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 8. Important changes in U.S. Geological Survey laboratory analytical methods, reporting conventions, and laboratory sample contamination from 1970-95

[NWQL, National Water Quality Laboratory; BSP, Blind Sample Program; SRWS, Standard Reference Water Sample; NASQAN, National Stream Quality Accounting Network. U.S. Geological Survey memoranda are identified by the originating office and a unique number QWyy_nn (Office of Water Quality) or NLyy_nn (National Water Quality Laboratory), where yy gives the year of the document and nn is a sequential number of the year's memoranda]

_____________________________________________________________________________
Constituent      Date        Description of change (memoranda or references)
_____________________________________________________________________________

Common ions   10-01-82 -     Sulfate showed a positive bias of about 2
                07-31-89       mg/L for concentrations less than 75 mg/L
                               because of interferences in the turbidimetric
                               method that were not blank corrected
                               (QW90_04).

              01-01-80 -     Residue on Evaporation (ROE) -- Small,
                12-31-89       statistically significant trends detected in
                               laboratory measurement bias based on BSP
                               reference sample data (Alexander and
                               others, 1993).  Trend is positive for the
                               NWQL and negative for the Atlanta laboratory.

              04-01-90 -     Sulfate showed a positive bias on the order 
                03-31-92       of 0.5-3 mg/L for samples with concentrations
                               of 15-75 mg/L.  BSP data demonstrate the bias.

              04-01-90 -     Chloride--Ion chromatography method showed
                10-20-92       lack of precision and positive bias on the
                               order of one standard deviation for BSP
                               samples.  The ion chromatography method was
                               switched to a colorimetric method in December
                               1992 (NL93_03).

              04-01-90 -     Fluoride--Ion chromatography method showed
                01-02-94       lack of precision on the order of four 
                               standard deviations for BSP samples.  The ion
                               chromatography method was switched to an ion-
                               selective electrode method in January 1994
                               (NL94_03).

Nutrients,    10-01-72 -     Values changed in the data base to maintain
 major ions,    09-30-78       consistency in reporting of non-detected
 and trace                     ("less-than") values (QW81_18; QW81_22).
  elements                     Zeros were replaced with the method
                               reporting limit and a remark code of "<".
                               Un-remarked values equal to the method
                               reporting limit were assigned a remark
                               code of "<".

Nutrients     Before 1972    The methods of preservation used by field
                               personnel are unknown, but may have
                               included chilling of samples, sulfuric acid,
                               or mercuric chloride.  Mercuric chloride
                               was officially recommended for use by the
                               USGS for nitrogen analyses as early as
                               November 1970.

              01-05-72       All field offices advised to discontinue use
                               of mercuric chloride as a preservative for
                               nitrogen, phosphorus, and organic carbon
                               because of concerns about instrument
                               interferences and problems of mercuric
                               chloride contamination and safe disposal of
                               wastes (QW72_09).  Chilling of samples to
                               1-40 C. and rapid transport of samples are
                               recommended alternatives.

              10-01-80       Mercuric chloride preservatives in tablet form
                               adopted after review of the literature
                               suggests benefits of the use of this biocide
                               (QW80_26).

              10-01-86 -     Mercuric chloride preservative in liquid
                09-30-94       ampoule form adopted (E.L. Skinner, U.S.
                               Geological Survey, written comm., 1990) 
                               because of detected ammonia contamination in
                               tablet preservative (QW85_07).

              03-27-92       The NWQL began replacing 250 mL brown nutrient 
                               bottles and 1 milliliter mercuric chloride 
                               ampoules with 125 milliliter brown nutrient 
                               bottles and 0.5 milliliter mercuric chloride 
                               ampoules for the purpose of reducing the 
                               amount of mercuric chloride that is being 
                               used in the field and that must be disposed 
                               of by the laboratory (QW92_08).

              10-01-94       Discontinued the use of mercuric chloride
                               preservative (QW94_16).  A USGS study
                               finds no significant differences in analytical
                               results based on samples treated separately
                               with mercuric chloride and chilling.

              10-01-94      Discontinued analyses of whole-water ("total")
                               samples for nitrite, nitrate + nitrite,
                               ammonia, and orthophosphorus (QW93_04).
                               From 1973-1994, analyses of these
                               constituents were performed on samples
                               "decanted" in the laboratory.  Statistical
                               review of all 1989 data processed in the
                               NWQL suggests that "dissolved" and "total"
                               determinations for these four constituents
                               are not statistically separable.

Kjeldahl      10-01-78 -     Kjeldahl nitrogen analyzed by method
 (ammonia       09-30-91       I2552/I4552 without blank correction.
 plus                          Positive bias could average more than 0.1
 organic)                      mg/L.
 nitrogen       
              01-01-79 -     Evidence of occasional periods of positive 
                12-31-85       bias based on SRWS reference sample
                               measurements at the Denver Central
                               Laboratory (Alexander and others, 1993).

              10-01-80 -     Positive bias as high as 0.5 mg/L found in
                09-30-86       blanks (Schertz and others, 1994; QW85_07)
                               at the Denver Central Lab.  The source
                               was believed to be the tablet form of
                               mercuric chloride preservative.  High
                               concentrations of Kjeldahl nitrogen at
                               western NASQAN stations served by the
                               Denver laboratory during the early 1980's
                               (Alexander and others, 1993) may relate,
                               in part, to this bias.

              10-01-91       Kjeldahl nitrogen "micro" method implemented
                               (Jirka and others, 1976) with blank 
                               correction.  Positive bias present in 
                               previous method no longer detected.

Phosphorus    10-01-73 -     Total phosphorus--Incomplete digestion in
                04-30-90       the potassium persulfate method produced
                               negatively biased concentrations (QW92_10).

              01-01-79 -     Dissolved phosphorus--Evidence of occasional
                12-31-81       periods of positive bias based on reference
                               sample measurements at the Denver Central
                               Laboratory (Alexander and others, 1993)

              05-01-90 -     Total phosphorus--Dilution procedure
                09-30-91       implemented with existing potassium
                               persulfate method (QW92_10).  Less bias
                               observed for this method; the method may
                               yield higher concentrations than previous
                               method (greater than or equal to 0.01 mg/L) 
                               for samples with with suspended-sediment 
                               concentrations above 50 mg/L.

              10-01-91       Total phosphorus--Kjeldahl "micro" method
                               implemented (QW92_10).  Comparative studies 
                               with the potassium persulfate method indicated
                               that the analytical results of the new method 
                               did not display the negative bias of the old
                               method; concentrations of the new method
                               are likely to exceed those of the old
                               method by more than 0.01 mg/L.

Trace         Before 1975    Selenium--The colorimetric diaminobenzidine
 elements                      method may have produced a positive bias as              
                               large as 5 ug/L (QW81_12).

              mid-1980s      Arsenic--A change in the rounding algorithm
                               increased the frequency of nondetected
                               values after this time.  Values reported
                               as 1.0 ug/L prior to mid-1980s were reported
                               as less than 1.0 ug/L after this time.
_____________________________________________________________________________

revised 01/15/97


The pathname for this page is <html/report/table8.htm>
Last modified: Thu Jan 16 15:00:27 EST 1997