1992-10-29 – NRC – West Lake Landfill – McLaren Hart – Summary Report – Analysis


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cn\<\ M E M O R A N D U M DATE: October 29, 1992 TO: Designated PRP Group West Lake Landfill Site FROM: McLaren/Hart Ray Forrester Brent Finley Jack Buddenbaum SUBJECT: NRC SUMMARY REPORT - ANALYSIS •1> 07 996^7
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Several key issues arise from the manner in which the radiological data have been
interpreted in the U.S. Nuclear Regulatory Commission (NRC) Summary Report concerning
the West Lake Landfill Site. This Memorandum briefly reviews the data that have been
collected, examines how that data have been interpreted, and identifies certain overestimates
on the part of the NRC concerning the nature and extent of radioactive contamination at
the Site.
NRC’s Beliefs Concerning Current and Future Health Risks
The NRC documents reviewed state that, under current conditions, the Site does not pose
a risk, nor has it in the past; however, those documents state that, in the future, levels of
radioactivity in soils located at the Site will eventually reach levels that pose a risk and,
therefore, remediation is required to protect against future risks. Indeed, the documents
conclude that levels of radioactivity will reach such magnitudes that on-site cleanup options
may not be feasible and that a costly off-site disposal option may be the only alternative.
These conclusions are based primarily on soil data that was collected over 10 years ago. A
careful review of the surficial and subsurficial boring data, and NRC’s interpretation of
those data, suggests that the radioactive inventory at the Site may have been exaggerated
to the point where the feasibility of less costly options, including no action, may have been
prematurely dismissed and may very well be acceptable for much of the material at the Site.
Data Collection and Interpretation
Briefly, the NRC contractor measured gamma emissions in 18 subsurface borings placed in
areas that were believed to be the most contaminated. The estimates of average levels of
Ra, Th and U and the total radioactive inventory were derived from the information
obtained from these subsurface borings and from approximately 10 surface samples. It is
interesting to note that the subsurface borings data showed that 7 of the 18 borings did not
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October 29, 1992
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have a single sample greater than background concentrations (>2.5 pCi/g Ra), and the
other 5 borings had levels above background only in the top 3 feet. Only the remaining 6
borings had levels of radioactivity that could be considered to be above background
throughout the boring. Apparently, the Site contains localized confined “hot spots”; it is not
contaminated in a homogeneous fashion.
The NRCs interpretation of the data collected, and its assumptions can be summarized as
Assumption #1: The average specific activity of Ra in all samples from all
of the borings is 90 pCi/g.
Assumption #2: Radiochemical analyses of 10 surface samples showed that
all Th:Bi ratios ranged from 4:1-40:1; other samples taken at Latty Avenue
reportedly had ratios ranging up to 300:1. As a conservative measure, it was
assumed by NRC that the average Th:Bi ratio in the landfill is 100:1. Since
Bi and Ra are assumed to be present in equivalent concentrations, the Th:Ra
ratios are also assumed to be 100:1. Therefore, the average Th concentration
in the landfill is 9,000 pCi/g.
Assumption #3: By ‘Visually integrating” the total volume of soil that contains
>5 pCi/g Ra and assuming that the average Ra concentration in the material
is 90 pCi/g, the total Ra inventory of the site is 14 Ci Ra. Therefore, the
total Th inventory of the site is 1400 Ci Th.
Assumption #4: Because of the large Th:Ra ratio, future decay of Th to Ra
will cause Ra activity to increase 5-fold over the next 100 years, 9-fold over
the next 200 years, and 35-fold over the next 1,000 years. Therefore, Ra
concentrations will eventually exceed soil criteria for most on-site disposal
options and off-site disposal may be required.
In summary, NRC has assumed, based on indirect measurement methods and conservative
assumptions, that Ra and Th levels are 90 pCi/g and 9,000 pCi/g, respectively, and that the
levels of Ra will eventually exceed acceptable concentrations because Th decay to Ra is not
in equilibrium.
However, a few critical assumptions made by NRC have resulted in gross overestimates of
the amount of Ra and Th present in the landfill. These assumptions may have a significant
impact on the disposal options. These are outlined below.
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Issue #1 – Ra Concentrations: A review of the data indicates that the actual
average activity of Ra in all subsurface borings is far less than 90 pCi/g.
Indeed, it is clear that well over half of the samples are at background and
approximately 90% of the samples are far less than 90 pCi/g. The NRC
apparently mistakenly took the arithmetic means of data to derive an average
concentration. However, arithmetic means are appropriate only for data that
are normally distributed. The Ra data at the Site are lognormally distributed,
and therefore, the geometric mean should have been calculated. Lognormal
distributions occur when the bulk of the data are grouped around a certain
value and a small percent of the samples have much higher concentrations
than the rest (outliers). This almost always occurs with “hot spot” data.
Several EPA guidance documents indicate that it is inappropriate to take the
arithmetic mean of data that are not normally distributed.
The impact of taking the arithmetic mean of these data is that the average Ra
concentration throughout the landfill has been overestimated. The geometric
mean, which is a more accurate measure of the central tendency of the data,
is less than 10 pCi/g. Hence, the NRC has overestimated the Ra
concentrations throughout the landfill by almost at least an order of
Issue #2 – Th:Ra Ratios: The 10 surface soil samples contained Th:Ra ratios
ranging from 4:1 to 40:1. However, only 2 ratios were greater than 30:1 and
the mean ratio was approximately 18:1. As with the Ra data, the Th:Ra ratios
are lognormally distributed because there are 1 to 2 “outliers”. Regardless,
NRC essentially ignored these ratios and instead referred to samples taken
off-site at Latty Avenue that were approximately 300:1. NRC arbitrarily
settled on a ratio of 100:1. As noted above, this is 5 times greater than the
true mean of the ratios measured on-site. Hence, NRC’s estimate that
average Th concentrations are 9,000 pCi/g may be at least 50-fold too high.
Issue #3 – Validity of Radiochemical Analyses: An apparent inconsistency
arises in the U:Bi ratios measured in the radiochemical analyses of the surface
soils and those measured by gamma analyses in the subsurface borings. As
is pointed out several times in the Summary Report, U:Ra ratios (and,
therefore, U:Bi ratios) in the subsurface borings are within a very narrow
range, approximately 1:2 to 1:10. However, in the radiochemical analyses of
the surface borings, the U:Bi ratios (and, therefore, the U:Ra ratios) range
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from 2:1 to 1:4,000. This may be indicative of poor data quality in the
radiochemical analyses (e.g., perhaps the instrument was not properly
calibrated). If so, then the thorium data, on which much of NRC’s
assumptions hinge, may not be entirely credible.
Issue #4 – Radiochemical Inventory: The radiochemical inventory for Ra was
estimated by taking the “mean” Ra concentration of 90 pCi/g and multiplying
it by the reported volume of soil that contains > 5 pCi/g Ra. This was done
by “visual integration”. The accuracy of the “visual integration” technique is
unknown. However, if one examines only the Ra data that are >5 pCi/g, the
values are once again lognormally distributed, with a geometric mean of
approximately 10 pCi/g. Hence, the total inventory of Ra at the Site is likely
to be less than 2 Ci, and not 14 Ci as stated in the NRC document. Similarly,
the total Th inventory may very well be 50-fold less than 1400 Ci
(approximately 28 Ci).
Issue #5 – Future Concentrations of Ra in the Landfill: The Summary Report
suggests that Ra concentrations will increase by 5-fold over the next 100 years,
9-fold over the next 200 years, and 35-fold over the next 1,000 years. This
would result in future Ra concentrations that exceed the NRC Branch
Technical Position (BTP) Option 4 of 200 pCi/g. If one assumes a 10 pCi/g
mean Ra concentration and a mean Th:Ra ratio of 18:1, then future Ra
concentrations are: 18 pCi/g in 100 years, 25 pCi/g in 200 years, 72 pCi/g in
1000 years, 161 pCi/g in 5000 years, and a maximum concentration of 186
pCi/g. These values are within Option 4 disposal limits. Accordingly, it is
entirely possible that soil concentrations of Th and Ra in much of the landfill
site will remain within the cleanup targets; only a few isolated “hot spots” may
require specific treatment.
In summary,
• The average Ra concentration and total radioactive inventory has been overestimated
by an order of magnitude.
• The average Th concentration and total radioactive inventory has been overestimated
by 50-fold.
• The radiochemical analyses upon which NRC has developed their estimates of
thorium concentrations may be suspect.
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It is entirely possible that a vast majority of the fill can be demonstrated to be within
guidelines now and for hundreds of years into the future.
Thorium Analysis vs. Th:Ra Ratios
The degree of remediation will be driven almost entirely by the established amount of
thorium in the landfill. Accordingly, during future site investigation activities, actual
radiochemical analysis for thorium should be performed, rather than to continue with the
indirect method of estimating thorium levels from radium levels. The elevated Ra currently
in the landfill has not been derived from the elevated levels of Th. The half life of Th is
in the thousands of years, hence, the elevated Ra is primarily due to the enrichment process
or portions of existing Ra being passed along in the ore refining process. In other words
any “ratio” of Th:Ra is strictly an artifact and has no bearing on any real relationship to Thto-
Ra decay segment. Therefore, the focus should be on Th analysis in future
measurements. Once accurate estimates of the Th inventory are established, the future
ingrowth of Ra and simultaneously decay-correct for existing Ra levels can be calculated to
yield a grand total for future Ra concentrations.
Baseline Risk Assessment
EPA will eventually require a baseline risk assessment. ChemRisk has successfully
negotiated with EPA to allow ChemRisk to prepare the baseline assessment on other sites
with full acceptability to EPA. ChemRisk’s experience with radiochemical risk assessments
should give it high credibility with Region VII. Also, few, if any, EPA contractors can match
ChemRisk’s experience in assessing health risks associated with exposure to radioactive
material. At an appropriate stage, ChemRisk is willing, at the request of the PRP’s, to
arrange a meeting with Region VII staff to introduce them to ChemRisk and to demonstrate
ChemRisk’s experience on the issues that must be addressed in performing a radiochemical
risk assessment.