NOTIFICATION OF IMPORTANT CHANGE IN NADP/NTN PROCEDURES ON 11 JANUARY 1994
Sample handling procedures at all NADP/NTN sites were changed substantially on 11
January 1994 in order to reduce contamination from the sample shipping container. This
notification alerts you that the data for samples before and after that date are different
and not comparable. See the table below for a tabular summary of the differences, based on
a special intercomparison study. The text below describes the rationale for the change of
procedures.
Prior to 11 January 1994, field samples were sent to the NADP/NTN Central
Analytical Laboratory (CAL) in the high density polyethylene buckets used for collection.
Only a 20-mL aliquot, used for field chemistry measurements, was removed before the sample
was sealed and sent to the CAL. This procedure had the advantage of minimizing the
handling of samples at field sites and of allowing the CAL to measure and record what was
in a virtually complete sample. Site operators' sample volume measurements and reports of
contaminants could be verified at the CAL and amended as needed. Despite these advantages,
a small but important source of contamination was found in the rubber o-ring that sealed
the lid to the sample bucket, preventing leaks during shipment. This o-ring was a source
of the same cations and anions that are measured in wet deposition. Most affected were
free hydrogen ion concentrations at sites with low ionic strength precipitation, causing
field pHs to be several tenths of a pH unit lower than CAL pHs at many western U.S. sites.
Efforts to improve lid cleaning procedures reduced but could not eliminate the
contamination. This persistent contamination and occasional changes in the o-rings
resulted in systematic time- varying biases that have the potential to interfere with the
interpretation of NADP/NTN data and long-term trends studies. Special studies had shown
that samples sent in bottles have a much smaller loss of hydrogen ions than samples sent
in the collection buckets. As a result, the NADP/NTN elected to change the field protocol
and use bottles to send samples to the CAL, pending the outcome of an intercomparison
study of old and new methods at 11 pre-selected sites.
This intercomparison study began in 1992 and ended in December 1993. Samples were
collected in side-by-side collectors set to open and close together. One sample was
handled the old way and sent to the CAL in the collection bucket. The other sample was
handled according to the new protocol and was taken from the field site to the field
laboratory in a bucket with a snap-on lid and no o-ring. At the field laboratory the
sample was poured into a one- liter, wide-mouthed, high density polyethylene bottle and
sent to the CAL. Sample in excess of one liter was discarded and an effort was made not to
transfer contaminants (floating, suspended, or settled) to the bottle. Results from this
study confirmed the larger loss of hydrogen ions in samples sent the old way to the CAL,
and the NADP/NTN implemented the protocol change on 11 January 1994.
Results of the intercomparison study are listed in Table 1.
If you have questions or comments please contact the
NADP/NTN Program Office,
Illinois State Water Survey
2204 Griffith Drive
Champaign, IL 61820
(217)333-7873
Table 1. Results of Intercomparison of Old and New Sample
Handling Procedures at NADP/NTN Sites. |
|
Median and 95%
Confidence
Interval of Change (1) |
Laboratory
Measurements |
Number of
Sample Pairs |
Direction of
Change |
Concentration (mg/L) |
Concentration
(Percent) |
Sulfate |
405 |
decrease |
0.02 +/- 0.004 |
2.6 +/- 0.5 |
Nitrate |
405 |
decrease |
0.01 +/- 0.004 |
0.9 +/- 0.4 |
Ammonium |
405 |
no change |
0.00 +/- 0.003 |
0.0 +/- 1.2 |
Calcium |
405 |
decrease |
0.002 +/- 0.001 |
2.5 +/- 1.2 |
Sodium |
405 |
decrease |
0.005 +/- 0.002 |
9.9 +/- 2.5 |
Chloride |
405 |
no change |
0.00 +/- 0.002 |
0.0 +/- 1.1 |
Magnesium |
405 |
decrease |
0.002 +/- 0.0003 |
9.5 +/- 1.6 |
Potassium |
405 |
decrease |
0.002 +/- 0.0005 |
11.1 +/- 3.1 |
|
|
|
(uequiv/L) |
(Percent) |
H+ for pH <4.6 |
112 |
increase |
3.2 +/- 1.1 |
7.2 +/- 2.3 |
H+ for 4.6<=pH<=5.6 |
191 |
increase |
2.5 +/- 0.5 |
35 +/- 7 |
H+ for pH> 5.6 |
102 |
increase |
2.1 +/- 0.8 |
220 +/- 91 |
|
|
|
(pH Units) |
(Percent) |
H+ for pH <4.6 |
112 |
increase |
0.03 +/- 0.01 |
n/a |
H+ for
4.6<=pH<=5.6 |
191 |
increase |
0.13 +/- 0.02 n/a |
n/a |
H+ for pH> 5.6 |
102 |
increase |
0.50 +/- 0.10 |
n/a |
|
|
|
(uS/cm) |
(Percent) |
Conductance |
405 |
increase |
0.9 +/- 0.1 |
9.3 +/- 1.7 |
|
(1) Changes are the median differences between the old (before 11
January 1994) and new protocols. Where a direction of change is
listed, the null hypothesis that the two protocols resulted in the same
concentrations could be rejected at the 0.05 level (Wilcoxon signed
rank test). The 95% confidence interval was calculated from the
median [1.58 (C75% - C25%)/SQRT(n)] (see Velleman and Hoaglin, 1981:
Applications, Basis, and Computing of Exploratory Data Analysis,
Duxbury Press, Boston, MA 02116).
