Guidance for Interpretation of NWQL Schedule 2020 and 2021 VOC Data
To: "A - Division Chief and Staff",
"B - Branch Chiefs and Offices",
"S - Special Distribution for Research",
"FO - State, District, Subdistrict and other Field Offices",
"PO - Project Offices",
" , NAWQA Leadership Team, Reston, VA ",
" , NAWQA National Synthesis Chiefs, Reston, VA ",
" , NAWQA 91 Study-Unit Project Chiefs, Reston, VA ",
" , NAWQA 94 Study-Unit Project Chiefs, Reston, VA ",
" , NAWQA 97 Study-Unit Project Chiefs, Reston, VA "
from: "Janice R Ward, Acting Chief, OWQ, Reston, VA "
cc: " , WRD Archive File, Reston, VA "
Subject: OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 98.04
Date: Wed, 15 Apr 1998 09:39:57 -0400
Sender: "Nana L Frye, Secretary (OA), Reston, VA "
In Reply Refer To:
Mail Stop 412 April 14, 1998
OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 98.04
Subject: Guidance for Interpretation of NWQL Schedule 2020 and 2021 VOC
On July 29, 1997, the National Water Quality Laboratory (NWQL) announced
in NWQL Technical Memorandum 97.11 that a new method for the
determination of volatile organic compounds (VOCs) had been approved as a
U.S. Geological Survey method. This method determines 86 VOCs at lower
reporting levels than were previously available, and is available as
Schedule 2020 which includes a search for unknown compounds and as
Schedule 2021 which does not search for unknown compounds. Details and
pricing of the method are available via the Water Resources Division
Intranet from the NWQL Home Page.
For pricing and the list of constituents and reporting levels, click on
"Spin" and request "2020" under a "Schedule" search. For a brief
description of Schedule 2020, page down to "Technical Information", find
"Schedules: Information and Frequently Asked Questions" and click on
PURPOSE OF MEMORANDUM
This VOC analytical method was available as Lab Codes 9090 and 9091
before being approved as NWQL Schedules 2020 and 2021. Based on a careful
analysis of data and procedures, data produced on or after October 1,
1996, are considered approved USGS data and are suitable to be included
in annual data reports. Data produced before October 1, 1996, using
custom Lab Codes 9090 and 9091 remain provisional and may not be included
in data reports. This memorandum presents guidance on how data from the
custom Lab Codes may differ from that of the approved Schedule, and how
those data may be used. The National Water Quality Assessment Program
(NAWQA) was the principle user of Lab Codes 9090 and 9091.
GUIDANCE AND INTERPRETATION
Provisional data, such as data from custom Lab Codes 9090 and 9091, may
be included in interpretive reports by referencing or describing the
analytical method in the interpretive report. Connor and others (1998)
describe the analytical method and its performance in Open-File Report
97-829. In addition, the attachment to this memorandum contains two
paragraphs that can be used in interpretive reports to describe the
custom method used before October 1, 1996. Data from custom methods
cannot be included in data reports.
In both custom Lab Codes 9090 and 9091, and approved Schedules 2020 and
2021, concentrations of analytes were determined only when each analyte
had been qualitatively determined to be present. Then concentrations were
determined using valid calibration curves. When concentrations are
reported for individual analytes from Lab Codes 9090, 9091, or Schedules
2020 or 2021, these concentrations may be used together for all
hydrologic interpretations. The difference between data produced using
Lab Code 9090 or 9091, and Schedule 2020 or 2021, is how low-level
detections were censored in relation to laboratory blanks.
Between April 1996 and October 1, 1996, for Lab Codes 9090 and 9091, some
environmental sample detections of common laboratory contaminants were
censored, regardless of daily blank values. These included: benzene,
ethylbenzene, toluene, xylenes, 1,4-dichlorobenzene, methylene chloride,
acetone, and carbon disulfide.
After October 1, 1996, a more complex and rigorous blank evaluation
procedure is used for data produced using Schedules 2020 and 2021. This
procedure results in substantially less censoring of common laboratory
contaminants. It compares additional quality-control blank sample results
to environmental sample results to determine whether the environmental
sample results can be considered valid. These comparisons are described
in Connor and others, 1998. This more rigorous procedure should show an
increase in the reporting of common laboratory contaminants after October
1, 1996, for Schedules 2020 and 2021, compared to results reported before
October 1, 1996, for custom Lab Codes 9090 and 9091.
In terms of interpretation of environmental results, if benzene,
ethylbenzene, toluene, xylenes, 1,4-dichlorobenzene, methylene chloride,
acetone, or carbon disulfide were determined to be present at a site
after October 1, 1996, and the same site yielded a nondetect (less-than)
value before that time, this difference could be an artifact of the
laboratory data evaluation procedures. Because of this, counting
detections both before and after October 1, 1996, can be misleading.
In order to count and compare numbers of detections over the period
before and after October 1, 1996, all comparisons of the numbers of
detections must exclude benzene, ethylbenzene, toluene, xylenes,
1,4-dichlorobenzene, methylene chloride, acetone, and carbon disulfide.
If those compounds are excluded, then comparisons of numbers of
detections before and after October 1, 1996 should be valid. However,
data users are cautioned not to over-interpret presence versus absence of
compounds at low concentrations.
Another caution to interpreting data at low concentrations is that as
concentrations decrease near the detection level, the relative standard
deviation becomes quite large. Therefore, data users are cautioned not to
over-interpret small concentration differences near reporting limits. To
interpret data at low concentrations, examine patterns of data rather
than individual quantitative measurements.
Connor, Brooke, Rose, Donna L., Noriega, Mary C. Murtagh, Lucinda K.,
Abney, Sonja R. 1998. Methods of Analysis by the U.S. Geological Survey
National Water Quality Laboratory-Determination of 86 Volatile Organic
Compounds in water by Gas Chromatography/ Mass Spectrometry, Including
Detections Less Than Reporting Limits. U.S. Geological Survey, Open-
File Report 97-829, 78 p.
Janice R. Ward /s/
Acting Chief, Office of Water Quality
This Memorandum refers to National Water Quality Laboratory Technical
Distribution: A, B, S, FO, PO
District Water-Quality Specialists
Regional Water-Quality Specialists
NAWQA_nlt, NAWQA_nst, NAWQA_91pc, NAWQA_94pc, NAWQA_97pc
Key Words: VOC, 2020, 2021, 9090, 9091, data interpretation
The following is a brief description of custom volatile organic compound
(VOC) Lab Code 9090 and 9091 analytical method for inclusion in the
experimental section of U.S. Geological Survey interpretive reports.
Water samples were analyzed by purge and trap capillary-column gas
chromatography/mass spectrometry at the U.S. Geological Survey's National
Water Quality Laboratory. Analyses were performed according to Connor and
others (1998). The analytical method provides reliable evidence of an
analyte's presence and is suitable for reporting concentrations at
sub-microgram-per-liter levels for 86 volatile organic compounds (VOCs).
The method also allows for reporting estimated concentrations for other
tentatively-identified VOCs. The analytical method began to be used in
April 1996, as a custom method and all data produced after October 1,
1996 are approved USGS data. Data produced prior to this date are
provisional. However, the principle difference between data produced
before and after October 1, 1996 is how low-level detections of VOCs
affected by laboratory contaminants were evaluated or censored in
relation to internal laboratory blanks.
Between April 1996 and October 1, 1996, a different evaluation and
reporting strategy was used for those VOCs affected by laboratory
contaminants. Environmental-sample results found to contain laboratory
contaminants were censored (that is, reported as less than the detection
limit) regardless of the magnitude of daily laboratory-blank values.
Additional laboratory blank samples were incorporated into the analytical
sequence after October 1, 1996, to help distinguish between true
low-level environmental concentrations and low-level environmental
concentrations caused by laboratory background contamination. Laboratory
contaminants that were censored were benzene, ethylbenzene, toluene,
xylenes, 1,4-dichlorobenzene, methylene chloride, acetone, and carbon
disulfide. The automatic censoring used between April 1996 and October 1,
1996, resulted in an underestimate of the occurrence of the censored
compounds in environmental samples.