Table 2.  Water properties and approximate sampling frequencies for 
National Stream Quality Accounting Network and Hydrologic Benchmark
Network water-quality data on CD-ROM

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        Water properties                                 Frequencies 1
                                                       (samples per year)
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 Physical/field measurements:
   Temperature, specific conductance (field and lab),
     dissolved oxygen, pH (field and laboratory),
     suspended sediment, turbidity, instantaneous
     streamflow...............................................4 to 12
   Streamflow, daily mean...................................... 365

 Major ions:
   Calcium, chloride, magnesium, potassium, silica,
     sodium, sulfate, fluoride, dissolved solids,
     hardness, alkalinity, bicarbonate, and carbonate.........4 to 12

 Nutrients and carbon (dissolved and total)2:
   Ammonia+organic (Kjeldahl) nitrogen, ammonia,
     nitrite, nitrate+nitrite, orthophosphate,
     phosphorus, and nitrogen.................................4 to 12
   Organic carbon (includes suspended also)....................4 to 8

 Radiochemicals (dissolved, suspended sediment):
   Gross alpha and beta, radium-226, tritium, uranium..........1 or 2

 Biological measurements:
   Fecal coliform and streptococci............................4 to 12
   Periphyton (chlorophyll A and B), periphyton
     biomass.....................................................4
   Phytoplankton (chlorophyll A and B),
     phytoplankton (total count), identification of
     predominant forms........................................7 to 12

 Inorganic trace elements (dissolved, total):
   Aluminum, arsenic, barium, beryllium, boron,
     cadmium, chromium, cobalt, copper, iron, lead,
     lithium, manganese, mercury, molybdenum, nickel,
     selenium, silver, strontium, vanadium, and zinc..........3 or 4
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     1 The range of sample frequencies for any constituent group reflect 
differences in frequency among the two networks, changes in frequency with 
time, and differences among stations within a single network.  See the 
section "Sampling Frequency" for a discussion of temporal changes in 
sampling frequency.

     2 Analyses for the "total" forms of nitrate+nitrite, nitrite, ammonia, 
and orthophosphate are based on samples "decanted" in the laboratory (see 
the section "National Network Laboratory Techniques" for additional details). 
Total nitrogen is computed as the sum of total Kjeldahl nitrogen and "total" 
nitrate+nitrite.