Data from Selected U.S. Geological Survey National Stream Water-Quality
Monitoring Networks (WQN)
USGS Digital Data Series DDS-37
By Richard B. Alexander, James R. Slack, Amy S. Ludtke, Kathleen K. Fitzgerald,
and Terry L. Schertz
QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 91.10
September 30, 1991
OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 91.10
Subject: PROGRAMS AND PLANS--Dissolved Trace Element Data
CONTAMINATION OF DISSOLVED TRACE-ELEMENT DATA:
PRESENT UNDERSTANDING, RAMIFICATIONS, AND
ISSUES THAT REQUIRE RESOLUTION
BACKGROUND
In 1986, the U.S. Geological Survey (USGS) Office of Water
Quality (OWQ) began a continuing evaluation of the methods and
equipment used to produce water-quality data, in general, and for
the National Stream Quality Accounting Network (NASQAN) program in
particular. The goal is to identify contamination and other
sources of variation in data introduced by sample collection,
sample processing, and analytical procedures, and then, to take
precautions or change methods to correct the problems. By 1989,
these activities had begun to focus to a considerable degree on
the quality of dissolved trace-element data. This focus stemmed
partly from concern over possible contamination in dissolved
mercury and lead results, and partly from reports appearing in the
scientific literature that differences in concentrations of
dissolved trace elements exist between data produced for NASQAN
and by several university projects. The initial observations and
comments about NASQAN data for dissolved trace elements were made
by Shiller and Boyle (1987) and Flegal and Coale (1989). In 1990,
the USGS conducted two studies to investigate aspects of the
quality of dissolved trace-element data: (a) a Blank Sample Study
(BSS) to detect potential contamination in water blanks processed
through precleansed field equipment, and (b) a Mississippi River
Methods Comparison Study (MRMCS), wherein dissolved trace-element
data were produced using three different protocols for collecting,
processing, and analyzing samples, namely, the protocols used by
NASQAN, Howard Taylor's National Research Program (NRP) project,
and Alan Shiller's project at the University of Southern
Mississippi. Then, the June 1991 issue of Environmental Science
and Technology contained an article written by Windom and others
entitled "Inadequacy of NASQAN Data for Assessing Metal Trends in
the Nation's Rivers." The article reports that based on recent
work, concentrations of dissolved cadmium, copper, lead, and zinc
in 18 East Coast rivers are considerably lower than values
reported under the USGS NASQAN program for samples collected
during "similar" time periods and at "similar" locations.
PURPOSES
The purposes of this memorandum are to: (a) present the
current understanding of whether USGS data for dissolved trace
elements are contaminated, (b) describe preliminary plans for
examining all aspects of the issue, (c) describe changes in NASQAN
analytical determinations for fiscal year (FY) 1992, and (d)
suggest how USGS District offices might proceed with dissolved
trace-element work in the Federal-State Cooperative Program while
important issues are being resolved.
The memorandum includes two appendices, two tables, and
16 figures to convey results of: (a) studies of dissolved trace-
element concentrations in North American rivers, and (b)
preliminary interpretations from the USGS-MRMCS and BSS. The
level of detail in the presentation is commensurate with the
importance of the findings with regard to the quality of USGS
dissolved trace-element data and the urgency for field and
laboratory studies to determine how to proceed with trace-element
work in USGS programs and projects.
PRESENT UNDERSTANDING
During 1991, newly available data from various studies have
enabled the OWQ to make initial evaluations of the quality of
NASQAN dissolved trace-element data. The OWQ has: (a) as noted
above, begun to evaluate dissolved trace-element data from the
MRMCS and the BSS, (b) reviewed selected trace-element projects
and protocols of Environment Canada, and (c) carefully reviewed
trace-element data recently reported in the literature.
The results of the MRMCS are presently incomplete, but it
appears that Howard Taylor's NRP data are comparable to Alan
Shiller's data. Further, the dissolved trace-element
concentrations found by Shiller and Taylor are comparable to
Windom's data, and to results generated by two separate Canadian
Studies (Table 1). Thus, similar concentration ranges exist for
various dissolved trace elements in the Mississippi River (Taylor
in the MRMCS, Shiller in the MRMCS, and Shiller and Boyle, 1987),
18 East Coast rivers (Windom and others, 1991), the St. Lawrence
River (Lum and others, 1991), and small Canadian Shield streams
(Robert McCrea, Environment Canada, written commun., 1991). The
dissolved concentrations reported by these studies are mostly in
the 10's of parts per trillion (ppt) for cadmium and lead, in the
low 100's of ppt for chromium, in the low to high 100's of ppt for
zinc, and between the mid 100's to 1,800 ppt (1.8 parts per
billion) for copper and nickel.
Table l.--Concentrations of Dissolved Elements from Selected Investigations [Parts per trillion (ng/L)]
(LT represents less than)
St. Streams &
USGS Mississippi River Mississippi Eastern Lawrence Lakes, Bruce
Methods Comparison Study River U.S. Rivers River Peninsula c
----------------------------------------- ----------- ----------- -------- ------------
Shiller & Windom Lum McCrea
Constit- Statis- District a Taylor a Shiller b Boyle et al. et al.
uent tics (N=9) (N=I0) (N=9) (N=7) (N=36) (N=52) (N=33)
---------------------------------------------------------------------------------------------------------------------------------------
Cd median 1,200 LT100 14 16 - - LT100
mean 2,600-3,000 d LT100 16 13 e 11 f 17 g LT100
Cr median 1,100 LT200 83 LT200
mean 950-1,100 d 40-200 d 73 e 20-200 d
Cu median 4,600 1,700 1,600 1,300 - - 500
mean 4,500-5,600 d 1,800 1,600 1,500 e 1,100 f 580 g 500
Ni median 1,800 - 1,500 1,400 - LT200
mean 1,800 - 1,700 1,400 e 770 g 100-200 d
Pb median LT500 LT60 - - LT200
mean 1,900-2,300 d 10-60 d 23 f 18 g 10-200 d
Zn median 5,300 900 190 240 - -
mean 6,100-6,700 d 980 290 200 e 850 f 550 g
Al median - 9,800 4,500
mean - 8,500 4,000
Fe median 24,000 - 1,700 1,700
mean 45,000 - 2,600 1,700 e
---------------------------------------------------------------------------------------------------------------------------------------
a) Depth- and width-integrated samples. The District samples were collected and processed by District crews and analyzed
at the National Water Quality Laboratory. Members of Howard Taylor's USGS National Research Program project collected,
processed, and analyzed the "Taylor" samples.
b) Grab samples.
c) Whole water samples with low suspended sediment content.
d) Data base contained "less than" values. The reported mean was calculated twice, once with the "less than" value(s)
set to zero and again with the "less than" values set at the reporting limit.
e) Discharge weighted means.
f) Unweighted mean of two sampling campaigns for 18 U.S. east coast rivers.
g) Estimated mean based on average concentration for each site and the number of samples at each site.
The six cited studies report comparable trace-element
concentrations in diverse river systems despite the use of five
different sampling methods (e.g., depth- and width-integrated
sampling using both conventional samplers and bag samplers,
surface grab sampling, surface pump sampling, and manual width-
integrated sampling using prepackaged contaminant-free equipment)
and five sample processing techniques to remove particulate matter
(e.g., conventional filtration, exhaustive filtration, cartridge
filtration, continuous flow centrifugation followed by chelation
techniques, and no removal of solids [for the virtually
particulate-free streams studied by McCrea]). The one common
feature of the six studies was the use of "ultra clean" protocols.
"Ultra clean" refers to: (a) avoidance of metal samplers, (b)
stringent precleansing of all containers, sampling equipment,
filtration equipment, and filters, (c) use of very high quality
water and acids for preparatory washing, blanks, preservation, and
analyses, (d) special precaution in the collection and field
handling of samples, including avoidance of all metal surfaces,
use of plastic gloves and forceps, and avoiding car exhaust and
atmospheric deposition (some projects conduct field processing of
samples in portable laminar-flow hoods), and (e) use of a class
100 clean room, or better, for laboratory processing and analyses
of samples. As one part in the overall ultra clean process, the
cleansing of sample bottles and glassware varies from a few to
many steps depending on the investigator. As an example, Appendix
1 describes the very detailed bottle and glassware cleansing
protocol used by Lum and others (1991).
In contrast to the cited trace-element concentrations for
these six studies, results obtained using standard USGS protocols
are much higher (Table 1, column 3). In the MRMCS, concentrations
for individual District samples (collected and processed by
District personnel and analyzed by the National Water-Quality
Laboratory (NWQL)) exceed concentrations in comparable Taylor and
Schiller samples (collected at the same time from the same cross
sections) by 4-fold for copper, to greater than 100-fold for
cadmium. Further, the design of the MRMCS allows the inference
that the differences in concentrations result primarily from
sample collection and field processing, rather than from
laboratory analysis.
Shiller and Boyle (1987) and Windom and others (1991)
attribute such observed differences in dissolved trace-element
concentrations to contamination introduced during the sampling,
processing, and analysis of USGS samples. This certainly could be
the cause of some, or all of the noted differences. However, the
noted differences might partially result from: (a) variations in
sample processing techniques (e.g., differences in particle
removal procedures might cause differences in the amount of
colloidal material incorporated and analyzed as "dissolved"); and
(b) removal of truly dissolved trace elements by adsorption during
use of ultra clean protocols processing due to sorption losses on
equipment and filters. The latter could occur if the exhaustive
acid cleaning opens active adsorption sites on bottles, equipment,
and filters which are not thoroughly equilibrated with excess
sample prior to processing aliquots collected for analysis.
Appendix 2 presents scatter plots and a table comparing
District and NRP data from the MRMCS. This is followed by a series
of box plots of data from the BSS. Based on these results, plus
those in Table 1, it appears that USGS operational program data for
rivers are significantly contaminated for arsenic, boron, beryllium,
cadmium, chromium, copper, lead, and zinc (see "Conclusion" section
of Appendix 2). The contamination appears to result from the sample
collection and sample processing steps. Based on the BSS data, and
additional laboratory comparison results from the MRMCS (not
presented in this memo), the NWQL does not seem to be a major source
of contamination at the relatively high (i.e., compared to NRP and
Shiller) reporting levels presently used in NASQAN for dissolved
trace-element data. No USGS data exist to evaluate the possible
contamination of dissolved trace-element data in ground-water
samples. However, because USGS dissolved ground-water samples are
filtered, it is possible that resultant data are contaminated for
several trace elements.
Until recently, nearly all water resource organizations used
methods similar to USGS conventional methods for collecting,
processing, and analyzing samples for dissolved trace elements.
About 15 years ago, the oceanographic community began adapting
knowledge of analytical research chemists to develop integrated
protocols that significantly reduced contamination in trace-
element results (Bruland, 1983; Windom and others, 1991). In
North America, these ultra clean protocols were later applied to
studies in the Great Lakes; however, they were not widely
implemented in rivers. As a result, for North America, it is
probable that most dissolved trace-element data collected from
rivers before about 1985 overestimate ambient environmental
concentrations. We know of no research or other agency data to
use for evaluating the quality of the historic data base for
dissolved trace elements in ground water.
WHAT NEEDS TO BE DONE
The OWQ has done the following:
1. Prepared this memorandum to: (a) describe the problem, (b)
raise questions that need to be answered, and (c) suggest how
Districts might work with cooperators on this issue.
2. Initiated development of a small capability for ppt analysis
within the NWQL. At present, for dissolved trace-element data,
the levels of contamination contributed by the NWQL are much lower
than those contributed by field activities. However, because our
goal is to develop ppt capability for sampling, sample processing,
and analysis, we must reduce reporting levels, and therefore
laboratory contamination, to much lower levels. Further, as we
systematically eliminate contamination from field activities, the
present laboratory contamination levels will become more
significant. We have established a target date of analyzing the
first environmental samples using ppt technology in October 1992.
The ppt capability is necessary to conduct studies: (a) on
dissolved trace-element sampling and processing, (b) that address
concentration differences resulting from factors other than
contamination, and (c) of the significance of dissolved trace-
element concentrations in natural systems with regard to various
program objectives. After we have developed the ppt capability,
resolved some issues, and determined costs, we will work with
individual programs to: (a) establish specific needs for dissolved
trace-element data, (b) determine whether ppt data are required,
or whether ppb data will suffice, and (c) if ppt data are
required, evaluate the alternative protocols for meeting the
specified needs.
3. Defined experiments that need to be conducted using ppt
technology to evaluate whether the ultra clean methodology causes
low results by removing trace elements from environmental samples.
This possibility is unlikely, but must be evaluated. These
experiments can be done now by NRP projects that have ppt
capability, and/or by the NWQL beginning in FY 1993.
4. Evaluated equipment for the sampling and processing of
dissolved trace elements in surface waters. At present, it
appears that the modified bag sampler (Leenheer and others, 1987)
is the most appropriate sampling device for dissolved trace-
element samples in large rivers. We may need to develop an
equivalent, or use a pre-existing design (e.g., McCrea's sampler,
Environment Canada) for shallow streams. Also, we must determine
the most appropriate equipment and procedures for collection of
dissolved trace-element data in ground water.
The OWQ will need to:
1. Develop and implement guidelines for comprehensive quality
assurance of all water-quality data, including trace-element data,
in all media. The guidelines must cover all aspects of sample
collection, sample processing, laboratory analysis, data
validation, and data storage/retrieval.
2. Develop and document suitable protocols for producing
dissolved trace-element data at both the ppb and ppt levels
for surface and ground waters. An improved protocol for surface-
water samples at the ppb level will be developed first and should
be available in early 1992.
3. Evaluate methods used to remove particles from water samples
prior to analysis for the "dissolved phase." At a minimum, we
need to explore and compare the "standard" filtration techniques,
chelation, dialysis, ultrafiltration, and super-centrifugation.
Moreover, the different filtration techniques used to produce the
data in Table 1 need to be compared to evaluate the possible
effects of processing artifacts on the reported concentrations.
4. Provide a more complete basis for decisions about which
dissolved and total recoverable trace-element data are suspect, so
the USGS can begin to make informed choices regarding what to do
about existing data (As previously noted, Appendix 2 provides
preliminary information on this issue).
5. Evaluate how to collect, process, and analyze whole water
samples for total recoverable determinations of trace elements.
Such samples are not filtered, but could be contaminated by
present sampling techniques. Data accuracy could be improved by
using: (a) noncontaminating sampling methods, and (b) laboratory
clean room capability developed for ppt determination of dissolved
trace elements.
6. Evaluate methods for collecting, processing, and analyzing
suspended sediment, bed sediment, and tissue samples for trace
elements. Because increased emphasis on these components is
likely in reconnaissance-type work, review of current procedures,
and possible methods development is warranted.
One problem the USGS does not face is procurement of new and
expensive analytical equipment. The graphite furnace atomic
absorption capability now in place in the NWQL and the ICP/MS
capability presently awaiting final approval are completely adequate
for ppt analyses. The challenge is to prevent contamination during
the sample collection, processing, and analysis steps to enable
production of accurate, reproducible data at the ppt level.
WHAT THE FUTURE MAY HOLD
We might have a future where:
1. For certain elements, historic data are limited to qualitative
value.
2. The cost of doing dissolved trace-element determinations at
the ppt level is so high that such analyses are inappropriate for
routine measurement in operational programs. We could be looking
at a future where the total number of samples for dissolved trace-
element determinations per year at the ppt level (for non-NRP
programs) are less than 500 compared to over 6,000 samples
analyzed now using conventional methodologies. Until research is
conducted and issues resolved, the number of samples appropriate
for ppb analysis of dissolved trace elements is unknown.
3. Suspended sediments, bed sediments, aquatic plant tissues, and
animal tissues are common matrices of study for trace elements in
operational (non-research) programs for assessing anthropogenic
effects on the chemistry of aquatic systems. Trace-element
concentrations in sediment samples typically are in the 10's to
100's of parts per million (ppm) for copper, lead, nickel, and
zinc; and in the single digit ppm for cadmium. Thus, prevention
of sample contamination will not be as difficult an issue as with
analysis of dissolved phase samples.
4. Dissolved trace-element data at the ppt level are produced by:
a. Special teams of individuals who conduct the entire
process of preparation, sample collection, sample processing,
and laboratory analysis. This would require a new direction
for the NWQL wherein lab personnel become members of project
teams with costs paid by the project, or
b. An approach wherein precleansed and disposable equipment
is used for sample collection and field processing. This
approach is typically used in the medical field and is being
used by McCrea of Environment Canada.
The observed similarity (in Table 1) of dissolved trace-
element concentrations in diverse river systems leads to several
hypotheses which need to be tested. Perhaps, dissolved
concentrations are controlled by thermodynamic limits which, under
the physiochemical conditions of fluvial systems, do not vary
substantially from one system to another. Perhaps the observed
low and fairly consistent concentrations of dissolved trace
elements result from kinetic controls, or a combination of kinetic
and thermodynamic controls. Regardless of the cause, the low
dissolved trace-element concentrations recently found in diverse
river systems indicate a relatively small contribution from
dissolved trace elements to total concentrations and fluxes.
Research is needed to determine: (a) if the concentrations of
dissolved trace elements are consistently low in freshwaters, (b)
if so, what the controls are, and (c) if so, how best to monitor
trace elements in operational programs to assess human impacts on
water quality, to determine trends, and to measure fluxes. If the
concentrations of dissolved trace elements are confirmed to be in
the ppt range, measurement will nevertheless continue in projects
devoted to understanding processes, rates, environmental controls,
and toxicology. However, because of cost, routine measurement of
dissolved trace elements in operational programs such as NASQAN
would probably cease, and other approaches to providing data for
assessing the effects of human activities on trace elements in
rivers, and trends thereof, would need to be employed.
RAMIFICATIONS FOR FISCAL YEARS 1992 AND 1993
As previously noted, the following elements exhibit
significant contamination for dissolved determinations: arsenic,
boron, beryllium, cadmium, chromium, copper, lead, and zinc. In
addition, some of the Division's recent dissolved mercury data are
contaminated. Therefore, at the beginning of FY 1992, the OWQ
will discontinue determining the cited list of elements plus
mercury at all NASQAN and Hydrologic Benchmark stations. This
list may grow as data are generated and interpreted from: (a)
methods comparison studies in other climatic-geohydrologic
regions, and (b) additional blank studies. For FY 1992, we will
continue to determine dissolved cobalt, lithium, molybdenum,
nickel, silicon, uranium, and vanadium in the NASQAN and Benchmark
Programs. We will also continue to determine the major ions, plus
aluminum, barium, iron, manganese, and strontium. For the future,
we will need to establish specific objectives for dissolved trace-
element data in the national networks and decide where, when, and
how it is appropriate to collect such data. We have advised the
National Water-Quality Assessment Program not to measure dissolved
trace elements until the USGS has resolved the various trace-
element issues and developed suitable protocols.
In the FY 1992 Federal-State Cooperative Program, Districts
should decide whether to determine dissolved trace elements by
conventional methods for dissolved and whole water samples on a
project-by-project basis, with full consideration of the
environment under study, the goals of the project, and the needs
of the cooperators. There may be hydrologic components with high
trace-element concentrations--such as acid mine drainage and urban
runoff--where present methodologies are acceptable. Although most
cooperators may be unaware of the ultra clean technology, or its
ramifications, it is important for Districts to advise affected
cooperators of the situation, and discuss options for specific
projects. In the short term, cooperators may continue to request
conventional sampling, processing, and analyses of whole water
samples geared to current drinking water standards, Maximum
Contaminant Levels (MCLs) for human health considerations, and
aquatic health criteria set by the U.S. Environmental Protection
Agency (EPA). As previously noted, an improved protocol for
producing data at the ppb level for surface-water samples will be
available in early 1992. This protocol will be applicable to both
dissolved and whole water samples and should meet the needs of
some cooperative projects. In the future, as more data are
generated using ppt technology, EPA may discover that regulatory
criteria and MCLs need to be revised.
The improved protocol for ppb-level work will include new
details for sample collection, field processing, and laboratory
handling and analysis. This protocol will build upon present
methods but make improvements based on work in progress by Art
Horowitz, the NWQL, Howard Taylor, Alan Shiller, and Environment
Canada. Simultaneously, with the writing of this protocol,
research will proceed on: (a) particle removal (phase separation)
procedures, and (b) possible chemical artifacts produced by the
ultra clean technology.
As noted, the work already initiated on development of ppt
capability in the NWQL has a goal of enabling initial analysis of
environmental samples in October 1992. However, experiments using
the new technology will extend through FY 1993. Thus, it will
probably be FY 1994 before we can reach final decisions on: (a)
how USGS operational programs should approach future work on
dissolved trace elements, and (b) appropriate caveats for the
historic trace-element data base. For existing data, the goal is
to provide a basis for decisions about which data are suspect. It
may be possible to establish the maximum levels of contamination
introduced during sample collection and processing. For example,
certain sampling devices may contaminate samples at levels
exceeding the present NWQL reporting levels, whereas other devices
may not. Perhaps no sampling devices contaminate samples above
the present reporting levels for certain trace elements. Such
possibilities need to be systematically defined.
As part of the process to sort through the dissolved trace-
element issue, the OWQ will convene a panel of experts to provide
advice. A number of USGS, Environment Canada, and university
scientists have expressed an interest and willingness to
participate. OWQ will work with the panel to address questions
of: (a) what do we know now, and what conclusions can we reach;
(b) what are the purposes in operational programs for collecting
dissolved trace-element data, (c) are acceptable substitutes
(media) available for certain purposes; (d) what additional
information--from experiments and other sources--do we need for
making decisions about future dissolved trace-element work in the
operational program; and (e) what institutional changes are
necessary to facilitate trace-element work in the USGS. The goal
is to produce a steady stream of information for the Districts
throughout FY's 1992 and 1993, leading to major decisions in
FY 1994.
If information in this memorandum prompts questions,
comments, or concerns, please enter them in QWTALK so that others
in the Division can share in the discussion.
REFERENCES
Bruland, K.W., 1983, Trace elements in sea-water, in Chemical
Oceanography: New York, Academic Press, v. 8, p. 157-220.
Flegal, A.R., and Coale, K., 1989, Discussion: trends in lead
concentration in major U.S. rivers and their relation to
historical changes in gasoline-lead consumption by R.B.
Alexander and R.A. Smith: Water Resources Bulletin, v. 25,
p. 1275-1277.
Leenheer, J.A., Meade, R.H., Taylor, H.E., and Pereira, W.E.,
1989, Sampling, fraction, and dewatering of suspended sediment
from the Mississippi River for geochemical and trace-element
analysis, in Mallard, G.E., and Ragone, S.E., eds., U.S.
Geological Survey Toxic Substances Hydrology Program--
Proceedings of the technical meeting, Phoenix, Arizona,
September 26-30, 1988: Water-Resources Investigations Report
88-4220, p. 501-511.
Lum, K.R., Kaiser, K.L.E., and Jaskot, C., 1991, Distribution
and fluxes of metals in the St. Lawrence River from the
outflow of Lake Ontario to Quebec City: Aquatic Sciences,
v. 53, no. 1, 19 p.
Shiller, A.M., and Boyle, E.A., 1987, Variability of dissolved
trace metals in the Mississippi River: Geochimica et
Cosmochimica. Acta, v. 51, p. 3273-3277.
Windom, H.L., Byrd, J.T., Smith, R.G., Jr., and Huan, F.,
1991, Inadequacy of NASQAN data for assessing metal
trends in the nation's rivers: Environmental Science and
Technology, v. 25, no. 6, p. 1137-1142.
David A. Rickert
Attachments
This memorandum does not supersede any previous Office of Water
Quality technical memorandum.
Key Words: Contamination, trace elements
Distribution: A, B, S, FO, PO
APPENDIX 1
(From Lum and others, 1991)
Procedure for Cleaning New Polyethylene Bottles
1. Wash with detergent (1-2 percent Extran) prepared with line-
distilled water (metal still) and rinse well with distilled
water. Shake off excess water.
2. Add sufficient distilled-in-glass acetone, cap and shake for
ca. 1 minute. Decant and shake off excess acetone.
3. Soak in 6 M hydrochloric acid bath (Baker Instra-Analyzed) at
50 degrees C for 2 days.
4. Next soak in 2 M nitric acid bath (Baker Instra-Analyzed) at
50 degrees C for 2 days.
5. Rinse five times with Chelex-water (deionized-distilled water
stored in contact with purified Chelex-100 resin, 50-100 mesh,
in reagent acid bottles previously rinsed with Chelex water).
Note that in preparing Chelex-water, the first batch is used to
clean and condition the inner surface of the reagent bottle.
This batch is discarded, and the bottle refilled with
distilled-deionized water and allowed to equilibrate overnight
before use.
6. Fill with 1 percent nitric acid (double sub-boiling distilled
and supplied in acid-cleaned teflon bottles) prepared with
Chelex water and store in zip-lock plastic bags.
Procedure for Cleaning Previously Used Bottles and
All Glassware
1. Rinse in deionized-distilled water.
2. Soak in 2 M nitric acid for at least 24 hours.
3. Rinse five times with Chelex-water.
4. Fill with 1 percent nitric acid prepared with Chelex water and
store in zip-lock plastic bags.
New sample bottles and glassware, once used, are reserved for the
same type of sample.
APPENDIX 2
PRELIMINARY INTERPRETATIONS FROM USGS STUDIES CONCERNING
CONTAMINATION OF DISSOLVED TRACE-ELEMENT DATA
The Mississippi River Methods Comparison Study
Figure 1 describes and Figures 2 through 9 present sets of scatter
plots for the six trace and two minor elements listed in Table 1.
For cadmium (Fig. 2), chromium (Fig. 3), copper (Fig. 4), lead
(Fig. 6), zinc (Fig. 7), and aluminum (Fig. 8), each set includes
two plots representing:
1. Sampling effects. The differences between pairs of samples
collected at nine locations wherein:
o One sample in each pair was collected by District crews,
and then processed (filtered) and analyzed by the NRP
personnel. This is identified as the District sample.
o The second sample of the pair was collected, processed, and
analyzed by NRP personnel. This is identified as
the NRP sample.
In each case, the sampling effects include those of the sampling
device, the act of collecting the sample, the churn splitter, and
the act of using the churn splitter.
2. Processing effects. The differences between pairs of samples
collected at 10 sites wherein:
o One sample was collected by NRP personnel, processed
by a District crew, and then analyzed by NRP personnel.
This is identified as the District sample.
o The second sample was collected, processed, and analyzed
by NRP personnel. This is the second sample in item 1
above, identified as the NRP sample.
Thus, for the sampling-effect plots, the only difference between
the District and NRP samples is who collected the sample, whereas
in the processing-effects plots, the only difference is who
processed the samples. For these six figures, note that all
laboratory determinations were made by the NRP.
Nickel (Fig. 5) and iron (Fig. 9) were analyzed by the NWQL,
rather than the NRP. Thus, for these elements (see Fig. 1), the
sampling-effect plot compares nine samples collected by District
crews, processed by NRP personnel, and analyzed by the NWQL versus
nine samples collected and processed by NRP personnel and analyzed
by the NWQL. The processing effects plot compares nine samples
collected by NRP personnel, processed by District crews, and
analyzed by the NWQL versus nine samples collected and processed
by NRP personnel and analyzed by the NWQL.
The eight sampling-effects plots exhibit positive biases (District
concentrations greater than NRP concentrations) for cadmium,
chromium, copper, lead, zinc, and aluminum, plus a negative bias
for iron (District concentrations less than NRP concentrations).
The eight processing-effects plots exhibit positive biases for
cadmium, chromium, copper, lead, zinc, aluminum, and iron.
Table 2 summarizes numerically the effects of sampling and
processing for 25 elements and also indicates the sign and
statistical significance (determined by the Sign Test) of the
effects. In Table 2, it is easy to see the magnitude of sampling
and processing effects relative to the median concentrations of
the samples collected, processed, and analyzed by the NRP (except
for Ag, Fe, and Ni, which were determined by the NWQL).
The Sign Test results presented in table 1 pertain only to the
signs of differences in the paired data sets; the magnitude of
difference is not considered. A one-tail test was run to
determine the significance of concentrations in the District
samples exceeding those in the paired NRP samples. The Sign Test
results are significant when concentrations of elements in the
District samples are consistently higher. As an example, for
zinc, the District processed samples exhibited higher
concentrations than the NRP counterparts in each of 10 pairs, and
the resultant p value was 0.002. For iron, the District processed
samples had higher concentrations than the NRP samples in 8 of 10
sample pairs, lower concentrations in the other two, and the
resultant p value was 0.11.
The Wilcoxen Test which evaluates both the sign and magnitude of
differences was run, but the results are not reported because
numerous "less than" values occur in the paired data sets for all
of the trace elements of concern.
The Blank Sample Study
With the results discussed to this point, consistent and sometimes
major sampling and processing differences are apparent between the
District and NRP protocols. Generally, District concentrations
are higher than NRP concentrations. However, no cause has been
illustrated. To address cause, Figures 10 through 16 show blank
data for all elements listed in Table 1 except aluminum (which was
not measured in the BSS).
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
In each figure (from left to right):
1. The DIW is the concentration of the noted constituent in the
deionized water used for the blanks.
2. The shelf blank is an aliquot of the original DIW and
represents analyte contamination/loss from storage.
3. The trip blank was DIW carried to the field; the container
was opened, part of the water was used, and an aliquot returned to
the laboratory. This represents analyte contamination/loss from
storage and the atmosphere.
4. The sampler blank was obtained by processing DIW through
the sampler. This represents analyte contamination/loss from
storage, the atmosphere, and the sampler.
5. The churn blank was obtained by processing DIW through the
sampler and the churn. This represents analyte contamination/ loss
from storage, the atmosphere, sampler, and churn.
6. The filter blank was obtained by processing DIW through the
sampler, churn, and filter/filter apparatus, then collecting the
filtrate after the first 500 ml were discarded. This represents
analyte contamination/loss from storage, the atmosphere, sampler,
churn, and filtration.
The DIW and shelf blanks were prepared, and the trip, sampler,
churn, and filter blanks were collected in 1990. All samples were
then frozen without acidification. In 1991, all samples were
thawed, acidified and held for one week, and then analyzed. In
Figures 10-16, note that the NWQL reporting limits for the BSS
were identical to the NASQAN reporting limits for nickel, zinc,
and iron, but lower than the NASQAN reporting limits for cadmium,
copper, chromium, and lead.
More conclusive interpretations would have resulted from the BSS
if DIW had been put through just one individual step in the field
handling sequence, rather than integrating multiple steps. New
experiments designed in this manner are planned.
At the reporting levels used in the BSS, Figures 10 through 16
show large amounts of contamination arising from the sampling step
for copper, lead, and zinc; and from the filtration step for
cadmium, lead, and zinc. Except for iron, the churn step did not
appear to be a source of contamination. The observed levels of
contamination in the BSS results are substantial at present NASQAN
reporting levels for copper, lead, zinc, and iron, but not for
cadmium, chromium, and nickel.
Conclusions
We have three lines of evidence to reach conclusions: (a) the
similarity of dissolved trace-element concentrations reported in
Table 1 for diverse river systems wherein the investigators used
five different sample collection methods and five different
particle separation methods, (b) the comparisons of District to
NRP data from the MRMCS as presented in Figures 2 through 9 and
Table 2, and (c) the observed contamination in the BSS data shown
in Figures 10 through 16. We also have information from many
discussions with hydrologists and chemists in the USGS,
Environment Canada, and several universities. Based on these
multiple lines of evidence, the OWQ has provisionally categorized
the USGS data for dissolved elements (trace, minor, major) as
follows:
1. Noncontaminated or minimally contaminated--barium, calcium,
cobalt, lithium, magnesium, molybdenum, nickel, sodium, silicon,
strontium, uranium, and vanadium.
2. Significantly contaminated--arsenic, boron, beryllium,
cadmium, chromium, copper, lead, and zinc. From other
studies and lines of evidence, the mercury data base is known
to contain contaminated results.
3. Significantly different from NRP data, but the differences may
result largely from filtration artifacts, rather than
contamination--aluminum, iron, and manganese.
4. As yet undetermined--selenium and silver.
Additional work is needed to confirm and further explore the
provisional listings. First, blank studies are needed of a
different design that cover more elements. Second, methods
intercomparison studies in surface waters are needed in different
climatic-geohydrologic regions to test for introduced trace
elements from contamination by aerosols during sample handling.
Third, studies are needed to test for contamination by
conventional methods for the sampling and processing of ground-
water samples.
The case of aluminum, iron, and manganese needs special work. In
Table 2, the median sampling and processing differences are large
for these elements relative to their median concentrations.
However, a soon to be published article by Art Horowitz
demonstrates that differences in the concentrations of aluminum
and iron of the magnitudes summarized in Table 1 can arise from
differences in how replicate samples are filtered. Additional
work is needed on the effects of filtration on the levels of: (a)
aluminum, iron, and manganese, and (b) the trace elements.
The pathname for this page is <html/wqn/qasure/qw91_10.htm>
Last modified: Thu Jan 16 15:16:56 EST 1997