of the Risk Analysis for the Round Two Biosolids Pollutants (Dioxins,
Furans and Co-planar PCBs), dated December 14, 1999
- By Ellen Z. Harrison, Director,
Cornell Waste Management Institute
- This review was performed by
Ellen Harrison at the request of Versar, Inc. under a contract
with the US EPA.
- This review of the Risk Analysis for
the Round Two Biosolids Pollutants (the RA) performed by Abt
Associates on behalf of the US EPA focuses on areas in which
I and some of the colleagues with whom I consulted have expertise.
- Overall this is a disappointing document
from both the point of view of ease of understanding and also
methodology and execution. The level of detail is scant as compared
to other related documents prepared for EPA. Working through
examples of the actual application of the various algorithms
would make it significantly easier to evaluate and this is done
in many similar risk analysis documents. Key assumptions are
not well explained or justified. Specific examples of these deficiencies
are noted below.
- Risk Assessment Issues
It is not clear why the
RA took the approach of assessing the impact of application of
sludges containing 300 ppt TEQ dioxins rather than working from
an "acceptable" risk level to derive a concentration
in sludge that would correspond to that level of risk (as was
done for Round 1 pollutant evaluation). This latter approach
lends itself more easily to adjusting calculations if cancer
risks are reassessed (as seems to be in the works). The approach
taken in the RA seems geared towards rationalizing the acceptability
of application of all but the most dioxin-contaminated sludges
since 95% of US sludges would meet the 300 ppt level.
- Use of different assumptions in the
risk assessment process leads to different standards. Using a
risk assessment process, the state of Wisconsin evaluated dioxin
risks from paper sludge land application and derived a cancer-based
standard of 1.2ppt in soil associated with a one-in-a-million
risk and a value of 0.19ppt where grazing is allowed (Goldring,
1992). If a higher allowable cancer risk of one-in-ten-thousand
is accepted by EPA in the sludge rules, acceptable levels based
on the Wisconsin assessment would be 19 ppt in soils where grazing
is allowed. Our understanding of the RA is that 30 ppt TEQ in
pasture soil was calculated from the application of sludges containing
300 ppt under the assumptions used in the RA (Exhibit 3-7). Thus
even based on cancer as an endpoint, using the Wisconsin risk
assessment assumptions and methods would result in a lower standard.
This makes the point that what assumptions are made is critical
to the RA results.
- As discussed below, however, cancer
may not be the most sensitive end point where risks greater that
one-in-a-million are accepted. If, however, the reassessment
of cancer risk for dioxins shows it to be 10-100 times greater
than assumed in the RA, then cancer would likely be the sensitive
end-point based on current information.
- Simultaneous exposure to multiple
- Realistically, the HEI (which is specified
appropriately in the RA as the farm family) would be exposed
to multiple pathways simultaneously. Only in the breast feeding
scenarios (15a and b) are multiple exposure pathways considered
in the RA, and then only for the mother and not the child. For
pathway 15a, the HEI breast-feeding farm family mother would
be exposed via pathway 2 (home gardener) rather than pathway
1 as analyzed. The farm family is likely to have a garden and
to eat from it.
- The RA provides no justification for
this failure to sum exposures to multiple pathways. Protocols
are suggested elsewhere by EPA for the assessment of human health
risk from hazardous waste combustion facilities (EPA 1998), where
dioxin-like compounds are a primary issue and exposure routes
would be similar. There, it is suggested that the appropriate
exposure scenario is defined as a combination of exposure pathways.
For the HEI, the following pathways are listed (Table 4-1, EPA
1998) as applicable: direct inhalation of vapors and particles;
incidental soil ingestion; ingestion of drinking water; and ingestion
of homegrown foods (including produce, beef, milk, chicken, eggs
and pork); plus potentially the ingestion of fish from on-farm
ponds or watercourses.
- Assessing the risk to the breast feeding
farm child should evaluate more than only intake via breast milk.
Inhalation, while representing a relatively low exposure route
for adults, may be higher for infants given their greater respiration
rate. This should be evaluated. In addition, a more holistic
evaluation of the exposure of young children is needed (see below).
- Without making adjustments to the risk
levels calculated in the RA based on the suggestions that follow,
it is difficult to estimate the cancer risk for the HEI based
on multiple pathway exposure. However, summing the cancer risks
for each of the relevant pathways calculated in the RA (Exhibit
4-1), namely pathways 2 (home garden), 3 (soil ingestion), 4
(animals eating forage), 5 (animals eating soil), 11 (inhalation
of dust), 12 (drinking water and ingesting fish), and 13 (respiration)
results in a 1.3 x 10-4 cancer risk. This does not include dermal
exposure, adult soil ingestion, nor consumption of poultry and
eggs. It also does not reflect the reassessment of cancer risks
reflecting a 10 to 100 fold greater potency than used in this
RA which is currently under discussion at EPA. The rationale
for including both pathways 4 and 5 as well as numerous other
modifications of the risk calculations are discussed below.
Additional pathways not included in the RA
The dermal exposure pathway is not addressed at all in the RA,
nor in the EPA 1998 document. It appears that this path may be
among the several more significant exposure routes when soil
levels are the concern. It represented 8% of the contribution
for farm family exposure as calculated in the useful evaluation
of exposure scenarios to only 1 ppt in soil in the EPA Estimating
Exposure to Dioxin-like Compounds (Table III-4, US EPA 1994).
- Exposure via poultry including particularly
eggs is not included in the RA. Recent work suggests a cancer
risk of 10-4 for individuals eating 3-4 eggs per week from chickens
exposed to soils containing 30-40 ppt TEQ (Harnly et al., 2000).
This is not an unlikely pathway for farm family exposure and
should be included. Free range poultry eat soil and would bioaccumulate
dioxins. Geese were found to consume 8% of their diet as soil
and wild turkey 9% (Beyer et al., 1994). Free range poultry are
suggested to exhibit similar behavior, with 10% of their diet
being soil (US EPA, 1998). Chickens accumulate dioxins and poultry
foraging on soils containing low ppt PCDD/PCDF levels are predicted
to bioaccumulate these compounds to unacceptable levels (Stephens
et al., 1995). The risks to those consuming poultry and eggs
should be assessed in the RA and the risks added to those of
other relevant pathways.
Uncertainty and variability
The point estimates used for a number of parameters in the RA
are questionable due to both variability and uncertainty. These
include many of the parameters discussed further below. The RA
should at least provide a sensitivity analysis to assess the
impact of the deterministic assumptions on the results. Better,
a revised risk assessment using probabilistic methods which incorporate
the various suggestions in these comments and which then calculates
a daily intake that includes background intake from non-sludge
sources could be conducted.
- Realistic data and scenarios incorporating
the variability and uncertainty should be used in a multi-media,
multi-exposure probabilistic analysis. EPA itself recognizes
the need to represent variability and uncertainty in risk assessments
and the value of probabilistic tools such as Monte Carlo analysis.
(US EPA, 1997a; US EPA, 1997b, US EPA, 1999).
- There are many examples of parameters
for which variability can play an important role in quantifying
risks. Using probabilistic methods to analyze impact of breast-feeding,
Hoover showed that fat content in breast milk is a key variable.
If an inappropriate single point value was used, results are
skewed. (Hoover, 1999). Dietary intake of key foods such as meat,
eggs, poultry and milk varies widely.
- Uncertainty also makes the use of single
point estimates questionable. Uncertainty is great regarding
dioxin cancer potency and toxicity due to very different responses
of different species. There are inadequate data on impacts on
humans during fetal development and infancy (Hoover, 1999) (also
inadequate data on wildlife for these same developmental impacts).
- TEFs are continuing to evolve, so their
use is important but a source of uncertainty. The recent WHO
consultation reevaluted them and recommends revisions that would
result in an approximate 10% increase in TEQ calculations compared
to using I-TEFs (WHO, 1998). There are a number of other halogenated
compounds that could contribute to the total concentration of
compounds exhibiting similar toxicity. These include brominated
analogues of PCDD and PCDFs (Van den Berg et al., 1998). Polybrominated
biphenyls and dioxins seem to pose similar risks to dioxins and
PCBs (US EPA, 1994; Hornung et al., 1996; Helleday et al., 1999,
Weber and Greim, 1997; Henck et al., 1994). They appear to be
carcinogenic (Hoque et al., 1998; Henderson et al., 1995). They
are detected in sludges (Hagenmaier et al., 1992a). If they act
in similar toxicologic or oncogenic mode to dioxins, they need
to be factored into the risk assessment or at least recognized
as contributing to the risk. Numerous other PCBs might also be
- In regard to the groundwater pathway,
the role of facilitated transport of dioxins bound to organic
matter needs to be investigated. Sorption onto organic matter
may give rise to the facilitated transport of these compounds
into ground water (Nelson et al., 1998). This is a particular
concern as complexation of hydrophobic chemicals with organic
matter can also inhibit the ability of microorganisms to degrade
these compounds even though they may still be available and therefore
toxic to higher organisms (Rinella, 1993 #4).
- In assessing the home garden scenario,
there does not appear to have been specific assessment of the
exposure due to eating of members of the genus Cucurbita. This
group has been shown to exhibit unusually high uptake (Hulster
et al., 1994). These squash and cucumbers are commonly grown
vegetables in the home garden. Whether their contribution to
exposure via this pathway is important or not needs to be evaluated.
Unclear assumptions and rationale
A number of the assumptions in the RA need to be more transparent.
For example, it is not clear why the application rates in the
RA is changed from Round 1 TDS assumptions. The soil ingestion
rate assumptions were also e changed. The rate of daily ingestion
changed from 200 to 400 mg/day and the assumption regarding what
is ingested changed from straight sludge to a sludge soil mixture.
The proportion of sludge in this mix and the concentration of
contaminants in that mixture is not clear in the RA. Since the
RA suggests that this is the highest risk pathway, these assumptions
are critical and need to be clear and well justified. Better,
a probabilistic look at this pathway would be important given
some of the uncertainty surrounding soil concentrations (see
below) as well as ingestion rates.
- A critical example about which we would
seek clarification has to do with how soil concentrations were
calculated in the RA (Exhibit 3-7). Various equations are displayed,
but the justification for the values used are not clear. It is
not clear how much of the dioxins is projected to leach, volatilize
and be lost through erosion. While the equations for loss are
given, working out of examples is needed for clarity and for
- The RA faces a complex task in trying
to simultaneously assess the risks posed by an array of related
chemicals that possess certain similarities, but also behave
differently from each other. The behavior of the different congeners
in regard to partitioning and fluxes, breakdown and metabolism,
etc. are significantly different (Douben et al., 1997; Cousins
et al., 1997). For some of the parameters, individual values
are used for each of the congeners, while for others a single
value is used to represent all the congeners. The RA should be
more explicit about when each of these different approaches is
- In the RA several algorithms include
a parameter which is the area receiving sludge. The figures used
are not consistent in different sections of the RA. For calculating
inhalation exposure, 1074 hectares are assumed to be sludged,
while only 28 hectares is used in the surface water and air to
plant pathways. No justification is provided for either of these
figures nor for why two different figures would be used in the
- The frequency of application of sludge
is assumed to be every other year in all parts of the RA except
the pasture, where it is assumed to be applied every 3 years.
There is no rationale or explanation for this difference in assumptions.
The RA appears to assume zero dioxin ingestion by animals in
yrs 2 and 3 when no sludge is assumed to be applied. It is not
reasonable to assume that levels fall to zero in pasture and
that there is no residual in the years following application.
- In the RA, many of the risk assessment
assumptions questioned in our analysis of Round 1 (Harrison et
al., 1999) also pertain to this assessment. For example, a very
low percent of watershed is assumed to receive sludge (0.006%).
For small watersheds, this is not a realistic assumption. In
our experience on a dairy farm in upstate New York, large acreage
can receive sludge and be the majority of the watershed of small,
but fishable, receiving streams. Other EPA assessments suggest
that on-farm ponds may be a source of farm family fish and should
be evaluated (US EPA, 1998). Another variable which we found
in previous analysis to underestimate potential exposure is the
produce intake of home gardeners. Use of a single point estimate
of dietary intake does not seem appropriate for the wide range
of consumption patterns.
Cancer is not most sensitive end point if 10-5risk
A wide range of non-cancer effects are found (Brouwer
et al., 1998). Non-cancer endpoints, especially developmental
impacts through fetal and nursing infant exposures, are more
sensitive end points than cancer where less than one-in-a-million
cancer risk is used as cancer end point (data in US Dept. of
Health and Human Services, ATSDR, 1998; WHO, 1998; Dr. Henry
Anderson, Wisconsin Department of Health and Social Services,
personal communication). The immune system is a sensitive target
for toxicity (US Dept. of Health and Human Services, ATSDR, 1998).
- Developmental behavioral impacts were
found at a does of 0.12 ppt/kg bw/day in rhesus monkeys and developmental
effects are found to be among the most sensitive LOAEL in animals
leading to a chronic oral MRL based on these effects (US Dept.
of Health and Human Services, ATSDR, 1998).
Impact to wildlife and soil organisms not assessed
Data on ecological impacts are limited. However, depending
on what level of human cancer risk is determined to be acceptable
(and what cancer potency factor is used), wildlife may be a more
sensitive endpoint. Ontario has developed a soil concentration
limit of 10 ppt TEQ based on protection of wildlife (earthworm
eating birds) (Birmingham, 2000). Shrews may be at risk from
TCDD and TCDF in land applied paper mill sludges (Abt Assoc.,
1994). Wildlife ingest significant quantities of soil (17% of
diet for nine banded armadillo, 10% for woodcock, 9% wild turkey)
(Beyer et al., 1994). Whether this represents a risk due to sludge
application is not clear. Thus risks to predators including raptors
higher on the food chain need to be assessed.
This RA does not assume any background exposure and is
only an assessment of incremental exposure from sludge application.
- A WHO panel recently completed a reevaluation
of PCB/PCDD/F and determined that: 1) a revised TDI of 1 to 4
pg/km body weight is established with 4 considered a maximum
for long-term exposure on a provisional basis and 1 as a goal;
2) existing background exposures may be causing subtle effects
at current intake levels of 2-6 pg TEQ/kg bw/day; 3) efforts
should be made to limit environmental releases to the extent
feasible. (WHO, 1998; Brouwer et al., 1998). Thus in an industrialized
country like the US, current exposure from background sources
may already put us at risk of unknown nature and extent. The
RA should include background exposure in the assessment.
Critical to the assessment of risks from a number of
pathways is the soil concentration of dioxins resulting from
the application of sludge. The RA calculates this by using models
for the loss via vaporization, erosion, leaching and degradation
(assumed to be zero for dioxins). Applying the models, a final
average concentration of approximately 30 ppt TEQ for agricultural
land is calculated for years 26-100 (Exhibit 3-7). It is unclear
why this average over years 26-100 was selected. This assumption
should be discussed and rationalized. Assessing risks at the
end of the 100 year application period would be an equally valid
assumption, especially since we will not be able to necessarily
identify locations where EQ sludges have been applied.
- Soil concentrations resulting from
application are a key variable. The approximately 10 fold reduction
in concentration predicted in the RA does not seem warranted
nor does it seem to agree with much of the literature nor with
projections in the Abt Associates document on risks to terrestrial
wildlife from papermill sludges (Abt Associates, 1994). Volatilization
and leaching are thought by most researchers to be insignificant
for dioxins (Fries and Paustenbach, 1990). Even following surface
application of sludge, when volatilizations and photodegradation
would be greatest, PCDD/Fs were found to be fully persistent
(McLachlan et al., 1996). Various field studies show little or
no loss of dioxins from soils (Orazio et al., 1992; Hagenmaier
et al., 1992 b). Half-life in soil is a critical parameter and
for TCDD estimates vary widely and are also very dependent on
whether the material is in the surface soil or subsurface (estimates
from 9 to 100 years in US Dept. of Health and Human Services,
ATSDR, 1998). In contrast, field observations of sludge-borne
PCBs show a diminution of concentration, likely due to volatilization
(Alcock et al., 1996).
- Use of models to predict key transfers
of compounds through the agricultural ecosystem, particularly
the air to plant transfer, is problematic for a number of reasons
(Douben et al., 1997). For some congeners of PCBs, model predictions
of volatilization are quite accurate, while for others predicted
fluxes underestimated by more than an order of magnitude (Cousins
et al., 1997). Since there is significant question about the
validity of the models, it has been suggested that use of measured
concentrations is a more accurate approach (US EPA, 1994). Such
data indicate very low disappearance rates for dioxins. With
a half life of over 20 years being realistic (McLachlan, 1997).
Research on related chlorinated hydrocarbons further indicates
that such compounds become more recalcitrant over time such that
the concept of a half life obeying first order kinetics is inappropriate
(Linz and Nakels, 1997).
- A calculation of soil concentrations
of dioxins on agricultural land where no loss through leaching,
erosion or volatilization is predicted and where dilution via
mixing into the top 15 cm of soil is the only factor diminishing
concentration would be an upper bound for soil concentrations.
At year 100, given 5 T/ac/yr application of sludges containing
300 ppt TEQ, a concentration of 70 ppt TEQ would result. The
average for years 26-100 would be approximately 55 ppt. While
not accepting that the average value over years 26-100 is appropriate
as the concentration to use in the assessment, the calculated
value of 30 ppt in the RA which shows nearly half of the applied
contaminants are no longer present seems questionable. This is
a place in which the uncertainty of the models is large and the
implications for the assessment are also large. While dilution
alone may be overly conservative, the models used appear to overestimate
A farm family is the appropriate HEI. Their exposure
is potentially higher than others for many of the pathways. They
tend to be on-site more, thus having a longer respiratory exposure
time (EPA, 1994). They are also likely inhale more particulates
and adult soil ingestion might be expected to be relatively high.
Farm families also eat significantly more home grown vegetables
than other populations (US EPA, 1999).
- A key set of parameters is proportion
of meat, milk, eggs and poultry ingested which are raised on
the farm. Since these are the primary dietary sources of dioxins
and since diet is the primary non-occupational exposure route,
this is a critical issue for the farm family. The RA states in
Exhibit 3-3 that for pathways 4 and 5, the HEI evaluated is the
farm household producing a major portion of the animal products
they consume and that the animals eat soil and plants grown in
soils amended with biosolids. However, the RA does not seem to
follow this approach. (This is an example of the lack of transparency
in the RA. While Exhibit 3-3 makes the statement above, Exhibit
3-12 shows a very small fraction from sludged sources.)
- The first question is how much dairy
and meat fat are consumed. No rationale is provided for why the
RA uses the mean non-metropolitan intake of beef, lamb and game
fat and the 95th percentile value for diary fat. Using figures
specific to the HEI farm family would make more sense and be
consistent with evaluating risks to the HEI. The Exposure Factor
Handbook (US EPA, 1999) shows that farm families consume a mean
intake of 2.63g/kg/day of home produced beef and the highest
group ingests 8.9 g/kg/day (Table 13-18). At 10% fat content
(US EPA, 1999), these levels correspond to a fat consumption
of 0.263 g/kg/day (mean) or 0.89g/kg/day (high end), far higher
than the 0.0818 g/kg/day in Exhibit 3-12. For dairy, mean consumption
for the farm family is 1.7 g/kg/day and the high end is 9.1.
With a fat content of 4% this translates into 0.68-0.364 g/kg/day
of fat from dairy, significantly lower than the 1.76 g/kg/day
used in the RA.
- The second question is the percentage
of the consumed foods coming from animals exposed to sludge.
While Exhibit 3-3 implies that the RA assumed all animals were
exposed, this does not seem to be how the RA was actually carried
out. Exhibit 3-12 shows that very low estimates for the fraction
of consumption derived from sludged soils were used. Contrast
the values of 0.097 for beef fat and 0.031 for dairy fat daily
ingestion from sludge amended soils used in the RA with 0.44
and 0.40 respectively suggested as home-grown percentages for
farm families in the EPA document Estimating Exposure to Dioxin-like
Compounds (EPA 1994). These estimates are derived from a
USDA farm survey, which while dated (1966), is likely more accurate
than figures derived from the general non-metropolitan population
(few of whom raise livestock) used in the RA. Data on the proportion
of meat and dairy eaten by farm families is provided in tables
13-71 and 13-72 of the Exposure Factor Handbook (US EPA, 1999).
The values given are that an average of 32% of all meat, 47%
of beef, 25% of all dairy, 21% of eggs and 15% of poultry consumed
by farm families are home grown. It is reasonable to assume that
all of the home-grown sources would be sludge-exposed.
- Not evaluated in the RA is exposure
from eggs. Farm families eat a mean of 9 g/kg/day of eggs (US
EPA, 1999). At a fat content of 8.4% (US EPA, 1999, this represents
0.75 g/kg/day of fat from eggs, a potentially significant exposure.
The farm family also eats an average of 1.54 g/kg/day of poultry
(US EPA, 1999) and these may be birds which free-range on sludged
- The farm family is also likely to inhale
air that contains particulates carrying contaminants as well
constituents volatilized from sludge. They may drink water that
has received sludge-leached inputs. Their diet may likely include
vegetables grown on sludge-amended soils and may include fish
caught from ponds or streams impacted by sludge. Thus analysis
of simultaneous exposure to multiple pathways is needed.
- A revised risk assessment based on
farm family health including prenatal and nursing exposure to
infants of mothers residing on dairy farms is needed in which
higher values are used for the percent of diet from sludged sources
as well as simultaneous exposure from multiple pathways.
Risk to children
Given the significance of the soil ingestion pathway
which alone represents a cancer risk of 8.3x10-5 according the
Exhibit 4-1 in the RA, a more comprehensive analysis of the risks
to children is warranted. Since the risks to a child from the
soil ingestion pathway alone is nearly one in ten thousand, it
is critical that the total risk to which an HEI child might be
exposed be evaluated. This should include summing exposure to
multiple pathways including: eating home-grown produce, using
realistic consumption data for a farm family; soil ingestion;
breast feeding for 6-12 months; drinking milk from farm raised
cows exposed via pathways 4 and 5; breathing air and inhaling
dust; and ingesting water. Dermal exposure and exposure via eating
of eggs and poultry should also be evaluated.
- The inclusion of an assessment of the
exposure of breast-feeding infants is appropriate and even with
the limitations in the RA pointed out in this review, that pathway
is potentially significant. Average daily intake of dioxins for
breast-fed infants on a body weight basis may be almost 1-2 orders
of magnitude greater than that of an adult (WHO, 1998). Milk,
a food in which bioaccumulated dioxins are shed and thus present
in significant quantities is a primary food for infants and children.
Over 95% bioavailability of most PCB, dioxin and furans from
breast milk has been reported. (McLachlan et al., 1993; McLachlan,
1996; Hoover, 1999). A 90% absorption was assumed in the RA which
may thus underestimate the risk. As pointed out above, the risk
may also be underestimated since the mother may be exposed via
additional pathways not included in path 15 a or b (including
the home garden pathway).
- Given the significance of the soil
ingestion pathway, which alone represents a cancer risk of 8.3x10-5
according the Exhibit 4-1 in the RA, a more comprehensive analysis
of the risks to children is warranted. Since the risks to a child
from the soil ingestion pathway alone is nearly one in ten thousand,
it is critical that the total risk to which an HEI child might
be exposed be evaluated. This should include summing exposure
to multiple pathways including: eating home-grown produce, using
realistic consumption data for a farm family; soil ingestion;
breast feeding for 6-12 months; drinking milk from farm raised
cows exposed via pathways 4 and 5; breathing air and inhaling
dust; and ingesting water. Dermal exposure and exposure via eating
of eggs and poultry should also be evaluated.
- Current levels of exposure to nursing
infants exceeds the 4 pg TEQ/kg level TDI recommended by WHO
(WHO, 1998) for nearly the entire population in Canada (Hoover,
1999) and we would expect similar results for infants in
the US. Thus background exposures cannot be ignored in assessing
risks to children. Most standards are developed based on adult
models. Some research has tried to improve on this by incorporating
pharmacokinetic adjustments for infants. "The breast-fed
infant's intake of organochlorines has been found in general
to exceed guidance values, raising the possibility that breast-feeding
may pose health risks" (Hoover, 1999, p 528).
- The RA should include assessment of
non-cancer risks to children, including fetuses. Sensitivity
to toxicity of PCDDs may be greater during the fetal/neonatal
period than for adults and may have an impact on male reproductive
system development (US Dept. of Health and Human Services, ATSDR,
1998). Cognitive functioning in preschool children is negatively
impacted by in utero exposure to PCBs and dioxins (Patandin et
al., 1999). Birth weight and postnatal growth until 3 months
of age were impacted by in utero exposure to PCBs and dioxins
(Patandin et al., 1998). Immune suppressive and delayed reproductive
effects are also a concern. A probabilistic risk evaluation of
organochlorine exposure through breast milk showed that for a
significant percentage of the population, PCBs and PCDD/PCDFs
provide the greatest concern for non-cancer health effects from
chemicals in breast milk (Hoover, 1999).
Exposure through animals ingesting sludge Pathways
4 and 5
- The RA underestimates the exposure
via pathways 4 and 5. The RA calculations suggest that pathway
5 represents a relatively significant source of exposure, as
such it is critical that it be closely analyzed for errors and
uncertainties. The assumptions regarding this pathway contain
- In assessing the impact of animal diet
there are numerous routes of exposure which need to be assessed
simultaneously. These include: 1) direct soil ingestion from
pasture (the soil ingested should use contaminant concentration
in the sludge and not diluted by tillage as discussed below);
2) ingestion of forage which has been grown in sludge-amended
pasture and which thus would have contaminant loads due to uptake
of volatilized contaminants (the model for this uptake should
be based on contaminant concentration in the sludge and not diluted
by tillage as discussed below); 3) sludge adhering to the foliage
of pasture-forage; 4) ingestion of forage which has been grown
on sludge-amended tilled soils and which would thus have contaminant
loads due to uptake of volatilized contaminants; and 5) soil
contained in harvested forage from tilled soils. These are further
- As mentioned above in the section on
farm family risk, the percentage of dietary intake of meat and
dairy coming from on-farm and thus from sludged sources is significantly
underestimated. In addition, pathways 4 and 5 for animals ingesting
forage and for animals ingesting soil need to be summed since
a farmer using sludge would likely use it on pasture and also
on field crops (US EPA, 1998). Thus the animals would be exposed
both through contaminants in forage (pathway 4) as well as through
soil ingested directly (pathway 5). Whether eating forage directly
from a pasture or eating harvested forage crops, the animal will
be ingesting forage to which sludge should be assumed to have
- In order to appropriately calculate
the exposure of these animals, the fraction of forage obtained
from harvested crops (pathway 4) and the fraction obtained from
eating while in pasture needs to be estimated. Since harvested
crops are likely to be grown where sludges have been tilled into
the soil, some dilution is expected. For pasture-grown forages,
however, the soil is not typically tilled (see discussion below)
and thus the plants would be exposed to the full concentration
of pollutants in the sludge. This is important because in modeling
the uptake of compounds into plants, the concentration of PCBs
in the soil surface is an important variable (Cousins et al.,
1998). This distinction between harvested and grazed forage consumption
is also important in assessing impacts of ingesting sludge which
adheres to the plants which is significant for pasture plants
(Chaney et al., 1996).
- Section 3.3.1 specifically mentions
that particulate pollutants are not considered, yet the literature
suggests that models used to calculate produce concentrations
should include exposure via plants due to small particles . Some
research suggests that re-entrainment and deposition of soil
particles is a minor contribution (Harrad and Smith, 1997). However,
most researchers suggest it is the major pathway for soil-bound
PCDD/Fs to aerial plant parts (McLachlan, 1996; Hulster and Marschner,
1995 in McLachlan, 1997; Smith and Jones, 2000).
- In rural areas, total dioxin deposition
may be 20-40% from re-entrained soil particles (Kao and Venkataraman,
1995 in Smith and Jones, 2000). In rural agricultural areas where
soils are tilled and animals disturb the soil, it would seem
that particulates are a critical aspect of dioxin exposure. The
relative importance of particulate vs. vapor-transfer into plants
is different for different congeners (McLachlan, 1997), suggesting
that a revised calculation of plant dioxin concentrations needs
to take particulate deposition into account using different values
for different congeners. The contribution of particulates to
total dioxin levels in vegetation was found to be significant,
with soil contamination increasingly significant with increasing
chlorination levels and with higher levels in soils (Smith and
Jones, 2000). For PCBs which are more volatile, this route was
less important (Smith and Jones, 2000).
- It would also appear that the RA does
not take into account the fact that significant amounts of soil
are contained in harvested field crops, with several percent
soil being estimated in harvested grass silage (Berende, 1990
in McLachlan, 1997). For grasses, it is, and a worst case situation
is 100 mg soil g-1 grass DW (Smith and Jones, 2000). This soil
which "tags along" with harvested forage needs to be
included in the RA pathways 4 and 5. Calculations of the contribution
of particulate contamination accounted for 30% of the total PCDD/Fs
found in grass containing a conservative background estimate
of 20 mg soil g-1 grass DW, while soil was responsible for a
majority of the pollutant load under a worst case situation of
100 mg soil g-1 plant DW (Smith and Jones, 2000). PCBs were less
related to soil load.
- In assessing the impact of soil contamination
of plants, it appears that for PCBs and possibly also PCDD/Fs
that there is an increasing concentration with decreasing soil
particle size. If so, then the use of the bulk soil concentration
to calculate the impact of soil in contaminating the vegetation
would underestimate soil contribution (Smith and Jones, 2000).
- The assumptions made regarding the
amount of soil ingested by grazing animals (1.5% of diet is value
used in RA for all grazing animals) seems to be based on best
management practices. Grazing cattle ingest from 1-18% of their
dry matter intake as soils and sheep may ingest as much as 30%
depending upon management and the seasonal supply of grass (Fries,
1996; Thornton and Abrahams, 1981). One or 2% intake was a low
value obtained when availability of forage was greatest in the
spring and an average yearly intake of 6% for cattle was observed
when exclusively pasture-fed (Fries, 1996). Other researchers
use estimates such as 6% of diet as soil and point out the critical
importance of assessing the amount of sludge ingested through
material adhering to vegetation and the lack of data on
appropriate values for that variable (Wild et al., 1994). A previous
EPA document suggests 4% or 0.5kg/day soil ingestion by grazing
beef cattle and 3% or 0.4kg/day for dairy cows (US EPA, 1998).
Assumptions regarding the percent of time spent in pasture and
percent of diet obtained from pasture are important in determining
what value to use for soil ingestion. In New York state, dairy
experts suggest that cows are in pasture approximately half of
the year (Galton, 2000). Use of a probabilistic method for risk
assessment would allow a more realistic range of values to be
used for these key parameters.
- A key parameter is the concentration
of pollutant in the soil ingested by the grazing animal. The
RA is not clear on what value is used. Again, an example of where
the RA needs to be much more transparent about values that are
used in calculations. The RA presumably used the values given
in Exhibit 3-7 which are the calculated average values for years
26-100 resulting from application of 5T/ac/yr mixed into 15 cm
- Sludge applied to pastures is generally
not incorporated into soils yet the risk assessment appears to
assume that the sludge is tilled into the soil to the depth of
15 cm. Tillage into pastures is not common practice, hence mixing
would be minimal (Fries and Paustenbach, 1990; Wilson et al.,
1997). Rather, pastures are top-dressed with sludge and are plowed
only every 4 years or less (very rarely in the case of steep,
rocky or otherwise marginal lands). This would have a significant
bearing on the concentration of sludge-borne contaminants ingested.
Since the RA assumes that the animals are ingesting not sludge,
but a sludge-soil mix, this is a critical mis-assumption. Even
where mixed into soils, the concentration may be greater than
that calculated in the RA (see section on soil concentration).
- In addition, when sludge is spread
on pasture or growing crops, a significant amount ends up adhering
to the leaves of plants. This residue is not easily removed by
rainfall. Thus it is likely to be an important route of exposure
to pastured animals and also to animals eating harvested forage
(Wilson et al., 1997). The RA does not seem to account for this
exposure. The amount of sludge ingested due not to uptake or
soil ingestion but to ingestion of sludge adhering to plants
needs to be included in the assessment and this material would
contain the full concentration of pollutants contained in the
sludge and not be diluted by any soil mixing.
- For pathway 4, it is not clear , but
it would appear that the RA may have underestimated by a factor
of 100. Section 3.3.4 refers to the use of methods presented
in section 3.3.1 to determine the concentration of pollutants
in the forage. That section makes use of a Vgag, a correction
factor of 0.01 which was obtained from the EPA Human Health Risk
Assessment Protocol document, Chapter 5 (US EPA 1998). While
that document recommends the use of that factor for calculating
concentrations in produce, it specifically recommends the use
of 1.0, no correction, for pasture grass. This is another example
of how the RA is so abbreviated that it is not possible to determine
the methods used.
Bioaccumulation through agricultural food chains is a
critical aspect of assessing human health risks of PCDD/Fs in
sludge application and data are sparse (McLachlan, 1996). In
research done on a dairy system where inputs did not include
sludge, but only airborne contaminants, it appears that the higher
chlorinated compounds become less concentrated as you move up
through the agricultural food chain although unfortunately the
last link in the chain humans seem to exhibit significant
biomagnification (McLachlan, 1996).
- The ability to use models to predict
final concentrations of dioxin congeners in produce (including
meat and milk) varies widely among the individual congeners as
shown by a comparison of modeled and observed concentrations,
with some of the congeners that comprise the largest share of
TEQ contributions in sludge (such as OCDD) found in significantly
higher concentrations than predicted (Harrad and Smith, 1997).
The bioconcentration factor for various PCB congeners was predicted
to vary greatly (by a factor of 32) in a study of the air-milk
transfer of PCBs and the predicted values agreed reasonably with
measured values (Thomas et al., 1998). Different congeners exhibit
very different rates of absorption, metabolism and bioaccumulation.
The higher chlorinated congeners are generally less well absorbed
(McLachlan and Richter, 1998).
- The concentrations of PCDD/Fs and PCBs
varies with different plant species exposed at the same sites.
For relatively non-volatile compounds, the variability is generally
less than a factor of 4 and can be explained by plant surface
area/volume, while for more volatile compounds (like PCBs), interspecies
variability of 30 fold or more was observed (Bohme et al., 1999).
This variation may be important in assessing the exposure through
pathways 2 and 4.
- In assessing uptake of dioxins into
animals, the duration of the experiments is important. For dairy
cows, a steady state concentration is predicted due to the excretion
of dioxins in milk after 9-11 weeks (Heeschen et al., 1994 in
McLachlan and Richter, 1998) while in beef cattle concentrations
will increase (although dilution due to increasing biomass is
important to consider) (McLachlan, 1997). In short term experiments
of less than 6 months duration, equilibrium may not have been
reached (Fries, 1996). The amount absorbed by cows is a critical
parameter and it seems to be very sensitive to the Kow of the
compound and these values are very uncertain (McLachlan, 1997).
The suggestion that absorption of PCDD/Fs are inhibited by being
in a sludge matrix does not seem to be justified by experimental
data (McLachlan and Richter, 1998).
This pathway is identified as the one posing the greatest
cancer risk (Exhibit 4-1). As such, it is one that should be
closely examined. Unfortunately the RA gives it only a brief
consideration. No rationale is provided for some of the assumptions
made nor is it clear what value is used for the critical soil
concentration parameter Cj.
- The RA has modified the soil ingestion
pathway analysis from the Round I assessment. The quantity of
soil ingested has been raised from 200 to 400 mg/day. At the
same time the assumption that the soil ingested is pure sludge
has been changed to ingestion of a soil/sludge mixture. The concentration
of that mixture is not specified (at least not that I could find).
Does the RA assume that the exposure is to a sludge/soil mix
containing the ~30 ppt concentration calculated in Exhibit 3-7?
If so, that is likely to be an underestimate even for sludges
incorporated into agricultural fields (see section on soil concentrations).
It might greatly underestimate concentrations for sludge products
which might be applied to home gardens (like composts) and not
- It is not only children who ingest
soil. Adults ingest soil as well and teenagers may ingest more
than adults. As recognized in Estimated Exposure to Dioxin-like
Compounds (EPA, 1994), failure to include soil ingestion
by older children and adults may underestimate lifetime soil
ingestion. Since this pathway represents the highest risk, it
would be important to include estimates of lifetime exposure
via soil ingestion. While I did not find literature on it, it
would be reasonable to suggest that farmers and home gardeners
ingest higher quantities of soil than average.
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