Data from Selected U.S. Geological Survey National Stream Water-Quality Monitoring Networks (WQN) on CD-ROM
Open-File Report 96-337

By Richard B. Alexander, Amy S. Ludtke, Kathleen K. Fitzgerald, and Terry L. Schertz

Table 4. Important changes in National Stream Quality Accounting Network and Hydrologic Benchmark Network field methods, constituent coverage, and field sample contamination from 1973-95

[NASQAN, National Stream Quality Accounting Network; HBN, Hydrologic Benchmark Network. USGS memoranda are identified by the unique number QWyy_nn where yy gives the year of the document and nn is a sequential number of the year's memoranda]

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Constituent       Date      Description of change (memoranda or references)
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Alkalinity     10-01-84     Field determinations of alkalinity by fixed-
                              endpoint titration (QW84_16) required for
                              all network samples.  Alkalinity
                              determinations between 1980 and 1984
                              were most likely made in the lab, but
                              parameter code 00410 allowed for storage
                              of field determinations (QW80_27, QW81_04,
                              QW82_05) that may have been made
                              by some network field personnel. 
                              Determinations prior to October 1, 1980
                              were made in the laboratory. 

               10-01-85     Filtered samples required for field
                              determinations of alkalinity by fixed-
                              endpoint titration (QW85_19).  Previous
                              field determinations of alkalinity were
                              made on unfiltered water samples.
  
                            Field determinations of bicarbonate and
                              carbonate in filtered samples by
                              incremental titration method (QW85_19)
                              implemented.   Significant differences
                              found between fixed-endpoint and
                              incremental methods during 1984-85
                              study.

Biological     01-01-73 -   Periphyton collected.
 parameters      10-01-80
          
               01-01-73 -   Phytoplankton collected.
                 10-01-81

               10-01-77     Change in phytoplankton sampling from 12
                              per year to 4 samples collected in June to
                              Sept. months and 3 samples during the
                              remainder of the year (QW77_12).

Carbon,        01-01-73 -   Total collected.
 organic         10-01-81

               12-28-76     Evidence of a negative bias of unknown
                              magnitude in total organic carbon resulting
                              from various loss processes (e.g.,
                              adsorption to container walls, biological
                              activity).  Questionable values were deleted
                              from the data files prior to this time
                              (QW77_02).

               10-01-77 -   Dissolved and suspended collected.
                 10-01-81

Fecal      
 bacteria      10-01-75     Began sampling.

 Coliform      10-01-76     Parameter code changed from 31616 to 31625
                              with increase in filter pore size from
                              0.45 to 0.70 micrometer (QW76_25T). 
                              Studies showed that more accurate results
                              are obtained using 0.70 micrometer pore
                              size filters. 

 Streptococci  10-01-76     Parameter code changed from 31679 to 31673
                              (QW76_25T) with change in media type from
                              M-enterococcus agar to KF agar (0.45- or
                              0.70-micrometer filter size accepted for
                              either parameter code; field personnel 
                              tended to use the same filter pore size
                              as used for fecal coliform samples).  

               04-15-91     A filter pore size of 0.45 micrometer is
                              required to comply with TWRI methods
                              (QW91_FS).  A study of the effects of the
                              two pore sizes on analytical results based
                              on data collected at NASQAN and HBN 
                              stations in 1990 indicates no statistically
                              significant differences in methods (QW91_FS).
                              The differences in analytical results appear 
                              to reflect random measurement errors with no
                              evidence of upward or downward bias based
                              on the symmetry of the distribution of
                              differences.   

Radio-         10-01-75     Began sampling.
 chemicals

Trace          04-01-78 -   Sample contamination with lead and cadmium
 elements        02-28-79     from paint ring on acid ampoules (QW79_12).

               10-01-82     Discontinued analyses for total recoverable
                              trace elements (QW82_18).

              Before 1991   Contamination detected in trace-element
                              samples from 1989 to 1991 related to field
                              sampling and processing procedures. 
                              Discontinued analyses for arsenic,
                              beryllium, cadmium, chromium, copper,
                              lead, mercury, and zinc in 1991 (QW91_11,
                              QW91_10, QW92_05).  The evidence of
                              contamination raised important concerns
                              about the accuracy of the data for these
                              constituents prior to 1991.

              Before 1993   Studies beginning in 1987 showed trace-
                              element contamination associated with the
                              interior components of some samplers
                              (Horowitz and others, 1994).  Previously,
                              the exterior of samplers had been routinely
                              coated with epoxy paint to avoid
                              contamination.  The following samplers
                              were phased out beginning in 1993:  DH48,
                              DH59, DH76, D74, P61, P63, and P72. 

               10-01-93     Capsule filters (0.45 micrometer) began to be
                              used at many stations.  Higher concentrations
                              of trace elements may be expected due to the
                              transmission of more colloid-sized particles
                              (QW93_05).

               02-01-94     New sampling and processing protocol 
                              implemented at some stations for low-level
                              inorganic analyses (QW94_09, QW94_05 ,
                              QW93_11).  Several USGS studies documented
                              the need for revisions in sample collection
                              and field processing procedures for trace
                              elements (Horowitz and others, 1994; QW93_06;
                              QW93_05; QW93_03; QW92_13, QW92_12, QW94_13). 
                              Samplers with Teflon or plastic components
                              (DH81, D77) were recommended for use.

Trace         Period of     Caution should be exercised in the interpretation
 elements      record         of mercury data (QW94_10).  Possible
 (mercury)                    contamination sources include mercuric chloride
                              tablets (QW80_26), manometers, thermometers,
                              and blue-silicon seals and stoppers of 
                              Kemmerer and Van Dorn samplers.
       
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revised 01/28/97              

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