Page 4 of 9 Examination of observed air concentrations

The volatilization and near-field dispersion models of the soil source category were developed from well established theoretical principals, and were developed as part of an effort to assess the impact of soils contaminated with PCBs (Hwang, et al., 1986). The virtual point source dispersion model for far-field dispersion estimates is also based on well established theory (Turner, 1970). However, these models have not been field validated for soils contaminated with dioxin-like compounds. Ideally, the algorithms for estimating air concentrations would be validated using data on soil concentrations of dioxin-like compounds and concurrently measured concentrations in air above and downwind of the contaminated soil. A discussion of the Gaussian plume dispersion theories used in the COMPDEP model is given in Chapter 3, Section 3.4.1.

Some sense of the reasonableness of model values can be made by comparing predictions to measured concentrations in ambient air in urban environments. Reports of such concentrations are summarized in Section 4.7, Chapter 4 of Volume II, and Tables B.14-B.16, Appendix B of Volume II. Sources other than soil are likely to be the cause of levels measured in urban air environments. Also, the dispersion model is designed for situations where the contaminated soil is surrounded by relatively clean soil. As discussed in Section above, off-site impacts have been noted in several sites of 2,3,7,8-TCDD contamination. These two points are made in order to establish a basis for comparing urban air concentration to concentrations predicted to occur from soils: one would expect urban air concentrations to be at least higher, if not higher by orders of magnitude, than soil emissions.

Tables B.14 and B.15 (Appendix B, Volume II) summarize average PCDD/PCDF congener-specific concentrations in urban air in the United States and in Europe. Results for two example compounds demonstrated in Chapter 5, 2,3,7,8-TCDD and 2,3,4,7,8-PCDF, are examined in this section. Observed concentrations of 2,3,7,8-TCDD were mostly non-detects with detection limits ranging from 0.01 to as high as 0.82 pg/m3. Occurrences were noted as high as 0.05 pg/m3 in Bridgeport, CT, and 0.004 pg/m3 in Wallingford, CT (both measurements as part of a study evaluating the impact of resource recovery facilities) and 0.06-0.08 pg/m3 in urban settings in Hamburg, Germany. In Stockholm, Sweden, occurrences in suburban, remote countryside, and coastal settings were listed at 0.0007, 0.0002, and 0.0001 pg/m3 respectively. Concentrations of 2,3,4,7,8-PCDF were detected in several reported studies. The range of 2,3,4,7,8-PCDF detections was 0.001-1.92 pg/m3. In the few reports where both compounds were detected, 2,3,4,7,8-PCDF was detected at 3 to 10 times higher concentration than 2,3,7,8-TCDD.

Eight air monitoring studies in the United States were used to arrive at a profile of air concentrations used for estimating background exposures to dioxin-like compounds through inhalation. These references were characterized as mostly urban and suburban, not background or rural. A summary of this compilation is in Volume II, Chapter 4, Section 4.7. and in Volume II Appendix Tables B-28 and B-29. The arithmetic mean concentrations (used for background exposure estimation) for 2,3,7,8-TCDD and 2,3,4,7,8-PCDF were 0.01 and 0.03 pg/m3, respectively. Section below discusses the use of this compilation to craft a profile of air concentrations that might be typical of rural, background settings where cattle are raised. Evidence suggests that urban air concentrations are 4-6 times higher than rural air concentrations. If so, than 2,3,7,8-TCDD and 2,3,4,7,8-PCDF concentrations in a rural environment might 0.002 and 0.006 pg/m3.

The on-site source category was demonstrated using concentrations of 1.0 ng/kg (ppt) for each example compound. This low concentration was assigned based on reports by researchers who measured concentrations of dioxin-like compounds in what they described as "background" and "rural" soils - they found non-detects to concentrations in the low ppt level. Modeled air concentrations of the example compounds 2,3,7,8-TCDD and 2,3,4,7,8-PCDF resulting from this level in soil were in the 10-5 pg/m3 range. For the stack emission source category demonstration, total (vapor + particle phases) concentrations of 2,3,7,8-TCDD simulated to arrive at points between 0.2 and 50 km were between 10-7 to 10-6 pg/m3. The off-site soil source category evaluated the impact of elevated soil concentrations to exposure sites that were located distant from the site of contamination. The example scenario demonstrating this source category had concentrations of this dioxin and furan congener set at 1 ppb, three orders of magnitude higher than the 1 ppt of the on-site source category demonstration scenarios. Air concentrations predicted in these example scenarios were in the 10-3 pg/m3 range.

Only this air concentration from the off-site soil contamination is generally in line with urban air concentrations of 2,3,7,8-TCDD, and/or a hypothesized rural air environment. It is at least plausible that elevated concentrations in soil would result in air concentrations that are in the same range as found in urban environments. A model result that would have questioned the model validity would have been, for example, that air concentrations resulting from soils of high concentrations would greatly exceed, or be very much lower, than urban air concentrations. In the same vein, it is certainly reasonable that air concentrations resulting from a single stack emission with generally a low release rate of 2,3,7,8-TCDD should be much lower than urban air concentrations.

It is not that clear that emissions and resulting air concentrations above soils at background levels should be lower by up to 2 orders of magnitude lower than what is hypothesized to occur in background setting. The argument has been made in Volumes I and II of this assessment that emissions from tall industrial stacks, followed by long range transport, are the ultimate source of these compounds in rural environments where the food supply is produced. The question remains as to how much of the contaminant in rural air is due to annual emissions and long range transport versus emissions from the soil reservoir source. If the modeling of this assessment is correct, than soils contribute very little to rural air concentrations. However, other evidence developed in this assessment suggests that the soil release and dispersion algorithms of this assessment may be underestimating air concentrations.

One piece of that evidence is discussed in the next section below. Plant/soil ratios, defined as the ratio of 2,3,7,8-TCDD concentration in plants divided by that in the soil, were found to be lower in model predictions as compared to literature values.

Two possible hypotheses were offered below:

1) the model is underpredicting air concentrations resulting soil releases, and/or

2) plant:soil ratios derived in experiments are not only the result of soil related impacts, but also from distant sources of air-borne release and long range transport - i.e., the air reservoir is not solely explained by soil releases.

One other possibility would be that the algorithms estimating air to plant transfers are not valid and estimating too low a transfer rate. However, the air to plant transfers algorithms were examined in the section further below, Section, describing an air-to-beef food chain validation exercise. There, air to plant transfers onto a leafy hay crop were examined with data and model was predicting hay concentrations right in line with observations.

In summary, three pieces of evidence suggest that the soil to air models, and/or the parameters values selected for this model, may be underestimating air concentrations. One is the comparison of predicted air concentrations for a background soil compared against air concentration data described above. The second is developed below where plant:soil ratios predicted by the model appear lower than measured under experimental conditions. Third, air-to-plant transfers appear to test well, leaving the soil-to-air algorithms questionable for predicting low plant:soil ratios. Impacts of contaminated soils to vegetations

There have been several studies which have measured plant concentrations of 2,3,7,8-TCDD for plants grown in soils with known concentrations of 2,3,7,8-TCDD, and more recently, studies with plant and soil concentrations for dioxin toxic equivalents or dioxin congener groups. One quantity that can be estimated from these studies is a plant:soil contaminant concentration ratio. The plant:soil ratio equals the concentration in the plant divided by the concentration in soil in which the plant is growing. Concentration ratios predicted to have occurred can be compared against those that have been measured in the various studies.

These ratio comparisons can be considered model validations, although none of the experimental or field conditions for the literature studies were duplicated in this exercise. The literature articles measuring soil and resulting plant concentrations of dioxin-like compounds are summarized in Table 7-6. This table also includes concentration ratios, and separates sections for above and below ground vegetations.

table Table 7-6. Summary of plant concentration versus soil concentration data for 2,3,7,8-TCDD.m.
In measuring both the soil and the plant concentration, several of the early literature articles, particularly those from Seveso (Wipf, et al., 1982; and Coccusi, et al., 1979) presumed that the soil in which the plant was growing was the ultimate source for the 2,3,7,8-TCDD contamination of above ground plant parts, if not from direct uptake than from deposition of suspended particles.

However, recent research has concluded that the contamination of above ground plant parts is due principally to air-to-plant transfers (Hulster and Marschner, 1993a; Muller, et al, 1993a; Muller, et al., 1993b; Welsh-Paush, et al (1993); and others).
expand table Table V3 7-6

These cited research efforts have concluded that there is no consistent relationship between soil concentrations of dioxin-like compounds and above ground vegetative concentrations of these compounds, which has led the researchers to conclude that air-to-plant transfers explain plant concentrations (a recent report did strongly imply a direct soil/plant for dioxin-like compounds for at least one family of above ground vegetables, the cucumber family (Hulster and Marschner, 1993b); this will be discussed below). This fact, coupled with the fact that sources of airborne contamination by dioxins include both distant sources and soil releases, make it difficult to compare literature reports of plant:soil contamination concentrations with those predicted by the soil contamination modeling of this assessment.

Recall that the "on-site" soil source modeling presumes that air concentrations and depositions to which the plant are exposed originate only from the soil in which the plant is growing. One would expect that the modeled plant:soil ratio for above ground plant parts would be lower than plant:soil ratios measured in field settings, since the field measured ratios are influenced by more than just the soil releases into the air.

On the other hand, the literature is consistent in concluding that soil provides the source for underground soil to root transfers. For this reason, Table 7-6 and the following discussions distinguish between above and below-ground vegetations.

The following plant:soil contaminant concentration ratios were estimated for the two scenarios demonstrating the on-site source category in Chapter 5, Scenarios 1 and 2:
below ground vegetables - 7x10-3 (dry weight basis, assuming vegetables are 15% dry matter),
above ground vegetables/fruit - 7x10-5 (dry weight basis, assuming vegetables/fruits are 15% dry matter),
grass - 6x10-3 (dry weight), and feed 3x10-3 (dry weight).

Some observations from experimental results found in the literature, and comparison with the results of the model, are:

1) The largest body of consistently developed experimental data on soil-plant relationships of dioxin-like compounds comes from a research group in Germany who have published numerous articles for different vegetations and experimental conditions in the 1990s (Hulster and Marschner, 1991; Hulster and Marschner, 1993a,b; Muller, et al., 1993a,b). Some of the earlier literature showed much higher impacts to vegetations than measured by these German researchers (Coccusi, et al., 1979; Facchetti, et al., 1986; Young, 1983), which in the judgement of the authors of this EPA assessment, renders them suspect. One early report, that of Wipf, et al. (1982), does show results consistent with the German research. The observations following will focus mainly on this research from Germany.

2) Experimental results for both above and below ground vegetations suggest that plant:soil ratios decrease as soil concentration increases. For below ground vegetations, this suggests that the movement into plants is not a passive and unimpeded process occurring with transpiration water, for if it were, plant:soil ratios would be constant as concentration increases. For above ground vegetations, the observations given above that air-to-plant transfers and not soil-to-plant transfers better explain plant concentrations, and that air concentrations include soil releases as well as long term transport, leads one to conclude that a consistent relationship between soil concentrations and plant concentrations is not to be expected. An explanation for this trend for below ground vegetative trends could not be found.

The models of this assessment - soil to below ground vegetation, soil to air to above ground vegetation, and air to above ground vegetation - cannot duplicate these observed trends, that is, the models will not show a decrease in plant:soil ratios as soil concentration increases. Above and below ground vegetation concentrations are a linear function of a biotransfer factor and an appropriate media concentration - air, soil water. For particle depositions, no transfer parameters are used, but plant concentrations are a linear function of model inputs, including deposition rates, plant interceptions and yield, and a plant washoff factor. Therefore, plant concentrations will be a linear function of soil concentrations for the soil source categories.

3) Plant:soil ratios for below ground vegetables for soil concentrations in the low ppt range would appear to be in the 10-1 to 10-2 range (Muller, et al, 1993; Hulster and Marschner, 1991), in contrast to the 0.007 predicted by the model. Much higher ratios were found in the earlier studies (Coccusi, et al., 1979; Facchetti, et al., 1986; Yount, 1983), which earlier had been speculated as being questionable. One earlier study, that of Wipf, et al. (1982), does report ratios similar to these later studies, as noted above. At higher soil concentrations in the sub to low ppb range, plant soil ratios are more in the 10-4 to 10-3 range (Hulster and Marschner, 1993a; Hulster and Marschner, 1991), even lower than the modeled 0.007 ratio.

4) The results for above ground bulky vegetations, fruits and vegetables, indicate plant:soil ratios that are lower than plant:soil ratios for bulky below ground vegetations, for comparable soil concentrations. The evidence for this observation is best found in the Hulster and Marschner (1993a) concurrent experiments for potatoes and pears/apples, as well as the earlier work of Wipf, et al. (1982) for several fruits and carrots. The same trend is also found for the grass results for 2,3,7,8-TCDD given in Young (1983). This trend is also duplicated by the models, which showed two orders of magnitude difference in below ground as compared to above ground vegetations. The plant:soil modeled ratio of 7*10-5 is similar to ratios found when the soil concentration was in the hundreds to thousands of ppt (Hulster and Marschner, 1993a). However, other data, particularly for leafy vegetations such as hay, grass, and lettuce, and for lower soil concentrations, indicate a soil:plant ratio of 10-3 to 10-2. ...

Two possible explanations are offered for this trend:

1) above ground vegetations in experiments are likely to be impacted by not only soil releases, but distant sources of release, and/or

2) the models could be underpredicting air concentrations resulting from soil releases.

cont 4) Several of the articles, both from the German work and the earlier work, noted that most of the concentration was in the outer portions of the below and above-ground vegetations, and not the inner portions. Despite significant increases in soil concentration from the ppt to the ppb range, inner potato tuber concentrations remained constant (Hulster and Marschner, 1991, 1993a). This evidence was the principal justification for the use of the empirical adjustment factors termed VG for soil to below ground transfers, VGbg, and vapor-phase air transfers to bulky above ground vegetations, VGag. The chemical-specific empirical transfers factors for both of these transfers were developed in laboratory experiments with several chemicals using thin vegetations - solution phase transfers to barley roots for below ground vegetation concentrations, and vapor phase transfers to azalea leaves for vapor phase transfers. For the dioxin-like compounds, direct use of these transfer factors would be most appropriate for the outer few millimeters, perhaps, of below and above ground bulky vegetations. The assignment of a VG of 0.01 for bulky above and below ground vegetations was based on an outer surface volume to whole plant volume ratio for a common vegetation such as a carrot or an apple. A VG of 1.00 was used for grass, since that is a thin vegetation.

Further evidence for the above ground VG came from a recent study by McCrady (1994), who measured the uptake rate constants of vapor-phase 2,3,7,8-TCDD to several vegetations including grass and azalea leaves, kale, pepper, spruce needles, apple, and tomato. The uptake rate for the apple divided by the uptake rate for the grass leaf was 0.02 (where uptake rates were from air to whole vegetation on a dry weight basis). For the tomato and pepper, the same ratios were 0.03 and 0.08. The VGag was 0.01 for fruits and vegetables in this assessment. McCrady (1994) then went on to normalize his uptake rates on a surface area basis instead of a mass basis; i.e., air to vegetative surface area uptake rate instead of an air to vegetative mass uptake rate. Then, the uptake rates were substantially more similar, with the ratio of the apple uptake rate to the grass being 1.6 instead of 0.02; i.e., the apple uptake rate was 1.6 times higher than that of grass, instead of 1/50 as much when estimated on an air to dry weight mass basis. The ratios for tomato and pepper were 1.2 and 2.2, respectively. Therefore, since the Bvpa in this assessment is an air to plant mass transfer, the McCrady experiments would appear to justify the use of an above-ground VG of a magnitude less than 0.10.

5) A recent experiment by the Hulster and Marschner (1993b) on vegetations of the cucumber family contradicted the conventional wisdom that direct soil to root to above ground plant impact would not occur for the dioxin-like compounds. Their results were most striking for zucchini, which showed uniform plant concentrations from inner to outer portions of the zucchini fruit, and the highest whole fruit concentrations and plant:soil ratios they had ever measured, despite careful experimental conditions which physically isolated the fruit from the soil. Pumpkins also showed high plant contamination and plant:soil ratios, with more expected plant concentrations measured for the cucumber. No explanation was offered for these results. It was assumed for this exposure assessment that the fruits and vegetables for human consumption, and the grasses, hay, and other vegetations animals consume, would not follow this pattern.

A principal conclusion that can be drawn from this examination is that the plant:soil contaminant concentration ratios developed by the soil contamination models of this assessment may be lower by perhaps an order of magnitude or more than measured ratios at lower soil concentrations, in the low ppt range, whereas they may be more in line and even higher when soil concentrations are the hundreds of ppt to the ppb range. This trend appears to hold for both above and below ground vegetations. This difference in the comparison of modeled and observed ratios as the concentration changes is because the data shows that plant:soil ratios decrease as soil concentrations increase. This cannot be duplicated by the model since the plant concentrations are a linear function of the source strength terms - the soil, soil water, or air concentrations and deposition. An explanation for this observed trend could not be found. The observation that plant:soil ratios for above ground vegetations are higher in the literature at lower soil concentrations (and more typical of background rather than heavily contaminated soils) as compared to the modeled ratios, has to be carefully considered. Two explanations are offered. For experiments conducted outdoors, the source of air reservoirs of dioxin-like compounds are the soil in which the plant is growing as well as from distant sources and long-term transport. Also, it is possible that the model is underpredicting air concentrations and hence underpredicting air to plant transfers. A validation exercise for the beef bioconcentration algorithm

The premise of this modeling exercise to test the beef food chain model for dioxin-like compounds is that air-borne reservoirs of these compounds in rural environments are the "source term" explaining concentrations found in beef. Further, this exercise probably would not qualify as a validation exercise in the traditional sense. Most environmental model validation exercises rely on data obtained from a single site. This exercise instead develops a representative rural air concentration profile and attempts to model a profile of average beef concentrations. The model structure, from air to beef, is shown in Figure 7-3. The algorithms for these components, and assignment of model ...

table Figure 7-3 Overview of model to predict beef concentrations from air concentrations.

... parameters, were described in Chapter 4, and are very briefly summarized here. The "observed" source, or independent, term in this modeling exercise are the air-borne concentrations of dioxin-like compounds shown at the top of Figure 7-3, and the "predicted", or dependent, results are the concentrations in whole beef shown at the bottom of this figure.

Both these quantities are developed from reported United States measurements. Section below describes the generation of these concentration profiles.

Section summarizes model algorithms and parameter assignments. Section presents the results and discussions from this exercise.

expand table Figure V3 7-3 Air and beef concentrations

Very little data are available worldwide on air concentrations of individual dioxin-like congeners in a rural setting. This is the kind of air concentration data that would be needed for this exercise. An evaluation of ambient air monitoring studies in the United States conducted for Volume II of this assessment showed that nearly all of the data was from urban or suburban settings. The purpose of this compilation was to determine an ambient air concentration suitable for estimating inhalation exposures to dioxin-like compounds. Measurements which were attributed to a nearby identifiable source, such as an incinerator, were not considered for this effort. From several studies around the country, a total of 84 air samples were available, from which a mean TEQ level of 0.095 pg/m3 was determined. Further detail on this compilation can be found in Chapter 4 of Volume II.

There are a few references which do have congener-specific data which might be characterized as rural. One is outside of United States in Sweden (Broman, et al., 1991). Air samples were taken in four areas, ranging from the Stockholm urban area to the open coastal area of the Baltic Sea. Results indicate lower TEQs when going from the urbanized area to the remote areas. The Stockholm city center was 0.024 pg TEQ/m3, a "suburb" was 0.013 pg TEQ/m3, a "countryside remote" area was 0.0044 pg TEQ/m3, and an "open coastal" area was 0.0026 pg TEQ/m3. Twenty-five PCDD/F concentrations were listed at the fg/m3 level (i.e., 0.001 pg/m3).

The only reference found for the United States with congener specific data for an area described as rural was from Ohio (Edgarton, et al., 1989). Six sites were tested, one of which might be considered rural. The data contained many non-detects, with detection limits between 0.033 to 0.82 pg/m3, although most non-detects had detection limits less than 0.3 pg/m3. The following TEQ concentrations were derived only from the positive listings: two sites in Akron - 0.077 and 0.079 pg TEQ/m3, two sites in Columbus - 0.092 and 0.179 pg TEQ/m3, a site near a highway - 0.065 pg TEQ/m3, and a rural site in a town called Waldo - 0.045 pg TEQ/m3. Like the data from Sweden, one can see a trend for lower concentrations in the Waldo site as compared to the sites in Columbus and Akron.

Other references did contain other pertinent data, such as total concentrations, TEQ concentrations, or congener group concentrations, in rural and urban settings. Eitzer and Hites (1989) took data from Bloomington, Indiana and a remote area in Wisconsin known as Trout Lake. TEQ concentrations were not given, but total congener group concentrations were reported. The sum of congener group concentrations, or total concentrations of dioxins and furans, equaled 2.2 pg/m3 for Bloomington, and 0.51 pg/m3 for Trout Lake. This 0.51 pg/m3 total concentration is similar to the total concentration found in the "countryside remote" area in Sweden discussed above, which is 0.41 pg/m3 (TEQ concentration was 0.0044 pg/m3, as noted above).

In an evaluation of air, soil, sediment, and fish in Elk River, Minnesota, a rural setting, again total congener concentrations in the air were reported (Reed, et al., 1990). Concentrations for three sites and for two sampling dates, one in the winter and one in the summer, were available. Two of the three sites were in rural settings and the third was near a refuse derived fuel incinerator. Total concentrations for the two rural sites in winter and in summer were 2.29 and 2.91 pg/m3 in winter, and 0.58 and 0.38 pg/m3 in summer. For the third site near the incinerator, winter and summer concentrations were 15.2 and 0.35 pg/m3, respectively. The average of the four data points for rural settings was 1.54 pg/m3, while the average of the two data points near the incinerator was 7.78 pg/m3.

Finally, Maisel and Hunt (1990) list TEQ concentrations only for monitoring networks including: a Connecticut coastal location described as urban (measurements described as "wintertime"), a southern California urban setting ("annualized"), and a central Minnesota rural setting ("annualized"). While not identifying it as such, this central Minnesota setting could be the one described above in Elk River, Minnesota. The TEQ concentrations for the two urban and one rural setting were: 0.092, 0.091, and 0.021 pg TEQ/m3.

Key points from this literature summary are:

1. Congener specific profiles for rural settings in the United States are generally not available. Based on several studies encompassing 84 data points with specific congener concentrations which best represent urban/suburban settings, but are not near identified emission sources, a mean TEQ air concentration of 0.095 pg/m3 is estimated.

2. Studies are available which do provide side by side data on urban and rural settings, although the literature references only list congener group concentrations or total TEQ concentrations (with the exception of the Edgarton, et al. (1989) described above). What this summary shows is that rural air concentrations of dioxin-like compounds appear to be 4-6 times lower than in urban settings, and that a TEQ concentration for rural settings appear to range from 0.004 to 0.04 pg/m3.

In order to develop a profile of air concentrations that will be considered representative of rural settings, what will be done, therefore, is to take the profile of congener-specific air concentrations for urban/suburban settings leading to a TEQ concentration of 0.095 pg/m3, and divide each concentration by 5. The resulting TEQ concentration is 0.019 pg/m3. The total concentration of PCDD/Fs in this rural profile equals 1.09 pg/m3. A uniform division by five for all congeners essentially assumes that the ultimate sources for an urban and a rural profile of air concentrations are the same. The specific concentrations used are shown in Table 7-7.

A review of data on concentrations of dioxin-like compounds in beef showed that very limited data was available worldwide, much less United States. Only three studies contained congener-specific data of dioxins and furans in beef in United States. In one study beef samples were composited with veal and the results described as beef/veal. The three studies only encompassed 14 samples. These studies include one conducted by the California Air Resources Board (CARB; Stanley and Bauer, 1989), the results of background analysis from a study conducted by the National Coalition for Air and Stream Improvement (NCASI; the study described in Lafleur, et al., 1990) and a survey of foods conducted in New York by Schecter et al. (1993).

These were the data used to estimate background exposures to dioxins in beef in Chapter 5 of Volume II. The total TEQ for beef and veal was calculated by using one-half the detection limits reported by the researchers to represent the concentration of nondetectable CDD/F congeners in the samples. Using this methodology, the TEQ concentration was estimated to be 0.48 ng/kg (ppt) for beef and veal on a wet weight basis. If nondetectable concentrations are assumed to be zero, the estimated TEQ for beef and veal is 0.29 ppt.

table Table 7-7. Observed air and beef concentrations, and fate parameters for individual dioxin and furan congeners.
The average whole beef congener-specific concentrations assuming non-detects were one-half the detection limit are to be used to represent beef concentrations, and they are shown in Table 7-7. All studies reported concentrations as lipid-based concentrations. Where lipid fractions were not supplied, 19% lipid content for beef was assumed to estimate whole beef concentrations. It is important to note that the United States samples came from commercial food outlets (grocery stores, e.g.). This fact will be used to imply that the data represents beef cattle that went through a feedlot fattening process prior to slaughter. As will be discussed below, this has implications regarding final concentrations.
expand table Table V3 7-7