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The U.S. Geological Survey maintains a quality-assurance program based on the analysis of reference samples for its National Water-Quality Laboratory located in Denver, Colorado. Reference samples containing selected inorganic, nutrient, and precipitation (low-level concentration) constituents are prepared at the Survey's Water Quality Services Unit in Ocala, Florida, disguised as routine samples, and sent daily or weekly, as appropriate, to the laboratory through other Survey offices. The results are stored permanently in the National Water Data Storage and Retrieval System (WATSTORE), the Survey's data base for all water data. These data are analyzed statistically for precision and bias. An overall evaluation of the inorganic major ion and trace metal constituent data for water-year 1988 indicated a lack of precision in the National Water-Quality Laboratory for the determination of 8 out of 58 constituents: calcium (inductively coupled plasma emission spectrometry), fluoride, iron (atomic absorption spectrometry), iron (total recoverable), magnesium (atomic absorption spectrometry), manganese (total recoverable), potassium, and sodium (inductively coupled plasma emission spectrometry). The results for 31 constituents had positive or negative bias during water-year 1988. A lack of precision was indicated in the determination of three of the six nutrient constituents: nitrate plus nitrite nitrogen as nitrogen, nitrite nitrogen as nitrogen, and orthophosphate as phosphorus. A biased condition was indicated in the determination of ammonia nitrogen as nitrogen, ammonia plus organic nitrogen as nitrogen, and nitrate plus nitrite nitrogen as nitrogen. There was acceptable precision in the determination of all 10 constituents contained in precipitation samples. Results for ammonia nitrogen as nitrogen, sodium, and fluoride indicated a biased condition. INTRODUCTION The National Water-Quality Laboratory (NWQL) of the U.S. Geological Survey, located in Denver, Colorado, routinely analyzes water, suspended sediment, and streambed and lakebed materials for inorganic constituents, many organic substances (including common pesticides), priority pollutants as defined by the U.S. Environmental Protection Agency (Keith and Telliard, 1979), and some physical properties. Results of the quality-assurance program used to monitor the quality of work at the NWQL are discussed in this report. Previous reports (Peart and Thomas, 1983a, 1983b, 1984; Peart and Sutphin, 1987; Lucey and Peart, 1988, 1989a, 1989b) document results from February 1981 through September 1987. Factors that need to be considered for interpretation of the results presented in this report include the following: 1. No effort was made to correct nonanalyqical errors, even when it was obvious which corrective measures were appropriate, so the data are preserved as produced by the laboratory. These errors include any made in logging the sample into the laboratory, transcription errors by the analyst, and data entry errors. Therefore, if the data reviewer in the Survey's office that collected the sample detects errors of this type, the quality of the data can be increased when compared to those data presented in this report. For example, two samples from different sites are submitted to the laboratory on the same day and are misidentified in a way that the analytical data reported for one would actually belong to the other. A data reviewer familiar with one of the sites or its historical data usually could detect the problem and make necessary corrections. 2. No quality-assurance samples had any constituents redetermined except those requested by the laboratory internal quality-assurance groups. Survey data reviewers in the offices that collected the samples are expected to scrutinize incoming new data for discrepancies and make requests for reanalysis. These requests may result in the detection of analytical and nonanalytical errors, and data quality would improve when compared to data quality presented in this report. 3. Figures included in this report may be used to determine analytical conditions at any given time for water-year 1988. Where control charts show that an analytical process has been in statistical control for most of the year, but the process also has been out of statistical control for a certain period, that period may be long enough that the statistical tests applied indicate lack of precision or significant bias for the entire year. The data from that period when the analytical process was in statistical control can be considered to have acceptable precision and bias. 4. Several data points seemed to be in error because of an incorrectly applied dilution factor. Sample dilutions are made routinely in the laboratory to bring sample concentrations into analytical range. If the dilution factor is not applied or is applied incorrectly, the reported value will be in error by the amount of the dilution factor. For example, if several analyses of a solution result in reported values of 250 mg/L each and one analysis results in a reported value of 25 mg/L, a 10X dilution may have been used and not applied to the final result. These kinds of errors are difficult to confirm. Their detection, confirmation through rerun requests, and correction in the field offices will increase the reliability of the data above that indicated in this report. 5. Non-analytical errors for nutrient analyses can result when the samples are not maintained at the ideal temperature of 4° C during shipping and receiving. During water-year 1988, the following sample categories, containing the indicated constituents in the dissolved phase, were included in this qualityassurance program: Inorganic constituents: alkalinity, aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, calcium, chloride, chromium, cobalt, copper, dissolved solids (residue on evaporation at 180°C), fluoride, iron, lead, lithium, magnesium, manganese, mercury, molybdenum, nickel, potassium, selenium, silica, silver, sodium, strontium, sulfate, and zinc. Nutrients: ammonia, as nitrogen; ammonia plus organic nitrogen as nitrogen; nitrate plus nitrite as nitrogen; nitrite as nitrogen; orthophosphate as phosphorus; and phosphorus. Precipitation: minute concentrations of ammonia, as nitrogen; calcium; chloride; fluoride; magnesium; nitrate, as nitrogen; orthophosphate as phosphorus; potassium; sodium; and sulfate. PROGRAM DESCRIPTION Standard reference water samples (SRWS's) (Skougstad and Fishman, 1975; Schroder and others, 1980; Janzer, 1985) are used as the principal component of the reference samples used in this program. The SRWS's are diluted with deionized water, mixed in varying proportions with other SRWS's, or used undiluted. A large range of concentrations of chemical constituents is achieved, thereby increasing the number of unique samples available for quality-assurance purposes. This, in turn, decreases the probability that quality-assurance samples will be recognized in the laboratory because of frequency of analyses or unique sample characteristics. The reference samples are prepared from the SRWS's in the Survey's Water Quality Service Unit in Ocala, Fla. , and are made to appear as much like environmental samples as possible. When the samples are prepared and proper forms are completed to ensure that appropriate constituents have been requested for the sample, the samples and the forms are shipped to selected Survey offices across the country. These Survey offices then ship the quality-assurance samples to the NWQL on a daily or weekly basis, as appropriate, with their regular samples. The number of quality-assurance determinations requested for inorganic constituents and nutrients are in direct proportion to the total number of requests for those determinations from all sources in the laboratory. The program goal is to have at least one quality-assurance sample analyzed daily for those constituents that are analyzed daily, and, similarly, to have an appropriate number of quality-assurance samples analyzed for those constituents determined less frequently. Natural precipitation samples were submitted once each week. All constituents in the reference materials are in the dissolved phase because the reference materials themselves have been filtered in the preparation process. Therefore, those constituents in this report that are designated as "total recoverable" are from reference samples that have undergone a digestion process (Fishman and Friedman, 1985, p 87-88) during analysis, rather than from unfiltered or whole-water samples. Differences that appear in this report between the dissolved analyses and the total recoverable analyses will be due largely or entirely to the digestion process rather than from any difference in the sampling techniques or sample source. Quality-assurance samples are processed by the NWQL as routine samples, including the normal laboratory quality-control and quality-assurance procedures. The data then are stored in the Survey's National Water Data Storage and Retrieval System (WATSTORE). After being processed by the laboratory, data from these quality-assurance samples will indicate the quality of the analytical data produced for environmental samples. STATISTICAL EVALUATION The SRWS's initially are analyzed by many laboratories throughout the United States, using several different analytical methods. The results are compiled by calculating the means, standard deviations, and 95-percent confidence limits and then applying a rejection routine (American Society for Testing and Materials, 1980). Resultant means are the most probably correct values or the most probable values (MPV's). These MPV's are used in this quality-assurance program for comparison with laboratory results. For reference samples composed of a mixture of two SRWS's, or SRWS's and deionized water, the MPV's for each constituent are weight-averaged according to their respective percentage contribu

Quality-assurance data for routine water analysis in the National Water-Quality Laboratory of the US Geological Survey for water year 1988