PROGRAMS AND PLANS--Dissolved Trace Element Data

In Reply Refer To:                          September 30, 1991
Mail Stop 412


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.

     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).  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.