# Conclusions and Recommendations 

## Introduction 

This chapter discusses conclusions and recommendations based on the
analyses described in the preceding chapters. Most of the conclusions
relate to the results of the analyses, not the methods used to obtain
results. The recommendations are intended to address unresolved issues
that have arisen during the course of the work. The Savannah River Site
Health Effects Subcommittee (SRSHES) raised some of these unresolved
issues during presentations and discussions on this work. Other
unresolved issues arise from the analysis but have not been resolved
because they are beyond the scope of the current task. In general, while
the resolution of these issues is not expected to change the major
conclusions of this work, it is expected to enhance confidence in the
conclusions.

## Conclusions

The following bullets summarize the major conclusions of this dose
reconstruction. These conclusions are then described in greater detail
in the subsequent subsections.

-   Doses and risks are small for all receptors and scenarios relative
    to doses and risks from background radiation over the 39-year period
    of the study.

-   For people who ate fish from the Savannah River or Lower Three Runs
    Creek, fish ingestion was the most significant pathway, and the most
    important radionuclides were generally ^137^Cs, ^32^P, and ^90^Sr.

-   For people who did not eat fish from bodies of water contaminated by
    releases of radionuclides to water, milk and beef were the most
    significant pathways and ^131^I and tritium were the most important
    radionuclides.

-   Immersion in ^41^Ar was a generally small, but constant contributor
    to dose; however, for some receptors, it was the largest fraction of
    a small dose.

-   Larger doses occurred in years corresponding to larger releases from
    the Savannah River Site (SRS), especially ^131^I; for the Adult
    Male, Adult Female, and Child Born in 1955, a large fraction of the
    total dose was received during the years 1955-1961.

-   There were important differences in doses, pathway significance, and
    radionuclide significance between children born in 1955 and children
    born in 1964---those born in 1955 experienced the large I releases
    early in the site history, while those born in 1964 did not
    experience them.

-   Doses caused by ingesting fish from Lower Three Runs Creek were
    significantly higher than doses caused by ingesting fish from the
    Savannah River.

-   For air releases, the variations in air dispersion of radionuclides
    from the site generally produced a significant, but not dominant,
    variation in estimated doses.

-   Consideration of uncertainty in the variables used to estimate doses
    could cause an estimated dose to be higher or lower than the
    corresponding point-estimate result. The mean of the distribution of
    total dose for any receptor ranged between 2.15 to 1.07 times the
    corresponding point-estimate dose; thus, the means of the uncertain
    doses were close to the corresponding point-estimate values.

-   The use of hypothetical scenarios to demonstrate the interactions of
    a range of receptor behaviors with the site and release
    characteristics was an effective analytical tool.

### Doses and Risks Are Small

Calculated doses and risks to the hypothetical receptors appear to be
small. The largest point-estimate dose was 9.4 milliSieverts (mSv) (0.94
rem) over the 39-year period studied for the Outdoor Family Child Born
in 1955; the corresponding risk of cancer incidence is 0.10 percent, and
the corresponding risk of cancer fatality is 0.024 percent. By way of
comparison, the annual average radiation exposure for a member of the
U.S. population is about 3.6 mSv (0.36 rem) (1), mainly from naturally
occurring sources of radiation and medical sources (e.g., x rays). An
annual background dose of 3.6 mSv over a period of 39 years would
produce a dose of 140 mSv (14 rem).

Although estimated doses (and the risks of cancer incidence) were higher
when uncertainties in variables were considered, the increases are not
sufficient to change this conclusion. For example, when uncertainties
were considered, the Outdoor Family Child Born in 1955 received the
largest mean dose of 13 mSv (1.3 rem) and the largest median dose of 11
mSv (1.1 rem). However, the maximum dose for the same receptor was 60.3
mSv (6 rem) and the minimum dose for the same receptor was 2.53 mSv
(0.25 rem).

### Important Exposure Pathways and Radionuclides 

[Chapter 11](../Chapter_11_ready%20for%20CDC%20031405.doc) discusses the
point-estimate doses for each receptor within each of the seven
scenarios, including the contributions to these doses by the various
radionuclides and exposure pathways. Although each scenario and each
receptor within the scenario demonstrated unique characteristics, some
generalizations can be made about the radionuclides and exposure
pathways contributing most to dose. For any given realization in the
uncertainty analysis, the importance of the various radionuclides and
scenarios could be different; however, the same general trends that
occur in the point-estimate analysis appear to be present. Other
conclusions from the uncertainty analysis are presented in Section
13.2.8.

For scenarios exposed to water releases through ingestion of fish taken
from the Lower Three Runs Creek and the Savannah River downstream of the
SRS (Delivery Family, Outdoors Family, and Near River Family), the
dominant radionuclides were ^137^Cs, ^90^Sr, and ^32^P. Table 13-1 shows
the fraction of dose resulting from fish ingestion for the 12 receptors
in the 3 scenarios exposed to water releases. In many cases (8 out of
12), the percentage of dose from fish ingestion is greater than 50
percent. For 83 percent of the receptors (10/12), fish ingestion was the
largest source of dose. Beef ingestion edged out fish ingestion for the
two remaining receptors.

Table 13-2 shows the percent of total dose over 39 years for a selected
set of radionuclides. ^137^Cs, ^90^Sr, and ^32^P were the dominant
radionuclides for exposures to water releases by fish ingestion. This is
shown most clearly by the large percentage of dose from these isotopes
for all members of the Delivery Person Family. However, for the Child
Born in 1955 for the Delivery Person Family, ^131^I (from ingestion of
terrestrial foods contaminated by air releases) was a significant source
of dose; for the Children Born in 1964 in the Outdoors Family and the
Near River Family, ^32^P was the most important isotope. For the Adults
in the Near River Family, ^137^Cs was the most important radionuclide.
For the Adults in the Outdoors Family, ^131^I was the most important
radionuclide.

Thus, it can be seen that the fish ingestion pathway accounted for a
large fraction of the dose for most of the receptors exposed to water
releases. The important radionuclides producing dose through this
pathway depended, in part, on the timing of the exposure; for Children
Born in 1964, important radionuclides tended to shift from ^137^Cs to
^32^P for water resources (see Section 13.2.5 for additional
discussion). In addition, for the Children Born in 1955, ^131^I, which
is not related to fish ingestion, was an important isotope (the most
important isotope for two scenarios).

Table 13‑1 Percent of Total Dose from Fish Ingestions for Various
Receptors in the Three Scenarios Exposed to Water Releases

  ----------------------- ----------------------- ------------------------
  Scenario                Receptor                Percent of Total Dose
                                                  from Fish Ingestion

  Delivery Person Family  Adult Female            92.8\*

                          Adult Male              90.2\*

                          Child Born in 1955      57.9\*

                          Child Born in 1964      92.9\*

  Outdoors Family         Adult Female            48.2\*

                          Adult Male              34.6

                          Child Born in 1955      11.8

                          Child Born in 1964      79.3\*

  Near River Family       Adult Female            71.0\*

                          Adult Male              66.2\*

                          Child Born in 1955      35.4\*

                          Child Born in 1964      82.3\*

  ^\*^For these                                   
  receptors, fish                                 
  ingestion was the                               
  largest source of dose.                         
  ----------------------- ----------------------- ------------------------

Table 13‑2 Percent of Total Dose from Fish Ingestions for Various
Receptors in the Three Scenarios Exposed to Water Releases

  ---------- ------------------ --------- -------- -------- -------- --------
  Scenario   Receptor           Percent                              
                                of Total                             
                                Dose                                 

                                ^137^Cs   ^90^Sr   ^32^P    ^131^I   Sum

  Delivery   Adult Female       77.0      9.7      4.3      4.3      95.3
  Person                                                             
  Family                                                             

             Adult Male         74.8      9.5      4.2      6.0      94.5

             Child Born in 1955 39.8      11.3     5.4      37.9     94.4

             Child Born in 1964 49.5      25.1     15.0     \*       89.6

  Outdoors   Adult Female       18.8      \*       17.5     42.2     59.7
  Family                                                             

             Adult Male         14.4      \*       12.6     45.3     57.9

             Child Born in 1955 2.5       1.6      6.0      82.9     93.0

             Child Born in 1964 8.5       5.0      57.5     4.7      75.7

  Near River Adult Female       29.7      6.5      25.4     10.8     72.4
  Family                                                             

             Adult Male         28.1      6.2      24.1     14.5     72.9

             Child Born in 1955 9.0       5.0      18.0     50.8     82.8

             Child Born in 1964 10.9      5.7      59.7     \*       76.3

  \*5% or                                                            
  less.                                                              
  ---------- ------------------ --------- -------- -------- -------- --------

Thus, it can be seen that the fish ingestion pathway accounted for a
large fraction of the dose for most of the receptors exposed to water
releases. The important radionuclides producing dose through this
pathway depended, in part, on the timing of the exposure. For the
Children Born in 1964, the important radionuclides tended to change from
^137^Cs to ^32^P for water releases (see Section 13.2.5 for additional
discussion). In addition, for the Children Born in 1955, ^131^I, which
is not related to fish ingestion, was an important isotope (the most
important isotope for two scenarios).

### Important Exposure Pathways and Radionuclides for Air Releases

(Note: the general comments made in the initial paragraph of Section
13.2.2 apply here as well.) For scenarios not exposed to water releases
by ingesting fish (Rural Families One and Two, Urban/Suburban Family,
Migrant Family), the most significant radionuclides were ^131^I and
tritium, and the most significant exposure pathways were ingestion of
milk and beef.

Table 13-3 shows the percent of total dose from beef and milk ingestion,
and the combination for scenarios not exposed to water releases from the
SRS. Of these receptors, 75 percent (12/16) obtained more than half
their dose from these two pathways. For the remaining four receptors,
these two pathways together accounted for the largest fraction of dose
with air immersion also an important pathway.

Table 13‑3 Percent of Total Dose from Beef and Milk Ingestion, and the
Combination, for Scenarios not Exposed to Water Releases from the SRS

  ---------------- -------------- -------------- -------------- --------------
  Scenario         Receptor       Percent of     Percent of     Sum of Percent
                                  Total Dose     Total Dose     from Two
                                  from Beef      from Milk      Pathways
                                  Ingestion      Ingestion      

  Rural Family One Adult Female   50.7           16.1           66.8

                   Adult Male     57.1           15.3           72.4

                   Child Born in  43.2           43.9           87.1
                   1955                                         

                   Child Born in  15.3           35.4           50.7
                   1964                                         

  Rural Family Two Adult Female   52.4           16.1           68.5

                   Adult Male     58.8           15.3           74.1

                   Child Born in  43.7           44.2           87.9
                   1955                                         

                   Child Born in  14.6           32.4           47.0
                   1964                                         

  Urban/Suburban   Adult Female   42.6           24.1           66.7
  Family                                                        

                   Adult Male     27.4           13.1           40.5

                   Child Born in  30.4           55.7           86.1
                   1955                                         

                   Child Born in  11.6           31.7           43.3
                   1964                                         

  Migrant Family   Adult Female   52.1           11.6           63.7

                   Adult Male     58.7           11.0           69.7

                   Child Born in  48.2           36.2           84.4
                   1955                                         

                   Child Born in  17.8           22.4           40.2
                   1964                                         
  ---------------- -------------- -------------- -------------- --------------

\*Since the Migrant Worker Family was only present near the SRS for six
months, it may be more appropriate

for comparison purposes to double these doses.

Table 13-4 shows the percent of total dose from ^131^I, tritium, and
^41^Ar, and their combination, for scenarios not exposed to water
releases from the SRS. In these families, the Adult Male, Adult Female,
and Child Born in 1955 had most of their dose from ^131^I from a
combination of ingesting milk and beef. In these same families, the
Child Born in 1964 had most of its dose from tritium from a combination
of ingesting milk and beef.

Table 13‑4 Percent of Total Dose from Beef and Milk Ingestion, and the
Combination, for Scenarios Not Exposed to Water Releases from the SRS

  ---------------- ----------------- ---------- --------- ---------- -------
  Scenario         Receptor          Percent of                      
                                     Total Dose                      

                                     ^131^I     Tritium   ^41^Ar     Total

  Rural Family One Adult Female      73.8       12.0      8.0        93.8

                   Adult Male        76.9       11.3      5.7        93.9

                   Child Born in     93.3       3.8       1.5        98.6
                   1955                                              

                   Child Born in     9.3        63.9      16.6       89.8
                   1964                                              

  Rural Family Two Adult Female      76.5       9.9       7.7        94.1

                   Adult Male        79.4       9.4       5.5        94.3

                   Child Born in     94.2       3.0       1.4        98.6
                   1955                                              

                   Child Born in     10.4       59.9      18.8       89.1
                   1964                                              

  Urban/ Suburban  Adult Female      75.5       14.1      5.1        94.7
  Family                                                             

                   Adult Male        50.0       10.6      34.8       95.4

                   Child Born in     93.5       3.5       2.1        99.1
                   1955                                              

                   Child Born in     9.9        63.9      18.8       92.6
                   1964                                              

  Migrant Family\* Adult Female      74.7       8.9       8.6        92.2

                   Adult Male        77.9       8.4       6.2        92.5

                   Child Born in     93.7       2.8       1.8        98.3
                   1955                                              

                   Child Born in     10.7       52.4      23.1       86.2
                   1964                                              

  \*Since the                                                        
  Migrant Worker                                                     
  Family was only                                                    
  present near the                                                   
  SRS for six                                                        
  months, it may                                                     
  be more                                                            
  appropriate for                                                    
  comparison                                                         
  purposes to                                                        
  double these                                                       
  doses.                                                             
  ---------------- ----------------- ---------- --------- ---------- -------

Thus, it can be seen that two ingestion pathways (milk and beef
ingestion) accounted for a large fraction of the dose for most of the
receptors exposed only to air releases of radionuclides. The important
radionuclides producing dose through these pathways depended, in part,
on the timing of the exposure; for the Children Born in 1964, important
radionuclides tended to shift from ^131^I to tritium for air releases
(see Section 13.2.5 for additional discussion); for the other receptors,
^131^I was the most important radionuclide.

### Significance of Immersion Dose from ^41^Ar 

Releases of ^41^Ar produced a small but persistent dose for all
receptors from immersion in the plume. Generally, doses of 0.02 to 0.09
mSv (2 to 9 millirem \[mrem\]) over 39 years were produced. An exception
was the Adult Male for the Outdoors Family who received 0.31 mSv (31
mrem) over 39 years. In most cases, this air-immersion dose was minor,
but for receptors with small doses from other sources, it could be a
significant contributor. For example, for the Child Born in 1964,
air-immersion dose from ^41^Ar accounted for almost 17 percent of the
total dose in Rural Family One and almost 24 percent of the total dose
in Rural Family Two and the Urban/Suburban Family. Immersion dose
accounted for almost 39 percent of the dose to the Adult Male in the
Urban/Suburban Family principally because he worked a significant amount
of time onsite. For scenarios exposed to water releases, the percentage
of dose from air immersion was generally small (less than 10 percent)
for all receptors.

### Major Dose Fraction for many Receptors from Large Releases in 1955-1961

Large releases of ^131^I that occurred at the SRS in a period around
1955 to 1961 produced a major fraction of the dose in many receptors.
This is shown in the tables of annual effective dose as a function of
time in [Chapter 11](../Chapter_11_ready%20for%20CDC%20031405.doc)
(i.e., Tables 11-4, 11-7, 11-10, 11-13, 11-16, 11-19, and 11-22).
Because the Children Born in 1964 did not experience these large ^131^I
releases, their doses tended to result from other radionuclides,
principally tritium and ^137^Cs. The Adult Female, Adult Male, and Child
Born in 1955 for those scenarios exposed only to air releases had a
majority of their dose from ^131^I releases during this time frame. For
those scenarios exposed to water releases, the Adult Female, Adult Male,
and Child Born in 1955 had a significant fraction of dose delivered in
later years (mid-1960's), when the liquid releases were greater. In some
cases, the majority of the dose came from these later water releases,
but this depends on the degree to which doses from fish ingestion
dominate.

### Differences in Children Born in 1955 and 1964

Significant differences exist between doses for Children Born in 1955
and Children Born in 1964, and these differences are observed to greater
or lesser degrees in all seven scenarios. These differences in doses
include the magnitude of the doses, the radionuclides primarily
responsible for producing the doses, and the exposure pathways through
which the doses are received. Table 13-5 compares for Children Born in
1955 to the Children Born in 1964 for each scenario: 1) the total doses,
2) the radionuclide/pathway pair contributing most to this dose, and 3)
the fraction of the total dose accounted for by this major
radionuclide/pathway pair.

For the first four scenarios in Table 13-5, which are only exposed to
air releases from the SRS, the differences between the Child Born in
1955 and the Child Born in 1964 are similar. Considering the Child Born
in 1955, for three out of four scenarios the pathway was beef ingestion
and the radionuclide was ^131^I; for the remaining scenario, the pathway
was milk ingestion but the radionuclide remained ^131^I. The
contribution to total dose was about 40 percent in three cases and 70
percent in one case. This shows that that the large ^131^I releases
early in the site history dominate the dose mechanisms for the Children
Born in 1955. Considering the Child Born in 1964, for three out of four
scenarios the pathway was milk ingestion and the radionuclide was
tritium; for the remaining scenario, the pathway was air immersion and
the radionuclide was ^41^Ar. The contribution to total dose was about 25
percent in three cases and 37 percent in one case. This behavior
illustrates that because the Children Born in 1964 did not experience
the large ^131^I releases early in the site history, radionuclides other
than ^131^I and its dominant pathways became more evident. However, the
dominance of doses caused by these other radionuclides is smaller than
those caused by the early releases of ^131^I. Both ^41^Ar and tritium
were more persistently released over time than ^131^I.

For the last three scenarios in Table 13-5, the children are exposed to
both air and water releases from the SRS. For the Outdoor Family and the
Near River Family, the air release of ^131^I caused the dominant dose
for the Children Born in 1955; however, the dominant pathway for the
Outdoor Family is beef ingestion and milk ingestion for the Near River
Family. For the Children Born in 1964 for the Outdoor Family and the
Near River Family, the dominant pathway is fish ingestion and the
dominant radionuclide is ^32^P; in both scenarios, about 60 percent of
the dose is produced this way. For the Delivery Person Family, unlike
the other scenarios in Table 13-2, the Children Born in 1955 and 1964
have the same dominant pathway and dominant radionuclide; for both,
about 40-50 percent of the dose is caused by fish ingestion and ^137^Cs.
This is the result of the very high doses produced by ingesting fish
taken from Lower Three Runs Creek.

Table 13‑5 Comparison of the Dominant Cause of Dose for Children Born in
1955 and Children Born in 1964 for all Seven Scenarios

  ----------------- -------- ------------ ---------------------- ------------ ----------
  Scenario          Child    Dose         Major                               Percent of
                    Born in  (mSv/mrem)   Radionuclide-Pathway                Total Dose
                                          Pair                                

  Rural Family One  1955     1.6/160      ^131^I                 Beef         42
                                                                 Ingestion    

                    1964     0.072/7.2    Tritium                Milk         29
                                                                 Ingestion    

  Rural Family Two  1955     3.8/380      ^131^I                 Beef         43
                                                                 Ingestion    

                    1964     0.14/14      Tritium                Milk         26
                                                                 Ingestion    

  Urban/Suburban    1955     2.7/270      ^131^I                 Milk         70
  Family                                                         Ingestion    

                    1964     0.11/11      Tritium                Milk         37
                                                                 Ingestion    

  Migrant Worker    1955     2.2/220      ^131^I                 Beef         37
  Family\*                                                       Ingestion    

                    1964     0.083/8.3    ^41^Ar                 Air          23
                                                                 Immersion    

  Delivery Person   1955     5.2/520      ^137^Cs                Fish         40
  Family                                                         Ingestion    

                    1964     2.1/210      ^137^Cs                Fish         49
                                                                 Ingestion    

  Outdoors Family   1955     9.4/940      ^131^I                 Beef         46
                                                                 Ingestion    

                    1964     1.8/180      ^32^P                  Fish         57
                                                                 Ingestion    

  Near River Family 1955     3.1/310      ^131^I                 Milk         27
                                                                 Ingestion    

                    1964     1.8/180      ^32^P                  Fish         60
                                                                 Ingestion    

  \*Since the                                                                 
  Migrant Worker                                                              
  Family was only                                                             
  present near the                                                            
  SRS for six                                                                 
  months, it may be                                                           
  more appropriate                                                            
  for comparison                                                              
  purposes to                                                                 
  double these                                                                
  doses.                                                                      
  ----------------- -------- ------------ ---------------------- ------------ ----------

In short, for all but the Delivery Person Family, the scenarios show a
distinct difference in the size and dominant causes of dose between the
Children Born in 1955 and 1964.

### Importance of Lower Three Runs Creek Fish Ingestion Dose

The annual concentration of a radionuclide in the Savannah River was
estimated by dividing the estimated total liquid release of the
radionuclide from the SRS in a given year by the annual volume of flow
in the Savannah River. For Lower Three Runs Creek, measured
concentrations were used for tritium, ^137^Cs, and ^90^Sr. Generally,
the concentrations in Lower Three Runs Creek were substantially higher
than in the Savannah River. For example, the ratios of peak
concentrations in the two bodies of water were 23, 10, and 6,
respectively, for tritium, ^137^Cs, and ^90^Sr. As a consequence, eating
fish taken from Lower Three Runs Creek produced higher doses for those
receptors so exposed. Table 13-6 demonstrates this trend quantitatively
and shows doses to the Adult Female in three scenarios: Rural Family
One, the Near River Family, and the Delivery Family.

These three families received, respectively, 0, 100, and 50 percent of
their fish from the Savannah River and, respectively, 0, 0, and 50
percent of their fish from Lower Three Runs Creek. When the doses from
water releases for the Adult Females in the Near River Family and the
Delivery Family are compared, there is a significant difference in dose
(i.e., 1.8 mSv \[180 mrem\] for the Near River Family versus 5.7 mSv
\[570 mrem\] for the Delivery Family). This difference in dose was
produced by ingesting the same quantity of fish in both scenarios.
However, for the Delivery Family Adult Female, 50 percent of the fish
ingested came from Lower Three Runs Creek; for the Near River Family
Adult Female, 100 percent of the fish ingested came from the Savannah
River. By replacing 50 percent of the fish consumed from the Savannah
River with fish from Lower Three Runs Creek, the dose from water
releases was increased by more than a factor of three. By assuming the
dose is linearly proportional to the quantity consumed from the two
sources, an algebraic manipulation shows that if a scenario obtained 100
percent of the fish it consumes from Lower Three Runs Creek, the dose to
the Adult Female would be 9.6 mSv (960 mrem). This implies that the
contamination level in the water (and therefore in the fish ingested) is
on average a factor of 5 greater in Lower Three Runs Creek than in the
Savannah River. This is generally consistent with the concentration data
presented in [Chapter 7](../Chapter_07_Ready%20for%20CDC%20031405.doc)
for Cs-137 concentrations in the two bodies of water.

Table 13‑6 Comparison of Fish Consumption and Dose for the Adult Female
by Family Location

  --------------- ------------ --------- ------------ --------- ------------ ---------
  \% of Dose From Rural Family           Near River             Delivery     
  Fish Consumed   #1                     Family                 Family       
  from Different                                                             
  Sources                                                                    

  Lower Three     0                      0                      50           
  Runs Creek                                                                 

  Savannah River  0                      100                    50           

                                                                             

                  Dose         Percent   Dose         Percent   Dose         Percent
                  (mSv/mrem)             (mSv/mrem)             (mSv/mrem)   

  Total Dose      0.3/30       100       2.1/211      100       6.1/610      100

  Water Release   0/0          0         1.8/180      85        5.7/570      93
  Dose                                                                       

  Air Release     0.3/30       100       0.3/31       15        0.4/40       7
  Dose                                                                       
  --------------- ------------ --------- ------------ --------- ------------ ---------

### Variations in Air Dispersion Significant but not Dominant 

In this study, careful attention was paid to the geographical locations
of the releases to air from the SRS, the geographical locations of
various receptors, and the air-dispersion patterns at the SRS. One might
be tempted to compare total doses from air releases to measure the
impact of air dispersion; however, total dose from air releases is not a
good indicator of the importance of air-dispersion variability because
it does not directly reflect the concentrations experienced at the
residence location for a particular receptor. The following are some of
the complicating factors that prevent total dose from being a good
indicator of air dispersion at the primary exposure locations:

-   For several scenarios, foodstuffs (milk, deer meat, and beef) were
    brought to the residence location from other exposure locations
    (including the SRS) as mandated by the scenario specifications.

-   In other cases, the receptor (especially the Adult Male) experienced
    exposure at a work location (including the SRS).

However, dose to the Adult Female by air immersion is a relatively good
indicator of the differences due to variations in air dispersion. This
is because the Adult Female was assumed to stay most of the time at the
residence location (except for small amounts of time spent in recreation
or at church) and because air immersion is a good surrogate for the time
integral of concentration of gamma-emitting radionuclides at the
residence location.

Table 13-7 compares air-immersion doses for the Adult Female in all
seven scenarios. Note that the ratio of these doses to the average dose
over the seven scenarios is generally different by less than a factor of
2. The largest deviation from the average is for the Urban/Suburban
Family, which has a dose smaller than the average by a factor of 2.6.

Table 13‑7 Air-Immersion Doses for the Adult Female in Each Scenario as
a Measure of the Effects of Air-Dispersion Differences

  ------------------------- ----------------------- ----------------------
  Scenario                  Air Immersion Dose for  Ratio of Dose to
                            Adult Female (mSv/mrem) Average Dose

  Rural Family One          0.024/2.4               0.544

  Rural Family Two          0.054/5.4               1.223

  Urban/Suburban Family     0.017/1.7               0.385

  Migrant Family\*          0.039/3.9               0.883

  Delivery Person Family    0.055/5.5               1.246

  Outdoors Family           0.084/8.4               1.903

  Near River Family         0.036/3.6               0.816

  Average                   0.044/4.4               

  \*Since the Migrant                               
  Worker Family was only                            
  present near the SRS for                          
  six months, it may be                             
  more appropriate for                              
  comparison purposes to                            
  double this dose.                                 
  ------------------------- ----------------------- ----------------------

Comparing the dose for each scenario to the average is parallel to
quantifying "tallness" by comparing a person's height to the average
height. Although comparing the maximum dose to the minimum dose
accentuates the differences in the variation across the site, the ratio
in this case is 4.9---a significant but not dominant contributor to
variations among receptors. The ratio of maximum total dose to minimum
total dose is over 130 as shown in Table 11-2. This implies that many
factors other than air dispersion have a significant effect on
determining total dose to each receptor. That is, the variations that
accentuate differences (e.g., time spent at different locations,
quantities of foods ingested from different locations) are more
significant than the air-dispersion differences related to average
meteorological conditions, receptor locations, and source locations.
However, this conclusion excludes consideration of onsite exposures. Due
to the proximity of the onsite location to the air-release sources,
significantly higher contamination and concentration levels were
experienced onsite.

### Variable Uncertainty Generally Raises Dose Estimates Slightly 

[Chapter 12](../Chapter_12_ready%20for%20CDC_031405.doc) provides a
description of the uncertainty analysis. Consideration of uncertainty in
the variables used to estimate doses could cause an estimated dose to be
higher or lower than the corresponding point-estimate result. Comparison
of the results from the uncertainty analysis to the point-estimate
analysis shows that on average the estimated doses from the uncertainty
analysis are higher. Table 12-9 compares the mean and median doses from
the uncertainty analysis to the point-estimate dose for each of the 28
specified receptors. The ratios of the mean doses to the corresponding
point-estimate doses range from a high of 2.15 to a low of 1.07;
similarly, the ratio of the median doses to the corresponding
point-estimate doses range from a high of 1.54 to a low of 0.86. Thus,
the means and medians of the uncertain doses were close to the
corresponding point-estimate values. These results illustrate the
behavior of the central tendencies (the means and medians) of the
estimated dose distributions.

As discussed in Section 12.5.1, the lognormal distribution is a
reasonable fit to the uncertain distributions of dose for each receptor.
It is not surprising that most of the uncertain dose distributions could
be well described by a lognormal distribution because most of the
variables modeled as uncertain in the uncertainty analysis were assigned
lognormal distributions based on the information available. Although the
receptor doses are not simple functions of the uncertain variables, the
general shape of the lognormal distributions used to describe those
variables is evident in the receptor dose distributions. Lognormal
distributions are skewed toward higher values. For this type of
distribution, the mean is always higher than the median. This behavior
is evident if the extreme values of the dose distributions are examined.
For example, on average, the ratio of the [maximum]{.underline} dose
from the uncertainty analysis to the point-estimate dose for the same
receptor is 7.8. The maximum value of this ratio is 11.5 for the
Urban/Suburban Family Child Born in 1955. In addition, on average the
ratio of the [minimum]{.underline} dose from the uncertainty analysis to
the point-estimate dose for the same receptor is 0.36; the minimum value
of this ratio is 0.13 for the Urban/Suburban Family Child Born in 1955.
In other words, on average, the maximum value is 7.8 time higher than
the point estimate while the minimum value is 2.8 times smaller than the
point-estimate value.

The medians of the lognormal distributions describing the uncertain
input variables were set equal to the variable value used to generate
the point estimate. However, except for the Urban/Suburban Family, all
the median doses were higher than their corresponding point-estimate
doses. This somewhat surprising result seems to be related to several
aspects of the analysis, including the properties of the models used to
compute the dose and the large uncertainty for some variables. In
particular, for scenarios in which exposure to liquid releases by fish
ingestion was a dominant pathway, variations in the uptake factors for
^137^Cs, ^90^Sr, and ^32^P seemed to combine in a manner that assured
the median doses would be higher than the point-estimate doses.

Although the dose estimates derived by considering uncertainty in the
input variables were larger than the point-estimate doses, the means and
medians of the uncertain doses were generally no more than a factor of 2
larger than the point-estimate doses. Although this underscores the
importance of performing an uncertainty analysis, the basic conclusion
of low doses and risks does not change when uncertainty is considered.

### Effectiveness of Scenarios

As described in [Chapters
1](../Chapter_01_%20ready%20for%20CDC%20031405.doc),
[3](../Chapter_03_ready%20for%20CDC%20031405.doc), and
[4](../Chapter_04_ready%20for%20CDC_031405.doc), the definition and use
of hypothetical scenarios to span the range of realistic receptor
behavior was an important aspect of implementing this dose
reconstruction. The scenarios defined sets of hypothetical receptors
whose doses and risks would reflect the interaction of the receptor
behaviors with the site and release characteristics. As indicated by
many of the conclusions listed previously, this strategy was successful
in illustrating how the released radioactive materials interacted with
different behavior patterns to yield a range of doses produced by
different radionuclides and pathways. In particular, the hypothetical
scenarios disclosed:

-   A range of 39-year doses that spanned two orders of magnitude.

-   Generally higher doses for scenarios in which receptors were exposed
    to liquid and air releases from the SRS than for scenarios in which
    receptors were exposed only to air releases. For scenarios exposed
    to liquid and air releases, fish ingestion was generally the most
    important pathway and ^137^Cs the most important radionuclide; for
    scenarios exposed only to air releases, milk and beef ingestion were
    the most important pathways, and ^131^I was generally the most
    important radionuclide.

-   The Children Born in 1955 had substantially different doses and
    causes for doses than the Children Born in 1964.

-   The scenario for obtaining fish from Lower Three Runs Creek
    experienced higher doses than scenarios for obtaining fish from the
    Savannah River or from waters not affected by liquid releases from
    the SRS.

Because use of the scenarios was able to provide this kind of
information, use of hypothetical scenarios in this context is supported
and the utility of the scenario definitions is confirmed.

## Recommendations

Unresolved technical issues have arisen during the course of the work
from two sources: 1) meetings of the SRSHES and 2) the analytical team
performing the work. Some of these unresolved issues were raised at
public meetings during which this work was presented and discussed.
These public meetings were held in the following cities:

-   Charleston, South Carolina---March 2003.

-   Savannah, Georgia---September 2003.

-   Columbia, South Carolina---August 2004.

Other unresolved issues arose during the development of the approach and
the analysis of results. These issues have not been addressed to date
because they fall generally beyond the scope of the present task.
Performing this work is not likely to change the major conclusions
listed in the previous section. However, performing this work can be
expected to resolve a number of technical issues and thereby enhance
confidence in the current conclusions. The following is a list of the
recommendations which are then described briefly in the subsequent
subsections:

-   Look at large, acute releases to see if the pattern of doses would
    be changed significantly.

-   Examine the buildup of long-lived radionuclides in soil to determine
    if terrestrial doses change significantly.

-   Model contamination in reservoirs to see if it causes significant
    doses.

-   Compare modeled concentrations in foodstuffs (fruit, vegetables,
    grain, beef, deer, etc.) with monitoring data for model validation.

-   Perform an auxiliary analysis to determine if the breast-feeding of
    infants changes dose substantially.

-   Perform an auxiliary analysis to determine how in-utero doses change
    total dose and cancer risk.

-   Model consumption of venison more carefully to see if the result
    changes.

-   Model dose from the consumption of drinking water taken from the
    Savannah River for municipal water supplies some distance downstream
    from the SRS (i.e., the municipal water intakes at Port Wentworth,
    Georgia, and Hardeeville, South Carolina, for Beaufort and Jasper
    Counties).

-   Obtain technical peer review of the study by publishing papers on
    the methods and results in peer-reviewed journals.

    The following sections discuss each recommendation briefly.

### Acute Releases vs. Annual Average Releases

This present analysis was performed assuming that the reported SRS
annual releases occurred uniformly throughout each year. In fact, the
Phase II report clearly documents that these annual reported releases
included some acute releases (i.e., relatively large releases over short
periods of time). Because the acute releases were subject to
air-dispersion conditions during the time of their release, the doses
from such acute releases may not have been accurately portrayed by using
the multi-year average meteorology that is more appropriate for routine,
relatively constant releases. It is recommended that the importance of
modeling air releases as acute instead of chronic, annual releases be
determined by computing doses for the largest acute releases in two
ways: 1) modeled as acute releases with air-dispersion conditions
corresponding to the time of release and 2) modeled as chronic releases
using the multi-year average meteorology. Differences in dose estimates
for the two computational approaches would help to determine whether the
approach used in this study has sufficient precision. Only a few of the
largest acute releases would need to be studied (10-25) to make the
determination. One problem in performing this evaluation is that
air-dispersion conditions were not measured onsite in the early years
when many of the larger acute releases occurred. This may introduce
additional uncertainty into the evaluation because onsite air-dispersion
conditions would need to be extrapolated from nearby but offsite weather
stations.

### Evaluate Soil Buildup of Long-Lived Radionuclides

A preliminary evaluation was performed while developing the analytical
approach to estimate the importance of the buildup of long-lived
radionuclides in soil. Because soil buildup did not appear to be
significant, the dose modeling was performed without taking into account
residual contamination that might have remained in the soil from one
year to later years (i.e., the dose from each year was assumed to result
only from the releases during that year). A more detailed examination
could confirm that this dose from residual radioactivity in the soil was
indeed negligible. Because dose pathways associated with such soil
buildup were not significant, it is unlikely that the residual doses
will be significant. For example, uptake of radionuclides by plant roots
from the soil was much smaller than uptake from radionuclides deposited
on plant surfaces; also, ground plane dose, which would be increased by
residual soil contamination, was a very minor pathway. It is recommended
that for a few scenarios (air-release-only scenarios will likely show
the largest effect) doses be computed by accounting for soil buildup of
radionuclides from one year to the following years to quantify the
importance of this effect.

### Evaluate Significance of Reservoir Contamination

Air releases of radionuclides could contaminate bodies of water used for
drinking in two ways: 1) direct deposition of radionuclides from the air
into the body of water (lake, pond, or reservoir) and 2) deposition of
radionuclides onto the surface water basin of the water body and
subsequent migration to the water body by surface runoff. These
transport pathways were not modeled because more direct pathways (i.e.,
deposition of airborne radionuclides onto crops in the food chain and
ingestion of fish from water bodies contaminated directly by SRS water
releases) appeared to be more likely to produce significant doses. It is
recommended that airborne contamination of otherwise uncontaminated
water bodies be modeled to determine the significance of the
contamination produced and the potential doses from the contamination.
This will indicate whether the dose estimates produced by this study
have sufficient precision.

### Compare Modeled Contamination in Foodstuffs to Monitoring Data

As a means of enhancing confidence in this intermediate-phase dose
reconstruction, a partial validation of the modeling of releases and
environmental transport may be desirable. A partial validation of these
aspects of the modeling could be accomplished by comparing intermediate
modeling results (such as contamination levels in soil and food) to
field measurements. These field measurements are contained in the annual
site Environmental Reports. Additional computer computations would be
required to obtain these intermediate results because they were not
permanently recorded during the previous calculations. It is recommended
that this partial validation effort be pursued.

### Evaluate Significance of Breast-Feeding Infants

The current study did not model the pathway of breast milk ingestion by
infants; instead, ingestion of contaminated cow's milk was modeled.
There are methods by which the breast-milk pathway could be modeled. The
modeling of the cow's milk pathway, as currently implemented, may be
more conservative (i.e., may produce a higher dose). It is recommended
that the breast milk pathway be evaluated by some comparative modeling
to determine its significance.

### Evaluate Significance of In-Utero Doses

Some approaches are available to estimate the fetal doses in-utero from
pregnant women experiencing environmental exposures to radiation and
radioactivity. The current approach did not include this pathway. It is
recommended that some of the scenarios with higher dose levels be
analyzed first to determine if the fetal doses are significant.

### Refine Modeling of Deer Meat Ingestion

The current approach models contaminated venison as "beef" taken from
cattle grazing at the various exposure locations incorporated into the
analysis. A more precise approximation to venison contamination may be
obtainable by modeling uptake of radionuclides by deer ingestion of
various plant species typically ingested by deer; such an approach would
require modeling the uptake of radionuclides by these additional plant
species. Because ingestion of contaminated beef produced large doses
from ^131^I, a likely outcome from this more precise approach dose from
venison ingestion may actually be lower calculated doses to humans.
However, it is recommended that this more precise approach be pursued to
remove a persistent criticism of the current approach.

### Estimate Doses from Drinking Water from the Savannah River

None of the scenarios specified for this study considered receptors
located far downstream on the Savannah River (e.g., Port Wentworth,
Georgia, and Hardeeville, South Carolina, for Beaufort and Jasper
Counties) where river water is used for municipal water supplies. The
radionuclide content of this drinking water is carefully monitored to
comply with the applicable limits on radionuclide concentrations in
drinking water promulgated by the U.S. Environmental Protection Agency.
Nevertheless, the residual radioactivity content will produce a dose,
although it is likely to be small. A problem in estimating doses from
this pathway is that the contribution of radionuclides in the drinking
water from the SRS may be difficult to distinguish from the contribution
of radionuclides from other sources. It is recommended that doses from
this pathway be estimated to address this concern raised by the SRSHES.

### Obtain Technical Peer Review

It is recommended that technical peer review be obtained for the methods
and results of this study. An important avenue to obtain this type of
peer review is by publishing papers on the methods used and the results
obtained in peer-reviewed technical journal. In addition, further peer
review may be acquired by presenting papers on this study at technical
conferences. By vetting this study in the technical literature,
confidence in the results will be enhanced.

## References

1\. National Commission on Radiation Protection and Measurements.
Ionizing radiation exposure of the population of the United States.
Report 93. ISBN 0-913392-91-X. 1987.

2\. U.S. Environmental Protection Agency. Calculate your radiation dose.
\[cited 2004 Sep\]. Available from URL:
[*http://www.epa.gov/radiation/students/calculate.html*](http://www.epa.gov/radiation/students/calculate.html).

3\. American Nuclear Society. Personal radiation dose chart. 2000.

4\. Centers for Disease Control and Prevention, National Center for
Environmental Health. Fact sheet: a summary of the Fernald risk
assessment report. 1998 Dec \[cited 2004 Sep\]. Available from URL:
[*http://www.cdc.gov/nceh/radiation/fernald/summary.pdf*](http://www.cdc.gov/nceh/radiation/fernald/summary.pdf).

5\. Centers for Disease Control and Prevention, National Center for
Environmental Health. Fact sheet: a summary of the draft phase II
Fernald risk assessment report. 1999 June \[cited 2004 Sep\].
[*http://www.cdc.gov/nceh/radiation/phase2/results.pdf*](http://www.cdc.gov/nceh/radiation/phase2/results.pdf).

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