals based on P rather than on N. Accordingly, the next five-year W-170 project proposes to address several emerging issues such as: 1) Utilization of new and novel residual materials; 2) Expanding the data base, when warranted, for trace elements such as Cr, Mo, As, and Se for risk assessment for biosolids and for all trace elements for other organic and inorganic residuals that may be used in agriculture, horticulture, or reclamation; 3) Methods development for estimating trace element bioavailability in residuals and residual-amended soils; 4) Assessment of long-term bioavailability of trace elements in residual-amended soils, and; 5) Chemistry and fate of plant nutrients, particularly P, in residuals and residual-amended soils.
New and novel residual materials are continually being considered for land application or for horticultural uses. Commercial blending of a variety of residual materials to produce synthetic soils for reclamation and horticultural uses is increasing dramatically. Unprocessed animal manures have been studied extensively, but these materials are being processed or amended more often in an attempt to improve aesthetic issues, reduce volume, or to decrease plant nutrient content or availability (e.g., alum amendment). Composting is being used on a wider variety of materials that are then considered for land application. These new situations warrant study as the appropriate land application guidelines are developed, and to fully understand the risk/benefit issues associated with each material.
Development of the USEPA 503 rule relied on an extensive data base for trace elements such as Cd, Cu, Cr, Ni, Pb, and Zn in biosolids-amended soils. A shortage of data still exists for elements such as As, Hg, Mo, and Se that needs to be addressed. For example, the original EPA 503 rule provided a cumulative load limit for Mo of only 18 kg/ha, based on limited data, which would have made Mo a very restricting element for land application programs. Conversely, there is a legitimate concern about Mo-induced Cu deficiency (molybdenosis) in livestock that could develop if the Mo limit were not restrictive enough. To set a limit that is sufficiently protective without being unnecessarily restrictive requires a data set that encompasses a wide range of soil and climatic conditions. The only study that has significantly added to this data base since the EPA 503 rule was written is being conducted by a member of the W-170 group (Nguyen and OConnor, 1997). In addition, there are growing concerns about trace elements in other organic residual materials that have not received much attention in the past. Examples include Cu and Zn in swine manure and As in poultry manure.
To improve our understanding of the fate and transport of trace elements in residuals and residual-amended soils, the methods for assessing trace element bioavailability need refinement and development. A variety of useful new methods are available that have either not been applied to the objectives of this project or have been applied only to a limited extent. The bioavailability, fate and transport of trace elements in residual-amended soils is influenced by the chemical form of the elements: organic versus inorganic, solid phase versus adsorbed, solubility of trace element solid phases, co-precipitation with other mineral phases, etc., and little is known about how trace elements are actually partitioned into the various chemical forms. Sequential extraction techniques and solubility equilibrium studies have been of some value, but recently developed or improved techniques, such as analytical electron microscopy and the synchrotron-based methods like microprobe extended x-ray fine structure (EXAFS), x-ray absorption near-edge spectroscopy (XANES), and microprobe x-ray fluorescence, offer considerable promise and have been used on trace element problems to a limited extent. In addition, procedures more specific to data necessary for risk assessment, such as the physiologically-based extraction technique (PBET, Ruby et al., 1996), which is an in vitro method for assessing the bioaccessibility of Pb and As in soils to humans, have not been adequately utilized for residual-amended soils and may be quite useful (Rodriguez et al., 1998).
The degree of protectiveness provided by the EPA 503 national sludge rule has been criticized from several fronts. One is the possibility that the organic C added with the biosolids will eventually oxidize allowing increased trace element bioavailability over time, a factor that was not considered in the risk assessment for the regulations (McBride, 1995). This phenomena has been called the time-bomb hypothesis and has generated considerable discussion in the scientific community and some public opposition to land application of biosolids. This hypothesis is already being considered by the W-170 committee by utilizing long-term biosolids studies (Chang et al., 1997; Brown et al., 1998) and will continue to be addressed in future work. A second concern relates to the possibility of more subtle effects of trace elements on soil microbial populations (McGrath et al., 1995). Based on these and other issues related to trace element bioavailability, we are required to improve our methods for assessing the bioavailability of various constituents in residuals and residual-amended soils. For example, trace element phytoavailability can be predicted fairly well with routine soil extractants for a given soil/residual combination, but we do not have a method that performs satisfactorily across a wide range of soil and climatic conditions. Much of the earlier work by W-124 and W-170 committee members focused on the availability of N in organic residuals for crop use. These efforts fulfilled the need of determining the agronomic loading rate for biosolids mandated by the EPA 503 regulations, and the methods that were developed are applicable to many residual materials. Further refinements are still needed as it becomes more important to accurately determine agronomic loading rates. It is well established that applying most residual materials based on N results in over application and accumulation of P in soils. This has become an important issue recently as concerns about P and water quality increase (Sharpley et al., 1994). One result has been the development of regulations stipulating that land application of residuals be based on P availability. Also of interest is the reduction of P concentrations in soils that have received large amounts of residuals so that the threat to surface waters is reduced. In both cases, our understanding of P chemistry is incomplete. Even on an applied level, there is a need for data relating P loading rates from residuals to changes in soil test P levels and then for determining the agronomic and environmental significance of those levels.
These emerging issues will be addressed with the following research objectives: 1) Characterize the chemical and physical properties of residuals and residual-amended soils; 2) Evaluate methods for determining the bioavailability of nutrients, trace elements, and organic constituents in residuals; 3) Predict the long-term bioavailability of nutrients, trace elements, and organic constituents in residual-amended soils.
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