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]
[This file is Table 8 in the accompanying Open-File Report]
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Constituent Date Description of change (memoranda or references)
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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.
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revised 01/15/97