AEPA
United States
Environmental
Protection Agency
Region III
Philadelphia, PA
19107-4431
Draft Guidance on Selecting Analytical Metal
Results from Monitoring Well Samples for the
Quantitative Assessment of Risk
Prepared by:
August 10, 1992
Dawn A. loven, Senior lexicologist
Technical Support Section
Superfund Program Branch
Hazardous Waste Management Division
L USE OF M 3NTTORING WELL METAL
DATA IN ilSK ASSESSMENT
A. BACKGROUND
From a purely toxkologkal perspective, the
principal purpose of collecting monitoring well
samples is to predict the potential impact that
ground water contaminants may have on
downgradient potable wells and, ultimately, on
human receptors. In this regard, the nitration
of monitoring well samples prior to metal
analyses is a critical issue that requires
resolution.
B. GUIDANCE FROM HEADQUARTERS
According to the Risk Assessment Guidance for
^ r^ P**!^
Evaluation Manual (Put Al CDecember 1989),
"data from unaltered (ground water] samples
should be used to estimate exposure
concentrations". However, the guidance is
unclear with regard to whether the cited quote
refers to monitoring well samples 21 potable
well samples. Further, no distinction is made
between metal and non-metal analyses, and
detailed justification for the position stated in,
the guidance is not provided.
C. REGIONAL POSITION
Due to the vague language presented in the
guidance, Region III has adopted guidelines, as
detailed below, for handling monitoring well
metal data. It must be emphasized, however,
that in order to ensure that the appropriate
monitoring; well metal data, Le. - filtered vs.
unfiltered, are selected for use in f]be risk
DHOODL a
nyiirogeoiogist is i
•1. In general, for the purpose of selecting
appropriate input parameters for the
quantitative assessment of risk, results from
both filtered and unfiltered metal analyses
should be evaluated.
a. If the results for mutual samples are
supportive, that is, if the filtered and unfiltered
data for samples collected from the same
monitoring well are similar, then unfiltered
results should be incorporated in the
quantitative risk assessment.
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b. [fa notable disparity exists between
filtered and unfiltered monitoring well data, as
demonstrated by vast differences in aluminum,
iron and manganese concentrations in mutual
samples, then results generated from filtered
samples should be used.
2. SUPPORT
Metals that are removed from monitoring well
samples by filtration with a 0.45 micron filter
are unlikely to be transported in the aquifer.
Consequently, if the migration of contaminants
in ground water is improbable, the potential
for impacts on downgradient potable wells is
remote and, therefore, not a toxicological
concern. Conversely, metals that remain in
solution following filtration with a 0.45 micron
filter are present in ground water in a
dissolved form, or are bound to paniculate
matter small enough to be transported in the
aquifer. In either case, these mobile
contaminants have the ability to affect
downgradient potable wells and should,
therefore, be evaluated in the risk assessment.
IL THE FILTRATION ISSUE
A. BACKGROUND
The primary objective of most ground water
sampling protocols is to obtain a representative
sample of ground water. With regard to
achieving this objective, a controversial issue
must be reckoned-with: Should monitoring
well samples be filtered prior to metal
analyses? Generally, there is a single key issue
that must be examined when deciding whether
to filter a monitoring weH sample prior to
analysis: What is being filtered out and to
what extent does the removal of this material
affect the overall representativeness of the
ground water sample?
As alluded to previously, metal levels measured
in unfiltered monitoring well samples are
typically representative of total metal
concentrations. Filtered samples, on the other
hand, represent dissolved metal concentrations
and, consequently, are often more predictive of
metal mobility, since free metals tend to
migrate in ground water to a greater extent
than particulate-bound metals.
HYPOTHETICAL ISSUES
During the process of sampling ground water,
the true value of a parameter in the collected
sample may be altered. The resultant
alteration is most likely attributable to the
process of well sampling itself. In this regard,
there are two basic types of effects that could
modify ground water samples: physical and
A. PHYSICAL EFFECTS
1. TURBIDITY
Turbid monitoring well samples are most often
attributable to the introduction of silt or clay
particles from the aquifer matrix. This effect is
usually associated with the sampling process
(pumping, bailing, etc.), which can produce
turbulent flow immediately adjacent to the
well screen, The presence of paniculate
matter, Le. • suspended silt and clay, in
monitoring well samples is a common situation
at industrial and landfill sites. A highly turbid
ground water sample can be visually identified
in the field, or can be evidenced by comparing
specific cation results from the unfiltered
sample with those from the respective filtered
sample. High levels of aluminum, manganese
and iron in unfiltered aqueous samples, as
compared to filtered samples, often indicate the
presence of paniculate matter.
a. CAUSES
1. Ground water samples containing
suspended solids are generated by improperly
developed or constructed monitoring wells.
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2. Properly constructed and developed
wells may bear turbid samples if the wells are
pumped or purged at rates in excess of their
yield, particularly if the geologic formation is
extremely silty. (Note that such a condition
typically characterizes aquifers with marginal
yields.)
3. Naturally occurring dissolved metals,
humic acid, or organic colloids may cause a
sample to be turbid.
B.
CHEMICAL EFFECTS
1. AERATION
When ground water is removed from the
aquifer and exposed to the atmosphere, an
alteration in the chemical environment of the
ground water ensues. This < vent causes a
chemical reaction to occur, thus altering the
ground water sample from its original state.
Such a reaction is illustrated by the formation
of iron precipitate in the sample bottle upon
exposure to air.
a. SHIFTS IN DISSOLVED GASES
Most chemical alterations are produced by
inadvertent aeration of the sample. The
aeration process results in radical shirts in
dissolved gases, thereby presenting two
fvndarn*»ptfl| problems;
1. Copredpitation Reactions
Shirts in rh«*rninil equilibria occur within the
sample due to the *><**1 or loss of gases.
This shift tnrvtifigf tfrf solubility of ionic
species in solution, potentially causing a
precipitation reaction to^ccur. Such a
reaction can impact the levels of other
dissolved species in the sample, thus generating
false negative results.
With the formation of precipitate, a solid is in
contact with the aqueous sample. A reaction
between the solid precipitate and the dissolved
species may cause the concentrations of other
ions in solution to decrease via cation or anion
exchange, postprecipitation reactions, or
adsorption. False negative results may,
therefore, be produced.
IV. FUNCTIONAL ISSUES
A. UNFILTERED SAMPLES
Ground water samples are typically preserved
to a pH < 2 with nitric acid (HNO3) prior to
analyses. If unfiltered ground water samples
containing suspended particles (usually silt or
clay) are acidified before analysis, then
substances not in solution can be placed into
solution, thereby falsely elevating their
concentrations in the sample. (False positive)
B. FILTERED SAMPLES
The filtering process can cause significant
aeration and subsequent loss of dissolved iron,
with coprecipitation of dissolved cations and
anions. Therefore, the filtering of ground
water samples can remove substances that
were in solution but have precipitated since
sampling, thereby falsely depressing their
concentration in the sample. (False negative)
1. POSSIBLE REMEDIES
The amount of iron precipitation is a function
of aeration. The effect of aeration on iron
precipitation could be significantly reduced at a
lower pH, at a greater redox buffering
capacity, or by using an in-line filter. (A lower
pH will slow ferrous iron oxidation kinetics. A
greater buffering capacity or an in-line filter
will reduce or eliminate the effect of aeration.)
2. Cation/Anion Exchange^Adsorption, or
Postprecipitation
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V. REGIONAL POSITION ON THE
COLLECTION OF SAMPLES
A. COLLECTION AND ANALYSIS
GUIDANCE
In Region in, as dictated by Quality Assurance
Directive No. 9 (QAD009), both filtered and
unfiltered samples are usually collected and
analyzed so that the distribution of metals in
ground water can be fully characterized.
B. EXCEPTIONS TO GUIDANCE
Listed below are exceptions to the Region in
guidance recommending the analyses of both
filtered and unfiltered monitoring well samples
for metals. Any deviations from the stated
guidance must be fully justified in the Quality
Assurance Project Plan.
1. If site-specific geologic conditions are
such that ground water may transport large
particulates (ex. • karst terrain, gravel aquifer,
etc.), then only unfiltered samples may be
representative of mobile ground water quality,
or
2. If there is sufficient historical data (i.e. •
a minimum of four consecutive quarters) from
the monitoring wells that are scheduled to be
sampled, then either filtered at unfiltered
samples may be collected, based upon
documented data trends. (Please refer to
QAD009 for specific details.)
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