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In the process of interpreting and analyzing trace element water-quality data for ground and surface water, it is important to determine the bias and variability that may be associated with these data. Trace element quality control samples (blanks and replicates) collected in the field for the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program from 1991 to 2002 were reviewed to determine the potential bias and variability that may be associated with the environmental samples. Bias in the data may be related to contamination from the field or laboratory during the collection, processing, shipping, or analysis of the samples. Sample variability can affect the interpretation of differences between individual measurements or mean concentrations. Trace element quality control data are available for 23 trace elements: aluminum (Al), antimony (Sb), arsenic (As), barium (Ba), beryllium (Be), boron (B), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), lithium (Li), manganese (Mn), molybdenum (Mo), nickel (Ni), selenium (Se), silver (Ag), strontium (Sr), thallium (Tl), uranium (U), vanadium (V), and zinc (Zn). In addition, replicate data for radon (Rn) in ground water were reviewed. Statistical analyses were used to estimate the likelihood of contamination bias and sample variability that could occur in the environmental samples. The 95-percent upper confidence limit was calculated at select percentiles to assess the potential for trace element contamination. The 95-percent confidence intervals were calculated for sample variability. The trace elements Sb, Be, and Tl in ground water and Sb, Be, Co, Mo, and U in surface water are unaffected by contamination. Limited quality control data (blanks) for Li and V in ground water and surface water do not allow for a good assessment on the potential contamination associated with these trace elements. Potential contamination was identified for Al, As, Ba, B, Cd, Cr, Cu, Fe, Pb, Mn, Ni, Se, Ag, Sr, and Zn in ground water and surface water. Evidence of potential contamination was shown for Co, Mo, and U in ground water; potential contamination was shown for T1 in surface water. In comparing the potential contamination for these trace elements with the U.S. Environmental Protection Agency’s (USEPA) drinking-water standards, the contamination for most of these trace elements is less than 10 percent of the drinking-water standard; therefore, contamination would have little or no effect when comparing trace element concentrations with the USEPA drinking-water standards. The exceptions are Al, Cd, and possibly Pb in ground water, and As and possibly Pb in surface water. Potential contamination identified for these trace elements is greater than 10 percent of the USEPA drinking-water standard, but affects only 5 percent or less of the As, Cd, and Pb samples. For most trace elements, the level of potential contamination is not large enough to significantly affect the measured concentration of the environmental sample. The exceptions may be Fe in ground water and Al in surface water, which have concentrations for at least 10 percent of the environmental samples that exceeded the USEPA drinking-water standards. Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002 By Lori E. Apodaca, David K. Mueller, and Michael T. Koterba Sample variability for some of the trace elements could not be determined because there were either no detected concentrations, or there were less than 10 replicate sets with detected concentrations. These trace elements are Be, Ag, and Tl for ground water and Sb, Be, Cr, Co, Pb, Ag, and Tl for surface water. For most trace elements, sample variability was less than 10 percent, which would have little or no affect on the reported concentrations. The exceptions are Al, Cd, Cu, Pb, Rn (at concentrations less than about 700 picocuries per liter), Se, and Zn in ground water and Cu, Se, and Zn in surface water, all of which have sample variability ranging from 10 to 20 percent. Sample variability should be considered when evaluating the potential error associated with a sample measurement. Collection of additional quality control samples for some of these trace elements to determine bias and variability is probably warranted particularly for those trace elements that the NAWQA Program did not begin sampling until 1998. Results obtained from the analysis of the quality control data can be applied to the interpretation of the environmental data collected from 1991 to 2002 and for water-quality data that are currently being collected as part of the NAWQA Program. Introduction The U.S. Geological Survey’s (USGS) National WaterQuality Assessment (NAWQA) Program was implemented in 1991 to improve the scientific and public understanding of water quality in the Nation’s major river basins and groundwater systems. The goals of the NAWQA Program are to describe current water-quality conditions and trends in the Nation’s rivers, streams, and ground water to understand the natural characteristics and human influences that affect water quality (Hirsch and others, 1988). In Cycle I (1991–2001), the first decade of extensive monitoring within 52 study units (fig. 1), work concentrated primarily on gathering comparable information on water quality in surface water and ground water. To interpret trace element environmental water-quality data, information is needed to determine the bias and variability in the water-quality data that can result from sample collection, processing, shipping, and analysis. Sample bias and variability can be evaluated by collecting quality control (QC) samples such as blank and replicate samples that are collected with the environmental samples. Bias is a systematic error and can be either positive or negative. An example of positive bias is contamination of the samples. Variability is a random error that affects the ability to reproduce an analysis. Purpose and Scope This report describes the results of the analysis of the trace element blank and replicate data collected with the environmental samples from 1991 to 2002 of the NAWQA Program. The QC data analysis is used to (1) describe the frequency and magnitude of trace element contamination using field blank data; (2) evaluate variability of the water-quality data using field replicate data; and (3) identify potential effects of bias and variability in interpreting the trace element data. Trace elements reviewed in this report include aluminum (Al), antimony (Sb), arsenic (As), barium (Ba), beryllium (Be), boron (B), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), lithium (Li), manganese (Mn), molybdenum (Mo), nickel (Ni), selenium (Se), silver (Ag), strontium (Sr), thallium (Tl), uranium (U), vanadium (V), and, zinc (Zn). In addition, radon (Rn) replicate sample analyses in ground water were reviewed. The results of the QC data analysis are compared with the U.S. Environmental Protection Agency’s (USEPA) drinking-water standards to assess the potential effects of bias and variability on the environmental data (U.S. Environmental Protection Agency, 2004). This report does not address QC or trace element data that are contaminated or trace element data errors related to the coding of trace element data. Acknowledgments The authors would like to thank Leslie DeSimone and George Groschen for their technical reviews of this manuscript. Also, the work done by the Cycle I NAWQA study unit hydrologists and hydrologic technicians who collected the trace element water-quality data is greatly appreciated. Without their efforts, the analysis of the trace element QC data would not have been possible. 2 Review of Trace Element Blank and Replicate Data Collected in Ground and Surface Water for the National Water-Quality Assessment Program, 1991–2002