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 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 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.