W1170: Chemistry, Bioavailability, And Toxicity Of Constituents In Residuals And Residual-Treated Soils
Statement of Issues and JustificationDisposal 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, the potential toxicity of pollutants in residuals to the soil and water ecosystem, 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. In addition, research will look at issues surrounding other residuals added to soils and the data needs associated with those residuals (e.g. manures, water treatment residuals). 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 biosolids 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.
Importance of chemistry, bioavailability, and toxicity of waste constituents in soils
In 1935 C.E. Marshall assessed the current understanding of the behavior of phosphorus in soils:
In view of the importance of the phosphates in plant nutrition it might be expected that some exact knowledge of their mode of occurrence in soils would have accumulated during their eighty years of ever widening use of fertilizers. It is not so. The agriculturist has determined the response of phosphatic manures of every conceivable crop on every type of soil. He has encouraged the chemists in the invention of a multiplicity of empirical methods for the detection of phosphatic deficiency in soils. And now from a prodigious mountain of literature one may cull only a few crude surmises, a mere thimbleful of facts which approach the heart of the matter.
It can now be argued that our understanding of the behavior of plant nutrients in soils has greatly improved, providing that the appropriate exceptions are made. These exceptions might include all forms of nitrogen, and the behavior of nutrients in different matrixes including manures and municipal biosolids. Even in these instances, substantial progress has been made in understanding the plant available fraction of total nutrient concentration.
However, as progress has been made to understand the relationship between total nutrient content and plant available nutrient content with sufficiency as a focus, research has been redirected towards cases of excess. Here excess is defined for a much broader range of compounds and receptors. For xenobiotic compounds and certain contaminants, excess can be defined as anything above background. It is also possible to define excess as the quantity capable of causing harm to a specified human or ecological receptor. As the focus has switched from potential deficiency to potential excess, the definition of receptor has also been altered from agronomic crops to ecosystems. For nutrients, excess amounts of a range of nutrients can severely compromise ecosystems. Examples of this include excess quantities of N and P in surface waters as a result of non-point source pollution from agricultural fields. For contaminants, excess can lead to chronic or acute toxic effects for a range of receptors.
A recent National Research Council report stated that bioavailability processes are already embedded in our regulatory and decision making processes (National Research Council, 2003). However, it noted that very often, the concept of bioavailability is hidden in risk assessment and rule making processes. The panel encouraged broad recognition of this concept and research to understand it more fully. The potential implications for use of bioavailability in decision making extends from remedial options at hazardous waste clean up sites to fertilizer recommendations on agricultural fields
W-170 members are proposing to use the initial task of the group and approach taken by the group: understanding the chemistry and bioavailability of waste constituents in soils, to address issues relating to the chemistry, bioavailability, and toxicity of chemical and biological constituents in soils, residuals and residual-treated soils.
Justification: Although the agronomic benefits of organic residuals have been clearly demonstrated, concern over the behavior of contaminants in the residuals has been the focus of public concern, regulation and a large body of scientific research. Much of the research has centered on the behavior of metals and organics in biosolids amended soils. This research formed the basis for the EPA Part 503 sludge rule. This rule was recently recognized as one of the only rules to include bioavailability assessments in the development of appropriate limits for a range of contaminants (National Research Council, 2003). In addition, the scientific basis for this rule and the approach used have been the focus of two National Research Council panels, both of which concluded that the approach used for rule development was valid. The most recent panel suggested that the limits set out in the rule be revisited based on recent developments in risk assessment and that the risk assessment approach used to set metal limits in biosolids be expanded to set pathogen limits. Members of the W-170 group, and its predecessor, W-124 have been extensively involved in the development of this regulation and continue to be involved in the promulgation of risk assessment for other residuals (i.e., the EPA risk assessment for land application of cement kiln dust) and for other elements not initially regulated in Part 503 (i.e., Molybdenum). The scientific approach that was initially used in the development of the 503 regulations has been utilized by members of the group as their research focus has expanded to include a range of residuals, contaminants and receptors. As the understanding of bioavailability has broadened, the group has also broadened its focus to develop linkages between a quantitative understanding of the form of the contaminant and its bioavailability. Research has also been altered to measure effects of contaminants on a range of receptors.
Two examples illustrate the altered focus of the group and its suitability for research on these broadened topics: metal and phosphorus bioavailability. In the initial development of the 503 regulations the difference between total and bioavailable metal concentration was clearly illustrated through field studies that used metals added in biosolids rather than metals added to soils as salts to determine appropriate metal loading limits. With this research as a starting point, cooperative work within the group has altered its focus to develop a quantitative understanding of the behavior of metals in biosolids amended and other soil systems.
In recent studies, the role of organic and inorganic components of biosolids in metal binding has been defined through a combination of laboratory incubations, greenhouse studies, and the use of x ray adsorption spectroscopy (Brown et al., 2003a; Hettiarachchi et al., 2003; Ryan et al., 2003,2004; Scheckel et al., 2004). Biosolids and other soil amendments have been used to reduce the bioavailability of metals in contaminated systems (Brown et al 2003b; DeVolder et al., 2003; Hettiarachchi and Pierzynski, 2002). Extracts to assess bioavailability for a range of receptors have been developed and linkages between mineral form of inorganic contaminants and bioavailable fraction have been made (Basta et al., 2003; Brown et al., 2004; Ryan et al., 2004; Schroder et al., 2003). As the potential for metals to affect a range of receptors is more fully understood, research has broadened to encompass a range of measurement endpoints. The goal of this research is to evaluate function of the restored ecosystem and utilizes tools such as in vivo and in vitro assays, toxicity assays, and measures of microbial function (Alexander et al., 2003; Basta et al., 2003; Brown et al., 2004b; Schroder et al., 2003). As a result of cooperative research conducted by members of W-170, alternative in situ remedial options have been included on a number of EPA Superfund National Priority List (NPL) sites. These include use of biosolids to restore metal contaminated ecosystems. The tools developed for this research have also been applied to gain a fuller understanding of the functioning of biosolids amended soils. The sustainability of biosolids application to agricultural lands has been demonstrated by evaluating the effect of biosolids application on soil function. Potential receptors have included earthworms and soil microorganisms. While important initial research has been done in this area and implications of this research are being recognized in the remediation of contaminated sites, this type of work is still at an early, developmental state.
Initial work on biosolids and nutrients focused on determining the plant available fraction of total N. Work was done to predict the mineralization rate of organic N over time, under different management practices, and in different soils and climate regimes (Gilmour and Skinner, 1999) Application rate recommendations for agronomic crops were based primarily on meeting the N needs of the crop. Determination of the plant available N (PAN) based on characteristics of the residual being applied is still a focus of research with better approximations potentially existing for different biosolids in comparison to manures (Andraski et al., 2000; Gilmour et al., 2003; Van Kessel et al., 2000) Initially, there was very limited research on the availability of P in biosolids amended soils as the Part 503 regulations based agronomic loading solely on N needs and utilization potential of a crop (McLaughlin and Weil, McLaughlin). With the understanding that excess soil P can negatively impact natural waters, there is a growing body of research and regulation that considers P solubility and potential for runoff from different P sources. Several states are moving towards P based loading limits for manures, biosolids, and fertilizers. In addition, the recently promulgated Confined Animal Feeding Operations (CAFO) regulations, and Natural Resource Conservation Service Code 590 Practice standards for Nutrient Management require consideration of P as well as N for utilization of manures and biosolids.
A large body of cooperative research, both in communication of information and collaborative laboratory and field studies is currently underway with members of the W170 group to evaluate characteristics of biosolids and how they alter the phytoavailable fraction of soil P, use of residuals to reduce P availability in soils, and appropriate tools to measure excess P in soil and water systems. The effect of chemical characteristics of biosolids on P availability and evaluating P availability in a range of biosolids and biosolids amended soils has been an area of emphasis within the group (Brandt et al., 2004; Elliot et al., 2003; O'Connor et al., 2004; Sakar and O?Connor; 2004). Use of residuals to reduce the availability and leachability of excess P in soils has also been a focus of the groups research (Codling et al., 2000; Dayton et al., 2003; Ippolito et al., 2003). This research has shown that characteristics of particular biosolids including high Fe and Al concentrations are able to reduce the solubility of biosolids P over both commercial fertilizers and animal manures. In some cases this reduction is sufficient to limit the utility of biosolids as a P source, even when applied to meet the N needs of a crop. In addition, other waste water residuals including water treatment residuals have a very high capacity to adsorb P and thereby reduce its potential to run off a soil surface or leach through a soil profile. These results suggest that tools are available to reduce the environmental hazards associated with excess P in soils and that nutrient management plans need to take into account the source of P in developing land application recommendations. It is still necessary to develop a fuller understanding of the longevity of the P sorption mechanisms and the role of soil properties in altering P availability. Additionally, the likelihood of loss of soil particulate matter under different tillage operations and management controls is required to determine the likelihood of runoff on a site specific basis.
Both these examples illustrate the change in the focus of the group to concerns on the bioavailability of inorganic and organic compounds in soils where excess or potential detrimental affects, rather than deficiencies are the focus. They also reflect the change in emphasis from a plant to an ecosystem focus. It is also clear that this type of research is in its early stages.
In addition to the cooperative research areas described above, there are emerging concerns on the fate of new classes of organic chemicals in soils. Classes of organic compound that are now the focus of concern include estrogenic compounds, personal care products, and pharmaceuticals (Topp and Clucci, 2004; Xia and Pillar, 2004)). A recent report by USGS noted the presence of a wide range of organic compounds in streams in the vicinity of waste water treatment plants and confined animal feeding operations. The fate and persistence of these compounds during biosolids stabilization processes or following land application of biosolids or manure is not known. Persistence in soils and potential for ecosystem effects as result of land application of residuals containing these materials is a topic that will require research.
In the recent NRC report on biosolids, the panel recommended that a risk based approach be used to set acceptable pathogen loading limits for land applied biosolids. The potential to understand the fate of pathogens following land application has increased with the development of new scientific methodology. Recent studies have focused on aerial transport of organisms from land application sites (Rusin et al., 2003). For both of these areas, large knowledge gaps exist. The methodology to answer these research questions falls within the context of the bioavailability/pathway based approach that was initially utilized in the W-170 group and remains a central theme of the groups? current research endeavors. Accordingly, the next five year W-170 project proposes to address these issues as well as other emerging concerns.
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