NWIS: New Remark Code (V) for Water-Quality Data
To: "E - All WRD Employees"
cc: " , WRD Archive File, Reston, VA"
from: "David A Rickert, Chief, OWQ, Reston, VA"
Subject: OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 97.08
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Date: Thu, 28 Aug 1997 08:00:46 -0400
Sender: "Nana L Frye, Secretary (OA), Reston, VA"
In Reply Refer To:
Mail Stop 412 August 28, 1997
OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 97.08
Subject: NWIS: New Remark Code (V) for Water-Quality Data
The use of a "V" remark code has been implemented in NWIS to identify
environmental values where the analyte was detected in both the
environmental sample and associated blanks. The code has been redefined
to accommodate this need. The code was previously defined as "variance,"
but no data in the data base has been flagged with a "V" for that meaning.
Remarking environmental values with a "V" code will indicate that the
environmental values have been affected by contamination. "V" codes are
not applied to the blank data, only the environmental data. The following
guidelines describe the conditions that warrant the use of a "V" code:
1. There is direct evidence of contamination.
2. The concentration of the contamination is significant relative to
the environmental concentration.
3. The contamination is sufficiently understood to allow some
interpretation of the environmental data. Generally, this will
require that the contamination can be shown to be systematic and
the source and magnitude can be identified.
A value in the data base with a "V" remark code not only identifies that
value as contaminated, but also signifies that something is known about
the nature of the contamination. Interpretations that include these
values will usually require the associated quality-control data.
A decision to use the "V" remark code is subjective and, ultimately, must
be made by those who are familiar with the particular cases and all of
the variables that must be considered. The goal, as a Division, is to use
the code as consistently as possible in the data base and to use it only
in cases that are well defined. Further discussion on the guidelines
listed above are included in attachment 1 of this memo. The Office of
Water Quality can be contacted for assistance on the guidelines and their
appropriate application.
David A. Rickert /s/
Chief, Office of Water Quality
This memorandum does not supersede any other OWQ Technical Memorandum.
Key Words: NWIS, water quality, remark code
Distribution: All WRD Employees
Attachment 1 - Further Definition and Discussion:
1. Associated blanks
Any number of blank sample types can fit this definition. There may be
blanks from the field that are associated with the environmental samples
by the same time period, site, basin, equipment, collection protocols,
etc. Or there may be blanks from the lab that are associated with the
environmental samples by the same method, sample set, time, etc.
2. Direct evidence of contamination
Contamination may occur in associated blanks that have characteristics in
common with only some of the environmental samples. Only the
environmental samples that can be directly linked with the contamination
should be remarked with a "V" code. For example, an analyte is found in
several field blanks and the source is traced to a specific set of
equipment, so the environmental values collected with that set of
equipment would be "V" coded. Other environmental samples that were
collected with different sampling equipment, but that are associated with
the same contaminated field blanks that were collected on the same day
from several sites, are not "V" coded. In this case, the reason for the
"V" code is the contaminated field sampling equipment which produced the
contaminated field blanks, not the site or the day of sampled.
3. Contamination is significant relative to environmental concentrations
Remarking environmental values with a "V" code should imply that the
environmental values have been affected by contamination. A very small
concentration in the blank may not have any effect on the environmental
data if the ambient concentrations are much higher. For example, a
concentration of 0.1 ug/L in a blank would have little effect on an
environmental sample with a concentration of 10 ug/L, but would have a
significant effect on an environmental sample with a concentration of 0.5
ug/L. Generally, if the concentration in the blank(s) is less than 10
percent of the concentration in the environmental sample(s), no "V" code
is used because the effect of the contamination is likely to be within
the precision of the method.
4. Random versus Systematic concentrations in blanks
There will be some variability in every measurement system. This type of
error, called random error, is inconsistent both in magnitude and
direction (positive or negative) and generally difficult to attribute to
any particular part of the measurement process. This type of error can
occur in the analysis of blanks and result in an occasional "hit" or
detected concentration that may be nothing more than variability in the
measurement process. Random errors in blanks may or may not occur in the
associated samples.
Systematic errors are in one direction and similar in magnitude.
Contamination from a single source has these characteristics. Systematic
errors usually occur in associated samples with the same direction and
similar magnitude. This is why it is best to determine that the
contamination found in the associated blanks is systematic before
attempting to account for the contamination in the interpretation of the
environmental values.
5. Number of blanks needed
There is no specific number of blank samples required before a decision
to use the V remark code can be made. Factors that can affect this number
include: (1) the strength and consistency of the concentrations in the
blanks, (2) the relation of the blank concentrations to the environmental
concentrations, (3) the strength of the association between the blanks
and the environmental samples.