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" 
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
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


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.