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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5026

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Appendix A. Quality Control and Quality Assurance of Water Samples

Thirty percent of the samples to be analyzed for dissolved nutrient concentration and 35 percent of the samples to be analyzed for total phosphorus concentration were for quality assurance purposes, which included field blanks (the first sample collected every week) and either a split sample or a method replicate (each type every other week). Field blanks are samples of reagent grade inorganic blank water processed onsite through clean sampling equipment, before an environmental sample is collected. Analysis of blank samples determines if the processes of collection, handling, transport, and analysis cause measurable contamination. Split samples are environmental water samples collected once and divided into two samples that are used to determine the variability in the analytical methods. Replicate samples are environmental samples collected twice in rapid succession from the same location and analyzed to determine both the variability of the system and the variability in the analytical methods.

The results of the quality assurance sampling indicated that precision and accuracy were acceptable and the variability in sampling and processing was lower than seasonal variability (table A1). The field blank concentrations of total phosphorus and orthophosphate were less than the laboratory minimum reporting level. The same was true for the field blank concentration of nitrite-plus-nitrate, except that one outlier sample held some contamination. The field blank samples of ammonia revealed some contamination, although it was minimal compared to the concentrations of the environmental samples. Ammonia concentration in the field blanks was considerably reduced, compared to that in previous years, by the use of capsule filters. Contamination of ammonia blanks in 2003 and 2004 was attributable to atmospheric deposition (Wood and others, 2006). Twice during the season, split samples were sent to two other laboratories in addition to NWQL and Aquatic Analysts. The interlaboratory split samples were analyzed to determine variability in analytical methods between the laboratories. In addition, some of the split samples were collected to examine variability that might be expected as a result of differences in the protocols used by USGS and those used by the Klamath Tribes in the collection of their long-term biweekly dataset. Box plots showing the statistical distribution of the difference between individual split sample concentrations and the median concentration of all of the split samples taken from the same churn splitter indicate that the variance in laboratory processing and analysis is generally low, except in the case of ammonia (fig. A1). The outliers in the distribution of the ammonia splits can all be attributed to the use of a non-USGS protocol and a single non-USGS laboratory. The reason for the high ammonia concentrations in these samples has yet to be determined, but the results do not indicate that the rest of the ammonia samples collected as part of this monitoring program are biased high.

Figure A1. Distribution of differences from the median concentrations among split sample interlaboratory measurements of samples from the water quality data collection program in Upper Klamath and Agency Lakes, Oregon, 2005.

Table A1. Quality-assurance results for water quality data collection program, Upper Klamath and Agency Lakes, Oregon, 2005.

[mg/L, milligram per liter; MRL, minimum reporting level. >, greater than; ≤, less than or equal to]

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