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Quality of nutrient data from streams and ground water sampled during 1993-95; National Water-Quality Assessment Program

Proper interpretation of water-quality data requires consideration of the effects that bias and variability might have on measured constituent concentrations. In this report, methods are described to estimate the bias due to contamination of samples in the field or laboratory and the variability due to sample collection, processing, and analysis. These methods are applied to quality-control data collected as part of the National Water-Quality Assessment (NAWQA) Program during 1993-95. Results are presented for seven nutrient analytes from stream samples and five nutrient analytes from ground-water samples. Contamination potentially affects measured concentrations of less than 0.4 milligram per liter for ammonia and less than 0.1-0.3 milligram per liter for various phosphorus analytes. Nitrite plus nitrate and the Kjeldahl nitrogen analytes are essentially unaffected by contamination at concentrations of environmental significance. Sampling variability can affect interpretation of small differences between individual measurements or mean concentrations. For the NAWQA data, there is essentially no effect of variability on measured concentrations of nitrite plus nitrate and orthophosphate, nor for ammonia at concentrations of about 0.2 milligram per liter or less. The potential errors due to variability are larger for dissolved and total Kjeldahl nitrogen and phosphorus, but the uncertainty in measured or mean concentrations caused by sampling variability is within a small range for all nutrients. Differences in concentration within these ranges have limited, if any, environmental significance. These results can be applied to interpretation of environmental data collected during 1993-95 in 20 NAWQA study units.