ALBERT G. MEDVITZ (P.O. Box 565, Rio Vista, CA 94571. Email: amedvitz@wco.com)
In 1993, the United States Environmental Protection Agency (USEPA) published new regulations (40CFR 503) for the management of sewage sludge, including requirements for pathogen reduction and pollutant limits in sewage sludge applied to farmland. These standards, dramatically different from earlier ones, have generated conflict and controversy in the scientific and agricultural communities and the general public. Sewage sludge is composed of the residue remaining after treatment of household and industrial wastes by urban sewage systems. In some municipalities sludge which may be land applied has been stored for decades. Pathogen levels vary depending on treatment practices. Thus, sewage sludges are heterogeneous complex entities with pollutant and pathogenic characteristics that vary across facilities and over time. The literature available to the EPA to assess risks associated with pollutant pathways relevant to livestock production is biased towards investigations of the concentration of pollutants in animal tissues and relatively short term toxicity effects. Organic pollutants, some of which may interfere with endocrine systems, were omitted from the EPA 503 risk assessment because of assumptions and policy decisions made about sludge borne pollutants. This presentation will discuss the implications of the limitations of the knowledge base and the EPA assumptions on assessing risks for sludge application to lands used for livestock production.
M.B. McBRIDE and D.R. BOULDIN (Dept. of Soil, Crop and Atmospheric Sciences, Cornell University, Ithaca, NY 14853)
Uptake coefficients (UC's), which express the amount of a metal taken up by a crop relative to the amount applied to the soil. are critical to the EPA 503 risk assessment for entry into the human diet of toxic metals (such as Cd, Hg and Pb) added to soils from the use of sludges as fertilizer. This UC approach of EPA, and the data upon which it was based, were reevaluated. It is shown that the final UC estimates employed in the risk assessment are biased toward low values by a number of factors. These include (a) a systematic analytical or contamination error apparent in the reported metal concentrations of the control crops, (b) the use of geometric means rather than arithmetic means or probabilistic methods to obtain single-point "averages" of UC for each crop group evaluated, (c) the fact that most of the UC values were derived for soils with pH 6 or higher. It is concluded that the EPA estimates of uptake coefficients, for Pb and Hg in particular, may have been compromised by sensitivity limits and errors in the analytical techniques used, and by crop contamination from atmospheric deposition and soil particles. Contamination of leafy crops by soil particles may explain why most of the Pb concentrations reported experimentally in control as well as sludge-grown crops are higher than for comparable farm-grown crops. Although the Cd UC's seem to be less affected by analytical or contamination error, they vary by a factor of 10,000 in leafy vegetables as a result of uncontrolled variables inherent to field experiments. The EPA point estimates of UC are low relative to most of the individual field measurements of UC, because of the technique of averaging by geometric means. For more than 50% of all soil conditions and crop variability represented in the risk assessment, the average UC used by EPA, and therefore the risk posed by Cd uptake into crops, is underestimated.
D. L. LEWIS (U.S.EPA, National Exposure Research Laboratory, Ecosystems Research Division, 960 College Station Road, Athens, GA 30605).
Determining concentrations of specific types of viable microorganisms in terrestrial and aquatic environments presents numerous challenges that should be taken into account with regard to a wide variety of public health concerns. One such area involves biosolids applied to land for agricultural and other purposes, which are treated to reduce levels of human pathogens, including Salmonella, Shigella. Vibrio, Yersinia, Campylobacter, hepatitis A, rotavirus, coxsackii viruses, and Cryptosporidium. Maximum allowable pathogen concentrations are based on epidemiological data indicating quantitative relationships between exposure levels and infection rates. Such relationships reflect the combined effectiveness of host defenses in a normal human population. They do not account for the increased susceptibility of a portion of the population that is immunocompromised. This portion is steadily increasing as the population ages and survival rates improve for a host of conditions including patients with chronic respiratory diseases, AIDS, and diabetes, as well as organ transplant recipients and patients undergoing radiation or chemotherapy. Accounting for risks of infection to this segment of the population would require further reducing the acceptable levels of pathogens in biosolids and monitoring indicator organisms that characteristically plague immunocompromised individuals. Candidate organisms could include Pneumocystis, Mycobacterium, Norwalk virus, or other environmentally stable infectious agents. Complicating the effort to assess exposure risks is the absence of any system for tracking and determining the source of occasional infections with environmental origins, and a myriad of issues pertaining to the effectiveness of standard culturing techniques for quantifying concentrations of microorganisms in the environment.
UTA KROGMANN (Rutgers University. Department of Environmental Sciences, 14 College Farm Road, New Brunswick, NJ 08901-8551) and ELLEN Z. HARRISON (Cornell Waste Management Institute, Rice Hall. Ithaca, NY 14853-5601).
For all contaminants except lead, the US EPA standards for land application of sewage sludges are significantly less stringent than standards in northern and central European countries. The cumulative pollutant loading allowed under EPA rules would result in contaminant levels approximately an order of magnitude higher than those allowed under rules in northern and central Europe. A significant fraction of European sludges are able to meet the standards and are thus being applied to land. Regulations in the US versus countries such as Denmark, Germany, the Netherlands. Sweden and Switzerland are based on different philosophies. The US regulations are based on a risk assessment which evaluates the potential risk to humans, other animals, and plants from selected pollutants in sewage sludge (14 pathways). The standard for a contaminant is set at the limit generated by the pathway resulting in the lowest concentration that represented an acceptable risk according to the assessment. Many European regulations are based on precautionary limits in which in the long-run a net balance between the input and output of pollutants in the soil is sought. Recognizing the persistence of inorganic contaminants, the goal is to prevent the accumulation of inorganic contaminants above levels in uncontaminated agricultural soils. Even when a similar risk assessment approach is taken (e.g. Netherlands soil intervention values), much. lower pollutant limits result from different assumptions and policy choices. In the long-run these different approaches could result in much higher pollutant concentrations in US agricultural soils than in European soils.