Concentrations and annual fluxes for selected water-quality constituents from the USGS National Stream Quality Accounting Network (NASQAN), 1996-2000

By Valerie J. Kelly, Richard P. Hooper, Brent T. Aulenbach, and Mary Janet

Data-Quality Criteria

• Definitions
  • IQR = Interquartile range
    • Measures the range of the central 50 percent of the data
    • Calculated as 75th percentile minus the 25th percentile
  • MRL = Minimum reporting limit
• Outliers
  • field data, nutrients, organic carbon, major ions
    • value > 75 percentile plus 1.5*IQR, and > 95 percentile, and >= 3*MRL
    • value < 5 percentile and >= 3*MRL
    • percent hits < 5 and value >= 3*MRL
  • trace elements and pesticides
  • value > 75 percentile plus 1.5*IQR, and > 95 percentile and >= 3*MRL
  • value < 5 percentile and >= 3*MRL
  • percent hits < 5 and value >= 3*MRL
  • pesticide surrogate recoveries
    • outside range 60 to 140 percent
• Replicates
  • difference > 2*MRL AND the relative percent deviation > 25
  • one replicate is detected at > 2*MRL AND the other is a nondetect (< MRL)
• Cation-anion balances
  • conductance < 100 and percent > +/- 15
  • 100 < conductance < 1000 and percent > +/- 10
  • conductance > 1000 and percent > +/- 5
• Logic checks for filtered and whole-water nutrients
  • the filtered values > whole-water values by 4*MRL for the whole-water analysis

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