The U.S. Geological Survey (USGS) collects and disseminates information about the water resources of the United States and its territories. Surface- and ground-water samples are collected and sent to USGS laboratories for chemical analyses. About 70 to 80 percent of all water-quality samples collected by the USGS Water Science Centers are submitted to the National Water Quality Laboratory (https://www.usgs.gov/labs/national-water-quality-laboratory/about; accessed February 2023) in Lakewood, Colorado. The National Water Quality Laboratory (NWQL) identifies and quantifies the constituents in the water samples through analytical chemical procedures. Inherent to these procedures are random and systematic errors otherwise known as uncertainty. Although some errors can be addressed and corrected, other errors are inherent to the overall analytical process and cannot be realistically eliminated. The magnitude of errors, or uncertainty; however, can be estimated and tracked over time.
The USGS began submitting blind quality-assessment (QA) samples to USGS analytical laboratories starting January 1973. The QA program went into full production mode in 1981 and is known today as the Inorganic Blind Sample Project (IBSP). The IBSP is an independent, external, quality-assurance project which purpose is to monitor and evaluate the quality of the inorganic analytical results through the use of double-blind QA samples. Starting in 1984, inorganic analytical results from blind samples submitted to the NWQL have been captured, assessed, and stored in the IBSP database. The Ocala Water Quality Laboratory was added to the project in 1985 and remained in the project until the laboratory closed its doors in 2004. In 2006, the IBSP took over the NWQL’s inorganic blind-blank project and effectively rolled that project into the IBSP. Inorganic analytical results from blind samples submitted to both the NWQL and the Ocala laboratory are readily available via the IBSP database. Data from the Albany, Atlanta, Denver, and Salt Lake City Centralized laboratories (1973 through 1983; dates vary depending on lab) exists in hard-copy form.
The IBSP maintains a database for the retrieval, assessment, and storage of blind QC sample analytical results. Currently, the database contains more than 367,000 blind-sample results for inorganic, nutrient, and physical-property measurements dating from October 1984 to present. New analytical data are added twice-weekly to the database. The IBSP web site, which draws directly from the database, provides tools to assist the NWQL in detecting and correcting problems in their analytical procedures in a timely fashion. In addition, the tools can aid the users of the NWQL's data by estimating the extent that laboratory bias and variability contribute to the overall bias and variability in their environmental data. Information is provided in three ways: 1. a variety of performance-assessment charts (see "Charts and data" link), 2. data-quality assessment summaries posted every other month (see "Data-quality assessments" link [internal access only]), and 3. raw data from the IBSP database. Raw data can be easily accessed via the “open data set” link on any performance assessment charts or data retrievals can be customized to document the NWQL's analytical bias and variability relative to the time period, analytical procedures, and concentration ranges of individual water-quality projects or programs. (Contact the project chief at 303.236.1872 [email@example.com] for a customized data retrieval.)
A "double-blind" QA sample is a QA sample submitted for analysis for which the identity of the sample as well as the concentration of the individual components within the sample is unknown to the analyst. Double-blind QA samples containing inorganic, nutrient, and physical property constituents at various concentrations are prepared and disguised as routine environmental samples. The QA samples used by the IBSP are very unique in that they are typically not synthetic reference materials; rather they are derived from snow-melt, surface-waters, or ground-water sources (Woodworth and Connor, 2003). These natural-matrix standard reference samples are used as-is (undiluted), or diluted with deionized water, or mixed in varying proportions with other standard reference samples in order to achieve a variety of concentrations within the range of concentrations that corresponds to those of typical environmental water samples.
The double-blind IBSP samples are made to appear as much like environmental samples as possible. The IBSP submits these samples in shipping coolers to the NWQL to mimic the process, as much as possible, by which actual environmental samples are submitted to the NWQL. All identifying information (except account number) is changed to that of actual customers. Bottle labels are even "soiled" go give the appearance that the bottles have been filled in the field. After the samples are logged in, they are subjected to the identical laboratory handling, processing, and analytical procedures as are the environmental samples. Once the laboratory analyzes the samples, the results are routed through the same channels as used by regular customers, at which point the IBSP personnel immediately compile and review the analytical results.
The NWQL's performance is evaluated by how closely its analytical results match the target value of the blind QA samples it analyzes. The target values are derived from the median concentrations listed for each constituent in the standard reference samples used to make the blind QA samples that are submitted to the NWQL. Blind-blanks have a target value of zero. The assessment of whether a non-blank analytical result is acceptable is based on the number of Quality Control Units (QCU) from the target value. QCU's are calculated three ways to account for expected changes in performance based on concentration and the most appropriate calculation applied. All results within +/-2 QCUs are considered acceptable. Blind-blank data should show no more than 1% of the results greater than the laboratory's detection limit.
Analytical errors fall into two major categories: bias and variability. Bias is systematic error that causes consistently positive or negative deviation in the results from the target value. Variability is random error that affects the ability to reproduce results. Repeated measurements of the IBSP samples over time provide estimates of both systematic bias and random variability in the laboratory's analytical procedures.
The QA samples used by the IBSP are very unique in that they are typically not synthetic reference materials; rather they are standard reference water samples derived from snow-melt, surface-waters, or ground-water sources (Woodworth and Connor, 2003). These QA samples contain inorganic, nutrient, and physical property constituents at various naturally-occurring concentrations. They are (generally) submitted as-is (2016-present) in order to reduce sources of bias and variability not inherent to the laboratory which performance is being measured. Blank samples, using independently prepared purified reagent grade water that exceeds ASTM Type I standards, are also submitted.A "double-blind” QA sample is a QA sample submitted for analysis for which the identity of the sample as well as the concentration of the individual components within the sample is unknown to the analyst. The double-blind IBSP samples are made to appear as much like environmental samples as possible. The IBSP submits these samples in shipping coolers to the NWQL to mimic the process, as much as possible, by which actual environmental samples are submitted to the NWQL. For non-blank, blind samples, all identifying information (except account number) is changed to that of actual customers. Bottle labels are even "soiled" go give the appearance that the bottles have been filled in the field. After the samples are logged in, they are subjected to the identical laboratory handling, processing, and analytical procedures as are the NWQL’s customers’ environmental samples. Once the laboratory analyzes the samples, the results are routed through the same channels as used by regular customers (through the National Water Information System [NWIS]). Blind-blank data are pulled directly from the NWQL’s Laboratory Information System (LIMS) in order to avoid rounding and censoring of data below the detection limit. IBSP personnel compile, review, and post the analytical results within two to six business days of the NWQL releasing the results.
The Inorganic Blind Sample Project of the USGS Quality Systems Branch has been in operation since 1973 and has measured inorganic analytical laboratory performance of several labs over the years. Currently (2018) blind Quality Assessment samples are sent only to the National Water Quality Laboratory (NWQL). Performance data for the NWQL are readily available for results as far back as 1985 (https://qsb.usgs.gov/ibsp/charts.php) and can be used to help better understand environmental sample results.
Woodworth, M.T., and Connor, B.F., 2003, Results of the U.S. Geological Survey's Analytical Evaluation Program for Standard Reference Samples Distributed in March 2003: U.S. Geological Survey Open-File Report 03-261, 109 p.