Proper interpretation of water quality requires consideration of the effects that contamination bias and sampling variability might have on measured analyte concentrations. The effect of contamination bias and sampling variability on major ion and total dissolved solids data in water samples collected in 48 of the 52 National Water-Quality Assessment Program study units from 1992–2010 is discussed in this report. Contamination bias and sampling variability can occur as a result of sample collection, processing, shipping, and analysis. Contamination bias can adversely affect interpretation of measured concentrations in comparison to standards or criteria. Sampling variability can help determine the reproducibility of an individual measurement or whether two measurements are different.

Field blank samples help determine the frequency and magnitude of contamination bias, and replicate samples help determine the sampling variability (error) of measured analyte concentrations. Quality control data were evaluated for calcium, magnesium, sodium, potassium, chloride, sulfate, fluoride, silica, and total dissolved solids. A 99-percent upper confidence limit is calculated from field blanks to assess the potential for contamination bias. For magnesium, potassium, chloride, sulfate, and fluoride, potential contamination in more than 95 percent of environmental samples is less than or equal to the common maximum reporting level. Contamination bias has little effect on measured concentrations greater than 4.74 mg/L (milligrams per liter) for calcium, 14.98 mg/L for silica, 4.9 mg/L for sodium, and 120 mg/L for total dissolved solids. Estimates of sampling variability are calculated for high and low ranges of concentration for major ions and total dissolved solids. Examples showing the calculation of confidence intervals and how to determine whether measured differences between two water samples are significant are presented.