United States
Environmental Protection
Agency
Region III
Office of Superfund
Hazardous Waste Management
Philadelphia, PA 19107
EPA/903/8-91/001
November 1991
Region III
Technical Guidance Manual
Risk Assessment
Chemical Concentration Data Near
The Detection Limit
EPA Contact: Dr. Roy L Smith
EPA
Region
Hazardous Waste Management Division
November 1991
Risk assessments often inappropriately report and handle data near the limits of detection. Common errors include (1)
omission of detection limits, (2) failure to define detection limits which are reported, and (3) unjustified treatment of non-
detects as zero. This guidance is intended to improve the quality and consistency of handling data near the detection
limit in risk assessments done in Region III. (EPAI903I8-91I001)
REPORTING DETECTION LIMITS
The practice of omitting information on detection limits
from risk assessments is inappropriate, both technically
and ethically, because it conceals important
uncertainties about potential levels of undetected risk.
For example, failure to detect TCE in drinking water at
a detection limit of 50 parts per billion (PPB) does not
establish acceptable levels of health risk; failure to
detect TCE at 0.05 ppb does. If risk assessors neglect
to consider detection limits for analytical data, they may
overlook serious health threats. Furthermore, detection
limits should appear both in data summary tables in the
body of the risk assessment, and in tables of raw data
in appendices.
In a generic sense, there are two types of analytical
lower limits: detection limits and quantisation limits.
The detection limit is the lowest concentration that can
reliably be distinguished from zero, but is below the
level which is quantifiable with acceptable precision. At
the detection limit, the analyte is proven to be present,
but its reported concentration is an estimate. The
quantisation limit is the lowest concentration which can
be not only detected, but also quantified with a
specified degree of precision. At the quantisation limit,
the analyte is both proven present and measured
reliably. The quantisation limit is always greater than the
detection limit, usually by a factor of about three.
NON-DETECTION v. ZERO CONCENTRATION
The routine assumption that site-related contaminants,
if undetected, are absent from samples is often unduly
optimistic. Some frequently-encountered carcinogens
(e.g.. vinyl chloride and tetrachloroethene in drinking
water, beryllium in soil) are significant potential health
risks at levels below detection limits. Risk assessors
should use professional judgment, augmented by the
decision path described below, to decide if hazardous
contaminants should be assumed present at levels
below the detection limit
-------
The detection limit is the lowest concentration that can
reliably be distinguished from zero, but is below the level
which is quantifiable with acceptable precision.
The quantitation limit is'the lowest concentration which can
be not only detected, but also quantified with a specified
degree of precision.
EXISTING GUIDANCE
Section 5.4 of the EPA Risk Assessment Guidance for
Superfund (USEPA, 1989) IA recommends that all data
qualifiers should be reported in the exposure
assessment, and that their implications be considered
before the data are used for risk assessment. Section
6.5.1 suggests use of models when monitoring data are
restricted by the limit of quantitation, and Section 5.3.1
contains guidance for re-analyzing samples and
determining which data should be treated qualitatively.
EPA's Guidance for Data Useabilitv in Risk Assessment
(USEPA, 1990) Section 3.3.4, subdivides generic
detection limits and quantitation limits, describing six
different lower analytical limits. Section 4.2 of DURA
describes a strategy for selecting appropriate analytical
methods, which includes consideration of risk at the
detection limit.
(1) The instrument detection limit (IDL) is three times
the standard deviation of seven replicate analyses at the
lowest concentration of a laboratory standard that is
statistically different from a blank.
(2) The method detection limit (MDL) is three times
the standard deviation of seven replicate spiked
samples handled as environmental samples.
(3) The sample quantitation limit (SQL) is the method
detection limit corrected for sample dilution and other
sample-specific adjustments.
(4) The contact required detection limit (CRDL) is the
sample quantitation limit which CLP laboratories are
required to maintain for inorganic anatytes.
(5) The contract required quantitation limit (CRQL) is
the sample quantitation limit which CLP laboratories
must maintain for organic analytes.
(6) The limit of quantitation (LOO) is the level above
which analytes may be quantified with a specified^m
precision, often +/- 30%. This precision is usually
assumed to occur at ten times the standard deviation
measured for the instrument detection limit.
Even with an optimum sample and analysis plan, risk
assessors still confront situations where significant risks
can occur below the detection limit. Neither RAGS nor
DURA presents a procedure for assessing risks from
undetected, but potentially present compounds, nor do
they suggest a specific reporting format for detection
limits. This Region III guidance document addresses
these gaps in national risk assessment guidance. It is
intended to augment, not replace, national guidance.
RECOMMENDED METHODOLOGY
A Reporting Detection Limits
Risk assessments should include analytical limits in all
data tables, including summary tables. One of the
following should be reported for all undetected
analytes, in order of preference:
Sample Quantitation Limit
Contract Required Detection Limit (or CRQL)
Limit of Quantitation (as described in DURA)
Each data table in the risk assessment should clearly
describe which limits are reported, and define them.
Risk assessments should use the format shown below
for all data tables. Undetected analytes should be
reported as the detection limit (i.e.. either the SQL,
CRDL/CRQL, or LOQ, in that order) with the code 'IT.
Analytes detected above the detection limit, but below
the quantitation limit, should be reported as an
estimated concentration with the code V.
Concentration In Sample (Code)
Compound
Sample Number
123 458
789
0.9(J)
Z2
Trichloroethene 0.1 (U) 15
Vinyl Chloride 0.2(U) 03(U)
Tetrachloroethene 5.5 3.1 (J)
Non-detects an reported aa the samp/a quantitation limit, defined as
three times the standard deviation of seven replicate spiked samples
handled aa environmental samples, corrected for sample dilution and
other sample-specific adjustments.
-------
B. Non-Detection v. Zero Concentration
Risk assessors have the following methods to choose
from, for handling data below the detection limit:
1. Non-detects handled as detection limits - In this
highly conservative approach, all non-detects are
assigned the value of the detection limit, the largest
concentration of analyte that could be present but not
detected. This method always produces a mean
concentration which is biased high, which is inconsis-
tent with Region Ill's policy of using best science in risk
assessments.
' 2. Non-detects reported as zero - This is the best-case
approach, in which all undetected chemicals are
assumed absent. This method should be used only for
specific chemicals which the risk assessor has
determined are not likely to be present, using tf?e
decision path below.
3. Non-detects reported as half the detection limit - This
approach assumes that on the average all values
between the detection limit and zero could be present,
and that the average value of non-detects could be as
high as half the detection limit. This method (or method
four, below) should be used for chemicals which the
risk assessor has determined may be present below the
detection limit, using the decision path below.
4. Statistical estimates of concentrations below the
detection limit - Use of statistical methods to estimate
concentrations below the detection limit is technically
superior to method three above, but also requires
considerably more effort and expertise than the three
simpler methods. Also, these statistical methods are
effective only for data sets having a high proportion of
detects (typically, greater than 50%). Therefore, statis-
tical predictions of concentrations below the detection
limit, as described by Gilbert (1987) and reviewed by
Helsel (1990), are recommended only for compounds
which significantly impact the risk assessment and for
which data are adequate.
C. Decision Path for Handling Data Near the Detection
Limit (PL)
Summarizing the discussion above, method one
(non-detects - DL) consistently overestimates
concentrations below the detection limit, and should not
be used. Risk assessors should use the following
decision path to select.among method two (non-detects
= 0), method three (non-detects = DL/2), and method
lour (specialized statistics) to achieve the least biased
estimate of reasonable maximum exposure.
The choice of method should be based on scientific
judgment about whether: (1) the undetected substance
poses a significant health risk at the detection limit, (2)
the undetected substance might reasonably be present
in that sample, (3) the treatment of non-detects will
impact the risk estimates, and (4) the database is suffi-
cient to support statistical analysis. The decision path
below, followed by examples of appropriate selections,
is recommended:
1. Is the compound present at a hazardous
concentration in any site-related sample?
If no. assume non-detects are zero: if ves. continue.
(Note that if the compound is not present in any sample
at a hazardous level (e.g.. 10* risk or a hazard quotient
of 1), it probably should be dropped from the risk
assessment.)
2. Was the sample taken down-gradient of (or, 9 no
gradient exists, adjacent to) a detectable concentration
of the chemical?
If no. assume non-detects are zero: if ves. continue.
3. Do the chemical's physical-chemical characteristics
(e.g.. water solubility, octanol-water partitioning, vapor
pressure, Henry's law constant, biodegradability, etc.),
permit it reasonably to be present in tiie sample? Are
other site-related compounds with similar
characteristics present in the sample?
If no (to both questions), assume non-detects are zero:
if ves (to either question), continue.
4. Does the assumption that non-detects equal DLJ2
significantly impact route-specific quantitative risk
estimates?
If no. assume non-detects equal DL/2; if ves. consider
using statistical methods to estimate concentrations
below the detection limit for that exposure route.
assuming data quality permits.
-------
EXAMPLES
1. TCE is present in groundwater on site at 500 pg/l, a
potentially hazardous concentration. Elevated TCE
concentrations are measured upgradient of a residential
well, but TCE is not detected in the residential well
itself. Other site-related chlorinated VOCs are detected
in the residential well. The detection limit for TCE was
5 ugll (equivalent to 5 x 10* risk under the exposure
scenario in the risk assessment).
Decision Path
Step 1 - continue
Step 2 - continue
Step 3 - continue
Step 4 - assume non-detects are DL/2. If multiple well
samples are available, and TCE is detected in some,
consider using specialized statistical methods:
2. Chromium is present in on-site soils at 10,000 mg/kg,
a potentially hazardous concentration under direct
contact exposure. Chromium is not detected In an
adjacent off-site soil sample, although other site-related
metals are. The detection limit for chromium in soil is
0.1 mg/kg, well below a hazardous concentration under
the exposure scenario in the risk assessment
Decision Path
Step 1 - continue
Step 2 - continue
Step 3 - continue
Step 4 - assume non-detects are DL/2; using
specialized statistics is unnecessary because the risk
assessment would not change appreciably.
3. PCBs are not detected in 20 on-site soil samples.
There is no history of PCS disposal at the site, and
PCBs were not detected in any other medium.
Decision Path
Step 1 - assume non-detects are zero.
4. Vinyl chloride, a site-related contaminant, is
measured in surface water downstream of the site
boundary at 10 ugll, a hazardous concentration for a
resident receptor. Five hundred meters upstream of the
site, vinyl chloride is not detected at a DL of 0.1
Decision Path
Step 1 - continue
Step 2 - assume upgradient non-detects equal zero.
5. 2,3,7,8-TCDD is detected in an unfiltered monitoring
well sample at 5 ng/l, a potentially hazardous
concentration. The next downgradient well has no
detectable TCDD. Pentachlorophenol, also detected in
the first well, is not detected in the second.
Decision Path
Step 1 - continue
Step 2 - continue
Step 3 - assume non-detects of both TCDD and PCP
equal zero because of low mobility in groundwater.
References
Gilbert, R.O. (1987). Statistical Methods for Environmen-
tal Pollution Monitoring. Van Nostrand Reinhold Co.,
New York.
Helsel, D.R. (1990). Less than obvious; statistical treat-
ment of data below the detection limit. Environ. Sci.
Technol. 24(12): 1767-1774.
USEPA (1989). Risk Assessment Guidance forSuperfu-
nd. Volume I, Human Health Evaluation Manual (Pan A).
EPAIS01I1-B9I002.
USEPA (1990). Guidance for Data Useability in Risk
Assessment EPA/540/G-90/008.
Foir additional information, contact (215) 597-6682.
Approved by:_
Thomas, C. Voltaggio,
------- |