End-Product Standards for Compost

Transcription

1 March 1996 RG-217 End-Product Standards for Compost printed on recycled paper Office of Air Quality TEXAS NATURAL RESOURCE CONSERVATION COMMISSION

2 End-Product Standards for Compost Prepared by Toxicology and Risk Assessment Section Office of Air RG-217 March 1996

3 Barry R. McBee, Chairman R. B. Ralph Marquez, Commissioner John M. Baker, Commissioner Dan Pearson, Executive Director Authorization for use or reproduction of any original material contained in this publication that is, not obtained from other sources is freely granted. The commission would appreciate acknowledgment. Copies of this publication are available for public use through the Texas State Library, other state depository libraries, and the TNRCC Library, in compliance with state depository law. For more information on TNRCC publications call 512/ or visit our Web site at: Published and distributed by the Texas Natural Resource Conservation Commission PO Box Austin TX The TNRCC is an equal opportunity/affirmative action employer. The agency does not allow discrimination on the basis of race, color, religion, national origin, sex, disability, age, sexual orientation or veteran status. In compliance with the Americans with Disabilities Act, this document may be requested in alternate formats by contacting the TNRCC at (512) , Fax , or RELAY-TX (TDD), or by writing P.O. Box 13087, Austin, TX ii

4 TABLE OF CONTENTS ABBREVIATIONS...v EXECUTIVE SUMMARY...vii FIGURE EX-1: Development of Recommended End-Product Standards for Grade 1 and Grade 2 Compost... ix TABLE EX-1: Recommended End-Product Standards for Grades 1 and 2 Compost...x INTRODUCTION...1 METHODS AND RESULTS...3 Chemicals of Potential Concern...3 Inorganic Chemicals...3 Organic Compounds...4 Exposed Populations...4 Exposure Pathways...4 Exposure Limits...5 Exposure Limits Based on Potential Ingestion of Compost...6 Exposure Limits for Other Relevant Exposure Pathways...7 Limiting Exposure Pathway...7 End-Product Standards...8 Grade 1 Compost...8 Grade 2 Compost...9 DISCUSSION...11 Chemicals of Potential Concern...12 Inorganic Chemicals...12 Organic Compounds...12 Combined Effects of Multiple Chemicals...13 Exposed Populations...13 Exposure Pathways...13 Exposure Limits...14 Toxicity Values...14 Parameters Used In Estimating Daily Intake...16 Net Accumulation of Chemicals in Compost...17 Limiting Exposure Pathways...19 End-Product Standards...19 Data Limitations...20 Grade 1 Compost...20 iii

5 Grade 2 Compost...22 Summary/Recommendations...22 REFERENCES...25 APPENDIX...29 TABLE A-1: Exposure Pathways...30 TABLE A-2: Oral Reference Doses (RfDs) and Total Background Dietary Intake (TBI) for the Chemicals of Concern in Compost...31 TABLE A-3: Exposure Limits Based on Potential Human Ingestion of Compost...32 TABLE A-4: Lowest Exposure Limit for Grade 1 Compost (residential scenario)...33 TABLE A-5: Lowest Exposure Limit for Grade 2 Compost (non-residential scenario)...34 TABLE A-6: Grade 1 End-Product Standard Selection Criteria...35 TABLE A-7: Grade 2 End-Product Standard Selection Criteria...36 iv

6 ABBREVIATIONS CFR EPA IRIS IEUBK MSW PCBs ppm RDA RfD RPc RSC TARA TBI TNRCC Code of Federal Regulations Environmental Protection Agency Integrated Risk Information System Integrated Exposure Uptake Biokinetic Model Municipal Solid Waste Polychlorinated Biphenyls parts per million Recommended Daily Allowance Reference Dose Reference Cumulative Application Rate Reference Concentration Toxicology and Risk Assessment Total Background Dietary Intake Texas Natural Resource Conservation Commission v

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8 EXECUTIVE SUMMARY Staff of the Toxicology and Risk Assessment (TARA) Section of the Texas Natural Resource Conservation Commission (TNRCC) have developed recommended end-product standards for municipal solid waste (MSW) compost. An end-product standard is defined as the maximum allowable concentration of a chemical in MSW compost that would not be expected to cause adverse affects in humans, domestic animals, or plants when MSW compost is used. A decision was made by the TNRCC to develop end-product standards for two grades of compost in order to provide the user with a choice of different qualities of compost. Grade 1 compost will be a "residential use" compost and the end-product standards have been developed based on a residential exposure scenario (e.g., compost is applied to home yards, gardens, and planting beds). Grade 2 compost is a "non-residential use" compost (e.g., compost is applied to agricultural, commercial, institutional, or governmental grounds) and will have less stringent end-product standards as human exposure is expected to be limited. Since Grade 1 compost will have the more stringent end-product standards, it can also be used for nonresidential purposes. However, both grades of compost must be mixed into the soil if used to grow animal or human food crops in order to prevent significant food chain effects. In addition, it is recommended that neither grade of compost be used as the sole growth medium since using only compost to grow plants (e.g., potted plants) may cause phytotoxicity. End-product standards were developed for 10 inorganic (arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc) and one class of organic compounds (polychlorinated biphenyls (PCBs)). All reasonably anticipated exposed populations (i.e., humans, domestic animals, and plants) and the respective expected exposure pathways were identified for the residential (Grade 1) and non-residential (Grade 2) usage patterns of compost. Exposure limits (i.e., the maximum allowable concentrations in compost that would not be expected to adversely affect the receptor) were determined for each potential pathway of concern for both grades of MSW compost. Some of the assumptions that were made in developing exposure limits are: 1) the compost is applied annually at the same rate and location for 30 years; 2) the pollutant concentrations are equal to the end-product standards each year; 3) the compost may be applied to areas where minimal or no mixing with soil would occur; although, as stated above, when applied to food crops or as a sole growth medium, it is recommended that the compost be mixed with soil; 4) 50% of the non-persistent organic matter in MSW compost decomposes prior to the next application; and 5) all of the chemicals of concern from the previous years' compost become incorporated into the most recently applied compost. A complete description of the rationale and uncertainties associated with these assumptions and others, is contained in the body of this report. Recommended end-product standards were determined as outlined in the flow chart (Figure EX- 1) and are listed in Table EX-1. Briefly, the Grade 1 end-product standard for each chemical was determined by selecting the lowest value of: 1) the lowest exposure limit; 2) the 99th-plus percentile concentration of the chemical in MSW compost; and 3) the standard listed in 40 CFR Parts 257, 403, and 503, Standards for the Use or Disposal of Sewage Sludge, February 19, 1993 vii

9 (please refer to Figure EX-1). The lowest exposure limit for arsenic was below the average background concentration of arsenic in soil. The TNRCC made the decision to use the 99th-plus percentile concentration of arsenic in MSW compost as the end-product standard, which is supported by the fact that the 99th-plus percentile concentration of arsenic is within the range of average background concentrations of arsenic in Texas soils. It is unreasonable to require arsenic concentrations in compost to be less than what would be expected in naturally occurring soil. The Grade 2 end-product standard for each chemical of concern was determined by selecting the lowest value of: 1) the lowest exposure limit; and 2) the 503 sewage sludge standard. A reasonable, yet conservative approach was used in deriving end-product standards for the chemicals of potential concern in compost. The approach taken in deriving end-product standards was warranted due to the uncertainties associated with the assumptions and methodology. Consequently, there is a substantial level of confidence that the recommended end-product standards will not result in adverse effects in humans, domestic animals (e.g., cows), and plants when compost is used as anticipated. Compost end-product standards are subject to change as more data become available. viii

10 FIGURE EX-1 Development of Recommended End-Product Standards for Grade 1 and Grade 2 Compost Grade 1 Compost Determine exposure limits for all residential exposure pathways Grade 2 Compost Determine exposure limits for all non-residential exposure pathways Compare lowest exposure limit with 99th-plus percentile concentration and 503 sewage sludge standard Compare lowest exposure limit with 503 sewage sludge standard Lowest value is the Grade 1 compost end-product standard * Lowest value is the Grade 2 compost end-product standard *Exceptions: arsenic, lead & PCBs ix

11 Chemical TABLE EX-1 Recommended End-Product Standards for Grades 1 and 2 Compost a Recommended End-Product Standard (ppm) a Grade 1 Compost b Grade 2 Compost c As Cd Cr (total) 180 1,200 Cu 1,020 1,500 Pb Hg Mo Ni Se Zn 2,190 2,800 PCBs 1 10 a b c End-product standards were determined as outlined by the flow chart (Figure EX-1) and the methods are described in the report. Grade 1 compost will be an "unrestricted use" compost and can be distributed without limitation to the general public. Grade 2 compost will be a "non-residential use" compost and will not be used where public contact is likely. Both grades of compost must be mixed into the soil if used to grow animal or human food crops in order to prevent significant food chain effects. In addition, mixing with soil is recommended in order to prevent plant toxicity for both grades of compost. The Grade 1 end-product standards for: 1) arsenic, cadmium, chromium, copper, mercury, nickel, and zinc equal the 99th-plus percentile concentration in MSW compost; 2) lead, molybdenum, and selenium equal the 503 sewage sludge standards; and 3) PCBs equals the lowest federal regulatory limit for residential exposure. Please note that as of February 19, 1994, EPA deleted the molybdenum pollutant limit (18 ppm) listed in CFR pending reconsideration of appropriate limits. In the interim, EPA has stated that sewage sludge must have a molybdenum concentration equal to or less than the ceiling concentration limit of 75 ppm. Grade 2 end-product standards for all of the chemicals are equivalent to the sewage sludge 503 standards. The Grade 2 end-product standard for PCBs equals the lowest federal regulatory limit for non-residential exposure. x

12 INTRODUCTION Approximately four million tons of yard waste in the form of leaves, grass, tree limbs, weeds and other organic debris such as municipal solid waste (MSW) are disposed of each year in Texas landfills. Composting is one of the six primary ways recommended by Clean Texas 2000, an environmental partnership program of the Texas Natural Resource Conservation Commission (TNRCC), to avoid landfilling. Compost, which is defined as a mixture of decaying organic matter, is nature's way of recycling organic waste. However, due to the nature of the feedstock streams that will be used for composting (i.e., materials which constitute compost), there is the potential for constituents to be present in the final compost product which may adversely affect humans, animals, and plants. The potential for adverse effects in humans, animals, and plants could be augmented if repeated application of compost resulted in the excessive net accumulation of chemicals in the compost and/or soil. Pursuant to the 73 rd Legislative Session, Senate Bill 1051 requirements, the TNRCC's Toxicology and Risk Assessment (TARA) Section developed endproduct standards for several inorganic and organic constituents in MSW compost. An endproduct standard is defined as the maximum allowable concentration of a chemical in MSW compost that would not be expected to cause adverse affects in humans, domestic animals, or plants when MSW compost is used as anticipated. The methodology used by the TARA staff in the development of end-product standards for MSW compost included: 1) identification of potential chemicals of concern (i.e., ten inorganic and one class of organic compounds); 2) identification of exposed populations (i.e., humans, domestic animals, and plants); 3) identification of potential exposure pathways (e.g., human ingestion of compost); 4) estimation of exposure limits (i.e., maximum allowable concentrations in compost); 5) selection of the limiting exposure pathway and subsequently, the lowest exposure limit; and 6) determination of end-product standards, which was dependent upon each chemical's lowest exposure limit, 99th-plus percentile concentration in MSW compost (Glenn, 1994; Johnson & Crawford, 1993; Tisdell, 1993; MDEP, 1992) (for Grade 1 compost only), and/or the 503 sewage sludge limit (40 CFR, 1993). Ten inorganic chemicals (arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc) that are thought to be present in compost were identified as chemicals of concern. Data on the actual concentrations of potentially toxic organic chemicals in compost was very limited or unavailable altogether, particularly for MSW compost generated in Texas. Staff of the TARA Section recommend that adequate data be collected to determine the concentrations of potentially toxic organic compounds in MSW. In the interim, the TNRCC decided to regulate the concentration of polychlorinated biphenyls (PCBs) in compost. This decision was supported, in part, by the fact that preliminary data collected by the Minnesota Pollution Control Agency (Worth, 1995) indicates that the concentration of PCBs in MSW compost may be at levels of concern. As more data become available, the selection of potentially toxic organic compounds of concern in compost may need to be revisited. 1

13 The TNRCC also made the decision to develop end-product standards for two grades of compost in order to provide the user with a choice of different qualities of compost. Grade 1 compost will be a "residential use" compost and the end-product standards have been developed based on a residential exposure scenario (e.g., compost is applied to home yards, gardens, and planting beds). Grade 2 compost is a "non-residential use" compost (e.g., compost is applied to agricultural, commercial, institutional, or governmental grounds) and will have less stringent end-product standards as human exposure is expected to be limited. Since, Grade 1 compost will have the more stringent end-product standards, it can also be used for non-residential purposes. However, both grades of compost must be mixed into the soil if used to grow animal or human food crops in order to prevent significant food chain effects. In addition, it is recommended that neither grade of compost be used as the sole growth medium since using only compost to grow plants (e.g., potted plants) may cause phytotoxicity.. The purpose of this document is to describe the approach used in deriving the recommended endproduct standards for the chemicals of concern in MSW compost. The methodology and results will be discussed first, followed by a Discussion section. The assumptions and uncertainties associated with the development of end-product standards will be addressed primarily in the Discussion section. 2

14 METHODS AND RESULTS End-product standards for compost were developed using methodology, with some modifications, employed by the EPA in setting Standards for the Use and Disposal of Sewage Sludge (40 CFR Part 503, 1993), hereafter referred to as the "503s". The general methodology employed by the EPA (EPA, 1992) was appropriate for developing end-product standards for compost as many of the chemicals of concern identified in sewage sludge are also present in compost. In addition, other states are currently using the 503 standards for sewage sludge as end-product standards for compost. Due to the difference in potential feedstocks and uses of compost, compared with sewage sludge, some of the assumptions and methodology used by TARA staff may differ from those used by the EPA in their development of the 503s (EPA, 1992). For example, TARA staff were asked to develop end-product standards for two grades of compost in order to provide the user with a choice of different qualities of compost. When the assumptions or methodology differ, the rationale for the deviation is explained. When assumptions or methodology are consistent with those used by the EPA, please refer to the Technical Support Document for Land Application of Sewage Sludge (EPA, 1992). The remainder of this section outlines the methodology used in the development of end-product standards for compost, along with the results. The following subsections describe the: 1) the identification of potential chemicals of concern; 2) the identification of exposed populations; 3) the identification of potential exposure pathways; 4) the estimation of exposure limits; 5) the selection of limiting exposure pathways; and 6) the determination of end-product standards. The uncertainties associated with each of these subsections are addressed under the Discussion section. Chemicals of Potential Concern Currently identified analytical data for chemicals detected in compost (Glenn, 1994; Johnson & Crawford, 1993; Tisdell, 1993; MDEP, 1992), the 503 rules and support documentation (40 CFR, 1993; EPA, 1992), and the scientific literature were reviewed to determine the list of potential chemicals of concern in compost. Ten inorganic chemicals (arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc) were selected as the inorganic chemicals of concern in compost. For reasons described below, PCBs were selected as the class of organic compounds of concern. Inorganic Chemicals. The EPA performed an incremental ranking of risk for all inorganic chemicals detected in sewage sludge based on levels of detection and relative toxicity (EPA, 1985). Ten inorganic chemicals (arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc) were identified by the EPA as "pollutants that may pose health or environmental hazards when sewage sludge is used or disposed" (EPA, 1992). A limited review of the literature to-date indicates that, with the exception of molybdenum and 3

15 selenium, all ten of the inorganic chemicals of concern in sewage sludge have also been detected at significant concentrations in compost (Glenn, 1994; Johnson & Crawford, 1993; Tisdell, 1993; MDEP, 1992). Since molybdenum and selenium were detected consistently in sewage sludge, a more comprehensive review of the literature is likely to identify these chemicals in compost as well. Similar to the EPA's rationale, the above ten inorganic chemicals were selected as inorganic chemicals of concern in compost because they may pose health or environmental hazards when compost is used or disposed. Organic Compounds. There is a paucity of data on current levels of potentially toxic organic compounds in compost, particularly for compost generated in Texas. Therefore, TARA staff recommend that adequate data be collected to determine the levels of potentially toxic organic compounds in compost. In the interim, preliminary data collected by the Minnesota Pollution Control Agency (Worth, 1995) indicates that the concentration of PCBs in MSW compost may be at levels of concern. Therefore, until more data on the concentrations of potentially toxic organic compounds in compost become available, a decision was made by the TNRCC to regulate the concentration of PCBs in compost. When data become available, it may be necessary to reassess the selection of organic compounds of concern in compost. Exposed Populations Humans, domestic animals (i.e., cows), and plants (e.g., vegetables) were all considered to be potentially exposed populations. These populations are the same as those evaluated by the EPA in the development of the sewage sludge 503 rules. Children were selected as the most sensitive residential human subpopulation for Grade 1 compost, where residential contact is likely, because they are expected to consume more compost per kg body weight, compared with adults. An industrial adult worker was selected as the most sensitive exposed human subpopulation for Grade 2 compost because the activities of an industrial adult worker most realistically represents the maximally exposed individual associated with the use restrictions assigned to Grade 2 compost. It should be noted however, that a recreational child (i.e., a child playing at the park) was also evaluated for Grade 2 compost, but was not the most sensitive subpopulation. Exposure Pathways An exposure pathway is defined as the route a chemical takes from a source to an exposed organism (EPA, 1989). Compost was considered the source of the chemcials of concern. All appropriate exposure pathways for humans, domestic animals, and plants considered by the EPA in the development of the final sewage sludge rules (40 CFR, 1993; EPA, 1992) were reviewed and selected as the exposure pathways of potential concern for compost (Table A-1). The exposure pathways considered by the EPA for development of the 503 standards (EPA, 1992) were selected because they also represent the most likely exposure pathways associated with the use of compost. As shown in Table A-1, the exposure pathways evaluated by the EPA (EPA, 4

16 1992) were categorized according to the potentially exposed populations based on the likely uses of Grade 1 or 2 compost (i.e., residential or non-residential, respectively). Exposure Limits Similar to the EPA's definition for sewage sludge (EPA, 1992), an exposure limit is defined as an estimation of the maximum concentration of a chemical in compost that would not be expected to adversely affect human health or result in domestic animal or plant toxicity. Exposure limits based upon potential human ingestion of compost were estimated as described below using the methodology employed by the EPA in the development of the 503 rules (EPA, 1992), with some modifications. Exposure limits for all other relevant exposure pathways were based upon values derived by the EPA for the 503 rules (EPA, 1992). The uncertainties associated with the assumptions and methodology used in estimating exposure limits are described in the Discussion section. In estimating exposure limits for compost, TARA staff assumed compost would potentially be applied annually for a period of 30 years. This assumption was based upon the national upperbound time (90th percentile) a person lives at a residence (EPA, 1989). Due to the concern that compost may be repeatedly applied to areas where little or no mixing with soil would occur (e.g., situations where compost is used as a mulch), TARA staff conservatively assumed that chemicals may potentially net accumulate in compost that did not get adequately mixed with the soil. The EPA did not consider the potential for chemicals to net accumulate in sewage sludge because they assumed that the sludge would be mixed with the soil. The EPA did, however, evaluate the potential for net accumulation of chemicals in the soil after repeated application and mixing of sewage sludge (EPA, 1992). If it is assumed that chemicals could potentially net accumulate in compost upon repeated application, a human receptor that ingests compost may be exposed to increasing concentrations of the chemicals of concern each year it is applied. One approach that could be used to account for the potential for net accumulation in repeatedly applied compost, is to simply divide all exposure limits based on human ingestion by the number of annual applications (30). However, this is extremely conservative because it is known that compost does not completely biodegrade. Therefore, in order to account for potential net accumulation in MSW compost, assuming that the compost biodegrades at a constant rate, the exposure limits were divided by the annual number of applications (30) times a biodegradation factor of (please refer to Attachment B; Model Development to Determine Organic Degradation in Compost, Pollution Prevention and Recylcling Division, Recycling Section for the assumptions and methodology used to derive the biodegradation factor of 0.061). The model provides a biodegradation factor based on 47% organic degradation which results in a biodegradation factor of However, a biodegradation factor of (which represents 50% organic degradation) was used since the two numbers are essentially the same and because it allows for erring on the conservative side. The uncertainties associated with assuming that chemicals have the potential to net accumulate in 5

17 compost that is repeatedly applied and does not get mixed with the soil are described in the Discussion section. The remainder of this subsection will describe the methodology for estimating exposure limits associated with potential ingestion of compost by a residential child (Grade 1 exposure scenario) and an adult worker (Grade 2 exposure scenario). Exposure limits for exposure pathways other than the human ingestion exposure pathway will also be discussed briefly. Exposure Limits Based on Potential Ingestion of Compost. Exposure limits for the human ingestion exposure pathway were estimated using Equation 1, which is similar to that used by the EPA in their development of the 503 rules (EPA, 1992), with slight modifications. where: EL = [(RfD x BW) - TBI] x CF (1) IR x FI x DE x AAP x BF EL = Exposure limit for chemical of concern in compost (Fg/g, i.e., ppm) RfD = Oral reference dose (mg/kg-day) TBI = Total background dietary intake rate of chemical from other sources (mg/day) CF = Conversion factor (Fg/mg) BW = Body weight (kg) IR = Ingestion rate (g/day) FI = Fraction ingested from contaminated source (unitless) DE = Exposure duration adjustment (unitless) AAP = Number of annual applications (unitless) BF = Biodegradation factor (unitless) The default values used in calculating exposure limits for an exposed residential child (i.e., Grade 1 compost) and an adult exposed in an occupational setting (i.e., Grade 2 compost) are provided below. The RfDs and TBIs for each chemical are listed in Table A-2.! RfD = Compound specific (Table A-2)! TBI = Compound specific (Table A-2)! CF = 1000! BW = Child - 16 kg (EPA, 1989); Adult - 70 kg (EPA, 1989)! IR = Child g/day (EPA, 1989); Adult g/day (EPA, 1989)! FI = Child (average time a child spends at home; EPA, 1989); Adult (100% occurs while at work)! DE = Child (i.e., 350/365 days/y (EPA, 1991)); Adult (i.e., 8 h/day, 250 days/y (EPA, 1991)) 6

18 ! AAP = 30 (based upon a 30 year residence time which is the national upper-bound time (90th percentile) at one residence (EPA, 1989)).! BF = (please refer to Attachment B) As shown in Table A-2, with the exception of copper and zinc, oral RfDs were used for all of the chemcials of concern (Integrated Risk Information System (IRIS), 1994). The Recommended Daily Allowances (RDA)(mg/day; NAS, 1989) were converted to mg/kg body weight for copper and zinc and used as oral RfDs. The results of estimating exposure limits based on potential human ingestion of compost are shown in Table A-3. The exposure limit for lead in Grade 1 compost was estimated using EPA's recommended screening level of 400 ppm for lead in soil for residential land use (EPA, 1994a) and dividing by 30 annual applications times the biodegradation factor of (please refer to Attachment B for a derivation of the biodegradation factor). An exposure limit for lead in Grade 2 compost was not determined because there is no oral RfD for lead. Exposure limits for total chromium were estimated by assuming that 50% of the chromium exists in the Cr(III) oxidation state and 50% in the Cr(VI) oxidation state. The EPA only lists RfDs for Cr(III) or Cr(VI). An adjusted RfD for total chromium (50% Cr(III); 50% Cr(VI)) was estimated by multiplying each RfD by 0.5 and adding the numbers (i.e., (1.0 x 0.5) + (0.005 x 0.5) = mg/kg-day). Exposure Limits for Other Relevant Exposure Pathways. With the exception of the human ingestion of compost exposure pathway, exposure limits for all other relevant exposure pathways were based upon values derived by the EPA in the development of the 503 rules (data not shown; please refer to EPA, 1992 for the results). The EPA reported exposure limits as either, reference cumulative application rates (RPc), or reference concentrations in sludge (RSC). An RPc was defined by the EPA as the maximum allowable amount of pollutant (kg) that could be applied to a hectare of land (EPA, 1992). Whereas, an RSC has been defined by the EPA as the maximum concentration (ppm) of pollutants in sewage sludge (EPA, 1992). Based on assumptions made by the EPA regarding the application of sewage sludge, an RPc was shown to be equivalent to an RSC (EPA, 1992). Since compost end-product standards are defined as the maximum allowable concentration of a chemical in compost, exposure limits for all other relevant exposure pathways (i.e., all relevant exposure pathways except human ingestion of compost) were based on the RSCs derived by the EPA for sewage sludge (data not shown; please refer to EPA, 1992 for the results). Limiting Exposure Pathway A limiting exposure pathway is defined as the exposure pathway with the greatest quantifiable risk associated with exposure to the chemical of concern in compost, compared with other exposure pathways. The limiting exposure pathway was estimated by selecting the exposure pathway with the lowest exposure limit, which indicates that exposure via that pathway results in the greatest 7

19 quantifiable risk. Thus, for each chemical of concern, the exposure limits for the human ingestion of compost pathway (Table A-3), estimated as described above, were compared with the exposure limits for all other relevant exposure pathways, as listed in Table 6-1 of the Technical Support Document for Land Application of Sewage Sludge (EPA, 1992). It is important to note that when comparing exposure limits shown in Table A-3 for Grade 1 or Grade 2 compost with the exposure limits derived by the EPA for the 503s (Table 6-1 of EPA, 1992), EPA's exposure limits were categorized according to the exposure pathways shown in Table A-1 and the appropriate exposure limit was used for comparison purposes for the relevant Grade of compost. The pathway with the lowest exposure limit was selected as the limiting exposure pathway for each chemical of concern for Grade 1 and Grade 2 compost (Table A-4 and A-5, respectively). End-Product Standards The following three pieces of information were used to assist in developing end-product standards for compost: 1) the lowest exposure limits (Tables A-4 & A-5); 2) the chemicals' 99th-plus percentile concentrations in MSW compost (Table A-6); and/or 3) the sewage sludge 503 standards (Table A-6 or A-7). The 99th-plus percentile value of the range of the chemicals' concentration in MSW compost (Table A-6) was estimated by adding three standard deviations to the average, based on the following data set identified to date: Glenn, 1994; Johnson & Crawford, 1993; Tisdell, 1993; and MDEP, The 99th-plus percentile estimation was based on the Empirical Rule which states that approximately all values used to derive an average for a normally distributed population are within three standard deviations from the average (Ott, 1988). A preliminary examination of the data identified to date suggests that the concentrations of the chemicals in MSW compost are normally distributed. As more data become available, the assumption of a normal distribution can be reassessed. The remainder of this subsection describes the methodology used to estimate end-product standards for Grade 1 and Grade 2 compost. The uncertainties associated with the selection of specific end-product standards are discussed in the Discussion section of this document. Grade 1 Compost. With the exception of arsenic, lead and PCBs, Grade 1 compost end-product standards were determined by selecting the lesser of the lowest exposure limit, the 99th-plus percentile concentration in MSW compost, and the 503 sewage sludge standard for each chemical of concern in compost (please refer Figure EX-1, Table A-6, and Table EX-1). Based on a decision by the TNRCC, the 99th-plus percentile concentration of arsenic in MSW compost was selected as the end-product standard. It should be noted however, that the 99th-plus percentile concentration of arsenic in compost falls within the range of average soil arsenic concentrations for the state of Texas (Texas State Soil and Water Conservation Board, 1991). The 503 standard of 300 ppm for lead was selected as the Grade 1 end-product standard because it is an appropriate federal regulatory limit which is not thought to be significantly different from the lowest exposure limit of 270 ppm shown in Table A-6. The Grade 1 compost end-product standard of 1 ppm for PCBs (Table EX-1) was based on the lowest federal regulatory limit for a residential exposure scenario (EPA, 1990). 8

20 As of February 19, 1994, EPA deleted the molybdenum pollutant limit (18 ppm) listed in CFR pending reconsideration of appropriate limits. In the interim, EPA has stated that sewage sludge must have a molybdenum concentration equal to or less than the ceiling concentration limit of 75 ppm. Grade 2 Compost. Grade 2 compost end-product standards were determined by selecting the lesser of the lowest exposure limits and the 503 sewage sludge standards for each chemical of concern in compost (please refer Figure EX-1, Table A-6, and Table EX-1). It should be noted however, that the Grade 2 end-product standard of 10 ppm (Table EX-1) for PCBs was based on the lowest federal regulatory limit for a non-residential exposure scenario (EPA, 1990). In addition, please note the above comments about molybdenum. 9

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22 DISCUSSION The purpose of developing end-product standards for the chemicals of concern in compost is to prevent adverse effects that may result from typical uses associated with compost. The methodology employed was similar to that used by the EPA in the development of the 503 sewage sludge standards (EPA, 1992). As stated by the EPA, the risk-based models developed for the Part 503 regulation were designed to protect individuals and populations that might be "highly exposed to reasonably anticipated adverse conditions" associated with the use of sewage sludge (EPA, 1992). Therefore, it is thought that the assumptions and methodology used in developing end-product standards for compost have resulted in recommended end-product standards that should prevent adverse effects that might result from the reasonably anticipated uses of compost. It should be noted however, that there is a significant degree of uncertainty associated with the methodology and assumptions used in deriving end-product standards. Discussion of the uncertainties is important for gaining a better understanding of the potential risks associated with the expected uses of compost and is a topic that the EPA continues to recommend be included in risk assessments (Habicht, 1992; Browner, 1995). The primary uncertainties in risk assessments are typically associated with the toxicity values and the exposure assessment. Sources of uncertainty in current toxicity information on the chemicals of concern include, but are not limited to: 1) the relative paucity of toxicological data; 2) the extrapolation from animals to humans; 3) the extrapolation from high to low doses; and 4) the lack of knowledge regarding potential interactions among various chemicals. The regulatory approach to dealing with these uncertainties has typically and continues to be one of applying conservative safety factors in deriving comparative toxicity values (e.g., reference doses [RfDs]). Sources of uncertainty associated with exposure assessment include, but are not limited to: 1) exposure scenario/pathway uncertainties; 2) parameter uncertainties; and 3) model uncertainties. Scenario uncertainties arise as a result of potential errors associated with fully defining the exposure pathway and dose. Scenario uncertainties also result from lack of descriptive information, aggregation errors (e.g., assuming subgroup homogeneity), differences in professional judgment, and incomplete analysis (e.g., failure to consider all exposed subgroups and significant routes of exposure). Parameter uncertainties may be present as a result of measurement errors, sampling errors, variability, and the use of generic or surrogate data. Additional uncertainty in the risk assessment model used in this analysis resulted from the generic nature of the model and from the use of many default exposure assumptions, such as exposure frequency and duration. The use of a generic model, however, was unavoidable and necessary since the assessment must ensure protection to human health and the environment in the entire State of Texas. The purpose of this Discussion section is to identify the toxicity and exposure uncertainties and discuss how these uncertainties can affect the derivation of the end-product standards. The 11

23 discussion is organized as follows: 1) Chemicals of Potential Concern; 2) Exposed Populations; 3) Exposure Pathways; 4)Exposure Limits; 5) Limiting Exposure Pathways; 6) End-Product Standards; and 7) Summary/Recommendations. Chemicals of Potential Concern Staff of the TARA Section reviewed currently located analytical data on the concentrations of chemicals in MSW compost (Glenn, 1994; Johnson & Crawford, 1993; Tisdell, 1993; MDEP, 1992), the EPA's final sewage sludge rules and support documentation (40 CFR, 1993; EPA, 1992), and the scientific literature to determine the list of potential chemicals of concern. This subsection provides a discussion of the uncertainties associated with the selection process which has been described in the Methods section of this document. Inorganic Chemicals. At the time the recommended end-product standards were developed, a very limited set of analytical data on the concentration of the inorganic chemicals of concern in MSW compost was available. Although there are uncertainties associated with the use of a limited data set on the actual concentrations of inorganic chemicals in compost (with the exception of molybdenum and selenium, for which data was not located), the data sufficiently identified the chemicals as chemicals that are present in MSW compost. The uncertainty associated with the selection of molybdenum and selenium as chemicals of concern in compost is related to the assumption that the inorganic constituents present in sewage sludge are also present in compost. This uncertainty will be reduced, if not eliminated, as more data on the concentrations of inorganic constituents in compost become available. Organic Compounds. Data on the actual concentrations of potentially toxic organic compounds in compost was very limited or unavailable altogether, particularly for MSW compost generated in Texas. In the development of the 503s, EPA eliminated the need to regulate potentially toxic organic compounds in part, because a significant amount of data (EPA, 1992) showed that the concentrations of select organic compounds in sewage sludge were below their respective riskbased concentrations of concern. However, since similar data showing the concentrations of potentially toxic compounds of concern in compost are not available, particularly for compost generated in Texas, TARA staff recommend that adequate data be collected to determine the levels of potentially toxic organic compounds in compost. When such data become available, TARA staff could evaluate the appropriateness of applying EPA's criteria (EPA, 1992) in determining whether there is a potential for adverse human health effects associated with exposure to potentially toxic organic compounds in compost. Specifically, the criteria used by the EPA are as follows: 1) the compound had been banned; 2) the compound was detected less than 5% of the time in sewage sludge; or 3) the 1-in-10,000 risk-based limit was less than the 99th percentile concentration of the compound in sewage sludge. In the interim, the TNRCC decided to regulate the concentration of PCBs in compost. This decision was supported, in part by the fact that preliminary data collected by the Minnesota Pollution Control Agency (Worth, 1995) indicates that the concentration of PCBs in MSW 12

24 compost may be at levels of concern. Until more data on the concentrations of potentially toxic organic compounds in compost become available, a significant degree of uncertainty will be associated with the selection of PCBs as the potentially toxic organic compounds of concern in compost. Combined Effects of Multiple Chemicals. The combined effects of multiple chemicals associated with exposure to compost were not considered in the development of end-product standards. Many of the chemicals of concern have different mechanisms of action or affect different target organs and may therefore, be expected to act independently. However, certain chemicals can affect the toxicity of other chemicals. For example, molybdenosis, a secondary copper deficiency disease in animals with elevated dietary intake levels of molybdenum, can be exacerbated in animals with copper deficient diets or suppressed in animals with high dietary levels of copper (Mills and Davis, 1987). If the combined effects of multiple chemicals associated with the exposure to compost could be evaluated, the risk of certain chemicals adversely affecting human health and/or the environment may either increase or decrease. Although the procedures for assessing the potential risks associated with exposure to a single chemical are relatively well established, the methodology for assessing potential risks associated with exposure to a mixture of chemicals is still in the developmental stage. Therefore, due to the inherent scientific methodologic limitations, TARA staff only assessed the risk that would result from exposure to each individual chemical of concern that is present in compost. However, it is believed that the end-product standards are sufficiently conservative to outweigh the uncertainty associated with the limitations in assessing mixtures of chemicals. Exposed Populations Humans, domestic animals (e.g., cows), and plants were all considered potentially exposed populations in developing recommended end-product standards for compost. Staff of the TARA Section acknowledge that not all sensitive ecological receptors and habitats have been considered. It is possible that sensitive ecological receptors and habitats may be impacted by chemicals present in compost at concentrations below the end-product standards. The likelihood of this occurring is unknown. However, thorough mixing of compost with soil should decrease the risk of detrimental ecological impacts. Exposure Pathways Staff of the TARA Section assumed that all relevant exposure pathways of concern for the use of compost were identical to the exposure pathways identified by the EPA in the development of the final sewage sludge rules (EPA, 1992). Since exposure pathways typically depend upon many site-dependent factors (e.g., meteorological conditions, soil type, depth of the groundwater table, etc.), it is possible that not all exposure pathways have been considered. However, the exposure pathways identified are thought to be the most significant routes of exposure associated with the reasonably anticipated uses of compost. The largest element of uncertainty associated with these exposure pathways and one that likely overestimates true exposure is the fact that 13

25 individuals/populations are unlikely to engage in all of the combined behavior patterns in the exposure pathways evaluated. Exposure Limits An exposure limit has been defined as an estimation of the maximum concentration of a chemical in compost that would not be expected to adversely affect human health and/or cause plant or domestic animal toxicity. Exposure limits were estimated as described under the Methods section and were based on calculations used by the EPA in developing the 503 rules for the human ingestion of compost exposure pathway, with some modifications. Exposure limits for all other relevant pathways relied on values estimated by the EPA (EPA, 1992). Additionally, end-product standards were derived for two grades of compost, whereas the EPA derived only one set of standards for sewage sludge. There are multiple uncertainties associated with estimating exposure limits based on: 1) the potential expected uses (i.e., residential [Grade 1] versus non-residential [Grade 2]); and 2) the assumptions and parameters used in calculating exposure limits. Reasonable expected uses of compost were considered when developing the exposure scenarios for the residential versus nonresidential uses of compost and were similar to those described by the EPA (EPA, 1992). One of the differences between the predicted uses of compost and those predicted by the EPA for sewage sludge was the assumption that compost may repeatedly be used in areas where little or no mixing with the soil occurs (please refer below for a discussion on how the uncertainties of this assumption can affect the calculation of exposure limits). As would be expected, there is a significant amount of uncertainty associated with predicting potential uses of compost. Since exposure limits are estimated based on assumptions that are dependent upon expected uses, the exposure limits may increase or decrease based on the actual uses of compost. The estimation of exposure limits depended upon the following factors: 1) the toxicity values; 2) the parameters used to estimate daily intake (e.g., body weight, ingestion rate, exposure duration, etc.); and 3) the potential for net accumulation of chemicals in repeatedly applied compost that does not get mixed with soil. Toxicity Values. The maximum daily intake for each chemical of concern in compost was assumed to be equal to its corresponding EPA-derived daily oral RfD, which is based upon noncarcinogenic effects (IRIS, 1994). With the exception of lead, zinc, and PCBs, EPA-derived oral RfDs were available for all of the evaluated chemicals of concern. The EPA states that, "In general, the RfD is an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime." Therefore, there is uncertainty associated with the use of RfDs. For example, although arsenic's RfD is listed as mg/kg/day (IRIS, 1994), the EPA also states that, "There was not a clear consensus among agency scientists on the oral RfD for arsenic. Applying the agency's RfD methodology, strong scientific arguments can be made for various values within a factor of 2 or 3 of the currently 14

26 recommended RfD value for arsenic, i.e., to mg/kg/day (IRIS, 1994)." Therefore, the use of EPA-derived oral RfDs may over- or underestimate the actual exposure limits for the chemicals of concern in compost. An "adjusted" RfD was used in the estimation of the exposure limits for "tota'" chromium (please refer to the Methods section for details). An "adjusted" RfD was used because EPA has derived separate RfDs for chromium (III) and chromium (VI), but has not derived an oral RfD for "total" chromium (IRIS, 1994). Since no data exist on the speciation of chromium in compost, it was conservatively assumed that 50% would be hexavalent chromium and 50% would be trivalent chromium. Although the speciation of chromium in compost is not known, chromium exists predominantly as trivalent chromium in most soils (ATSDR, 1993a). In addition, trivalent chromium is significantly less toxic than hexavalent chromium (ATSDR, 1993a). Predicting the oxidation state of chromium in compost is a complex issue, as the fate of chromium in soil and/or compost will depend on factors such as the ph, the oxygen content, the form of chromium (e.g., organic complexes), the temperature, etc. Therefore, it is likely that the assumption of 50% trivalent and 50% hexavalent chromium in compost has overestimated the actual risk. Recommended Daily Allowance (RDA) values for copper and zinc were used as reference doses in calculating the exposure limits. An RDA is defined as the daily intake level of essential nutrients that adequately meets the known nutrient needs of practically all healthy persons, as judged by the Food and Nutrition Board (NAS, 1989). The RDA was used as a reasonably protective dose for copper since there is no EPA-approved oral RfD. Although there is an EPAderived RfD for zinc, it is lower than the corresponding RDA, suggesting that the RfD may be an overestimation of the actual toxicity of zinc. Therefore, the RDA for zinc was used as a reasonably protective dose. As with the RfDs, the level of uncertainty associated with the derivation of an RDA can be affected by factors such as, the amount of available data, the quality of the studies, the length of the study, the lack of knowledge regarding the potential interactions between chemicals, etc. However, since it is thought that the RDA is a level that is essential and therefore, not likely to be toxic, use of an RDA as an RfD is relatively conservative. Presently, the EPA has not derived an RfD for lead. The exposure limit for Grade 1 compost was based on the EPA's recommended screening level for lead in soil under a residential scenario (EPA, 1994a). The lead soil screening level (EPA, 1994a) was derived using an integrated exposure uptake biokinetic (IEUBK) model for lead in children developed by the EPA (EPA, 1994b). For a discussion of the uncertainties associated with the derivation of the lead soil screening level for children, please refer to the guidance manual of the IEUBK model (EPA, 1994b). The potential for carcinogenic effects that could result from exposure to the chemicals of concern in compost have not been included in this report. The following inorganic chemicals have been listed by the EPA as potential human carcinogens: arsenic, cadmium, chromium (VI), lead, nickel, and PCBs (IRIS, 1994). Carcinogenic exposure limits for arsenic based on the potential for human ingestion of compost were not reported since the noncarcinogenic exposure limit for 15