Information developed over the years suggests that sustainable land application systems can be established and maintained under a wide range of conditions. Optimal crop yields can be achieved using effluents as a water supply and source of nutrients, and effluents and organic residuals can serve as a source of nutrients and soil conditioner. Contaminants (e.g., excess trace metals, toxic organics, nitrogen and phosphorus, and pathogens) can be controlled and effectively managed by land application systems. A stream or lake responds to specific nutrient, trace metal, inorganic, and organic contaminant additions in a similar manner whether the input comes from point- or nonpoint-source municipal, industrial, agricultural, or atmospheric sources (Reid, 1961; National Research Council, 1969; Burns, 1985). Likewise, many biological and chemical reactions in the soil are independent of the source of the contaminants (municipal or industrial effluents, sludges, manures, or other organic residuals), unless they are derived from material with low organic matter content (e.g., metal solutions such as plating industries, grit from highway runoff).
The Role of Soil Reactions
Soil reactions with contaminants in waste represent the key to sustainable land application systems. The soil and associated microorganisms and vegetation react to the specific contaminants in land-applied residuals and modify the contaminants through direct oxidation–reduction reactions, adsorption–desorption, biodegradation, and plant uptake. In some cases the reactions are temporary, while in other cases they are essentially permanent, or nearly so unless the overriding factors controlling the soil properties are changed by external sources (Page et al., 1987; National Research Council, 1996; Power and Dick, 2000). Thus, scientists need not "reinvent the wheel" by studying the fate of every contaminant that may be present in every waste source to predict how the contaminant behaves in land application systems over time. The extensive body of available technical information can be effectively applied in the development of technical guidance and regulatory requirements for all sustainable land application practices (USEPA, 1992, 1993, 2004a; National Research Council, 2002). However, such technical information is only part of what goes into developing sustainable land application projects that are guided by the applicable regulatory requirements and real-world management practices.
Establishing Controls Based on Science and Risk Assessments
The controls imposed on land application practices seek to protect public health and the environment, but also consider factors such as available control technologies, cost effectiveness, public policy objectives, public acceptance, and political realities. Early land application requirements were focused on good management practices and the general consensus of the scientific community (best professional judgment), softened by the realities of best available control technologies and affordability. The "soil–plant barrier" was regarded as providing an effective means of protecting humans from exposure to excessive levels of most chemical contaminants in the food chain (Chaney and Giordano, 1977; Chaney, 1980, 1983a, 1983b). This "barrier" was viewed as limiting the transmission of heavy metals through the food chain as a result of soil chemical processes that limit solubility thereby preventing plant uptake (e.g., soil barrier), immobility in fibrous roots preventing translocation to edible plant tissue, or phytotoxicity (e.g., plant barrier) occurring when concentrations in edible plant tissues are below that injurious to animal consumers. More recently, loading limits for specific chemicals have been developed by various means, ranging from not allowing any increase in background chemical concentrations (nondegradation strategy) to establishing acceptable levels based on various risk assessments and modeling approaches. Pathogen controls have primarily been based on treatment through process technology controls and waiting periods to allow for natural die-off.
The basic paradigm used for human health risk assessment of hazard identification, dose–response assessment, exposure assessment, and risk characterization (National Research Council, 1983) has become the usual framework for development of many of the regulations in the United States, although less so in Europe. The approach was used to establish limits on annual and total chemical contaminants applied with biosolids, taking into account such factors as the bioavailability of contaminants in sewage sludge (USEPA, 1993, 1995c), to assure that land application practices are sustainable. The risk assessment–based approach is data intensive, and often leads to the use of conservative default values and sensitivity analyses or Monte Carlo simulations to address areas of uncertainty. Concerns raised over emerging pathogens and chemicals for which little or no data are available tend to be put off for future consideration when more adequate scientific data are generated. Groups following the precautionary principal advocate limitations based on rates that do not lead to increases in background chemical concentrations or banning or prohibiting land application until the documented assurance that negative effects on health and ecosystems will be avoided (Marchant, 2003; Harrison et al., 1999; Mindfully. org, 2004; Ag BioTech InfoNet, 2004). As a result, future efforts to better address pathogen concerns and potential ecological effects associated with land application practices will likely use recently developed methodologies for conducting pathogen and ecological risk assessments (Colford et al., 2003; World Health Organization, 2001; National Research Council, 2002; USEPA, 1998), for example, the USEPA's Guidelines for Ecological Risk Assessment and the ongoing Water Environment Research Foundation (WERF)-sponsored project, "A Dynamic Model to Assess Microbial Health Risks Associated with Beneficial Uses of Biosolids" (Project 98-REM-1A). Bioassay techniques for better evaluating the effects of multiple stressors, complex mixtures, and compound by-products resulting from degradation in the environment as well as contaminants that may be present below effective analytical detection limits may also play a role in future research efforts and may be used eventually as a part of future regulatory strategies.
The Role of Active Stakeholder Involvement and Risk Communication
The importance of public involvement and public acceptance in maintaining sustainable land application projects simply cannot be overstated. Early efforts to use land application practices were usually conducted in relatively isolated areas with few neighbors. However, most modern projects are faced with the realities of local neighbors. In many cases, individuals and/or public interest groups regard even the concept of land-applying wastewater effluents, biosolids, manures, and other residuals as unacceptable. Many states have adopted standards to accommodate effluent reuse (e.g., Florida, California, Texas, Arizona, and Washington) as well as broad goals calling for increased recycling of organic wastes in an effort to conserve landfill capacity (e.g., New Jersey Department of Environmental Protection, 2003; North Carolina Department of Environmental Regulation, 2000). In general, these standards and goals strongly support the objectives of land application practices. However, "not in my backyard" (NIMBY) reactions by local neighbors to notices of proposed land application projects and to the start-up of new projects are now the norm. Unless citizen concerns and interests are taken into account and accommodated by planned projects, such local concerns can easily grow into formal project opposition. This may result in the involvement of external groups and individuals with even broader agendas to create political and legal barriers to moving projects. Public notification requirements associated with most regulatory programs are often viewed as exacerbating these reactions. However, they also create opportunities for active public education and involvement in programs that can help lead to long-term support of well-run land application projects.
Addressing public concerns through active stakeholder involvement and effective risk communication is an important aspect in developing sustainable land application projects. The basic human health risk assessment model developed by the National Research Council (1983) emphasizes the role of "risk management" in assimilating nonscientific factors into making policy decisions and the need for effective "risk communication" to communicate such policy decisions. While scientists generally view risk as the nature of harm that may occur, the probability that it will occur, and the numbers of people that will be affected (Groth, 1991), most citizens are concerned with broader, qualitative attributes: whether risk is imposed or voluntarily assumed, the equitable distribution of risk over a population, alternatives, and the power of individuals to control the risk (Sandman, 1987). Research has identified numerous factors that influence the "perception of risk" including the origin of the risk (natural vs. technological), benefits realized from accepting risk, and trust. Such factors are important aspects of effective risk communication and underlie the two basic considerations that typically drive effective risk communication (National Research Council, 1989): (i) perception can change even if the actual risk does not and (ii) perception is the reality you have to deal with.
As a result, models for effective risk management and the risk communication cycle (Soby et al., 1993) generally call for the concerns of the public and other stakeholders to be actively sought at each stage of the risk management process, including risk assessment (Fig. 2) .
The Effect of Odors
The initial basis for local concerns is often linked to the production of odors and/or other nuisance conditions (e.g., noise, dust, flies, truck traffic) associated with land application projects. Where such concerns have been effectively addressed through changes in treatment processes, on-site storage practices and/or field operations, local acceptance problems have often been overcome or avoided (e.g., Hamel et al., 2004
). When such concerns are ineffectively addressed, complaints about nuisance conditions often escalate to complaints about potential health effects that may result from the odors, potential bioaerosols, dust, and/or runoff from the land application site. The question, When does exposure to odors or dust and the compounds and/or potential bioaerosols affect health?, probably varies widely with individual sensitivities (Schiffman et al., 2000
). Highly sensitized individuals can react to exposures to smoke, perfume, cleaning agents, and certain foods (e.g., sources containing lactose, gluten, citric acid, shellfish, nuts, and even chocolate). Regulating land application of wastewater effluents, biosolids, industrial residuals, and manures based on the potentially most sensitive individuals to odors may lead to very restrictive practices.
Going Beyond Meeting Minimum Regulatory Requirements
Meeting Multiple Legislative Mandates
Efforts to actively involve potentially affected and interested stakeholders in the development and implementation of sustainable land application practices has lead to the establishment of various types of voluntary partnerships. Environmental Management Systems, ISO 14,001 Standards, and various other coalitions that follow Dr. W. Edward Demming's basic "Plan/Do/Check/Act" total quality management model principles (Demming, 1986) have been established (Fig. 3) . These approaches encourage achieving continuous system improvement and exceeding minimum regulatory requirements, along with the use of third-party verification. The purpose is to resolve and solve issues and concerns associated with land application practices to attain less controversy and greater sustainability.
Legislative mandates in the Clean Water Act (CWA), Resources Conservation and Recovery Act (RCRA), National Environmental Policy Act (NEPA), and various other federal and state statutes contain numerous provisions encouraging the safe and beneficial recycling of wastewater effluents, biosolids, and other organic residuals. The legislation also requires establishing guidance and regulatory requirements for various land application practices as well as other use and disposal practices. All of the mandates, as well as well-established formal rule-making process requirements, must be considered during the development of regulations that affect land application practices.
In addition to formal regulations, both federal and state agencies often develop and issue policy and guidance documents to help explain regulations and voluntary programs, as well as to provide technical assistance. While the agencies are generally committed to using sound science in their decision-making, many other factors affect development of policy, regulations, and guidance documents. These factors include implementation costs, technical feasibility, economic effects on small businesses, legal requirements, and social and political considerations. The agency staff and contractors involved with conducting risk assessments and the development of background documents associated with rule-making efforts on behalf of the regulatory agency are not experts in all of the areas of science involved with a rule-making effort. Commonly, highly conservative approaches are taken when addressing technical issues where there are, or appear to be, technical references with conflicting conclusions in the published literature. For example, information used to develop the proposed Part 503 rule concerning land application of sewage sludge included data from greenhouse studies involving sludges spiked with metal salts. These data were included despite the availability of better (more scientifically desirable) data from field studies involving sludges with elevated levels of the same trace metals. In other situations, attempts have been made to use the risk assessments and other analyses conducted in developing the Part 503 requirements for land application of sewage sludge as the basis for controlling contaminants in fertilizers and other nonorganic materials without consideration of the biosolids matrix effects and default parameters. Initial efforts to develop binding guidance or regulations for controlling certain practices are sometimes based on conservative assumptions rather than the latest science. Such requirements are open to relaxation if and when adequate technical information is provided to justify such changes in future versions of guidelines or regulations. Several sections of the Part 503 requirements for land application of sewage sludge were based on the 98th percentile quality of biosolids in the National Sewage Sludge Survey (USEPA, 1990) rather than risk assessment-based values, only to be deleted from the rule after successful court challenges after the rule was promulgated in February 1993. A good example of nontechnical influences affecting the rule-making process is reflected in the changes to the initial version of the standards issued by USDA for "Organic Foods" (7 CFR Part 205) in December 2000 under the authority of the Organic Food Production Act of 1990. The final version excluded use of sewage sludge, genetically modified crops, or irradiation in response to 40774 public comments received in response to the more flexible initially proposed standards.
Improving Disturbed and Contaminated Sites
At least some of the concerns about and resistance to land application can be overcome by projects that help address other environmental problems such as the restoration–revegetation–rehabilitation of highly disturbed and contaminated sites. Thirty years of field experience and well-documented research and demonstration projects prove that land-applied organic residuals are effective tools in reducing the bioavailability of trace metals and establishing sustainable vegetative cover on a number of highly contaminated sites, including a number of Superfund and Brownfield sites (Ryan et al., 2004; Brown et al., 2003a, 2003b, 2004; Rocky Mountain Water Environment Association, 2000; USEPA, 1995b; Sopper, 1993; Jacobs et al., 1993).
The Importance of Nutrient Management
Sustainable land application projects should strive to effectively match the nutrient needs of the crop or vegetation to be grown on the site. Thus, comprehensive nutrient management planning (CNMP) is critical for sustainability both agronomically and environmentally. Done well, CNMP avoids the buildup of nitrates, phosphorus, or other nutrients in amended land to the point where they become contaminants in the soil, storm water runoff, and/or underlying ground water (Sharpley et al., 2003; USDA Natural Resources Conservation Service, 1992, 2003).
The Role of Systematic Research Programs
Systematic research programs have advanced the understanding of how soil-based treatment systems work, addressed new areas of concern when they arose, and improved the overall design, performance, and reliability of land application systems as sustainable soil treatment and recycling systems. Notable examples are shown in Table 1.
These, and other facilities across the country, continue to collect important data associated with the performance of operating land application projects. However, few programs are collecting the type of data needed to effectively respond to the questions being raised about emerging pathogens (World Health Organization, 2001, 2003a) and new chemicals of concern (National Research Council, 2002; Stevens et al., 2003; Hale et al., 2003) associated with land-applied wastewater effluents, biosolids, industrial residuals, and manures. Research projects supported by the Water Environment Research Foundation, USEPA, USDA, and others will help fill these information gaps over time, but more effort is needed. Systematic research programs to support studies aimed at producing information that advances our understanding of how soil-based treatment systems work, to address new areas of concern as they arise, and to continue to improve the overall design, performance, and reliability of land application systems as sustainable soil treatment and recycling systems are as important today as ever.
The views expressed in this paper are those of the author and not official USEPA policy.