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Table 2. Summary of current (1995) required, recommended, and optional water-quality constituents to be measured in the three National Water-Quality Assessment Program ground- water components of the Occurrence and Distribution Assessment--Continued 'Selection of constituents for measurement in Flowpath Studies is determined by Flowpath-Study objectives. During at least the first round of sampling, however, the broad range of constituents measured in Study-Unit Surveys and Land-Use Studies would be measured. 2Schedules and laboratory codes listed are required for Study Units that began their intensive phase in 1991 or 1994, and apply until changed by National Program directive. Schedules for radium and uranium can be selected by the Study Unit, but require NAWQA Quality-Assurance Specialist approval. A detailed discussion is found in Koterba and others (in press). 3ANC (formerly referred to as unfiltered alkalinity) is measured on an unfiltered sample. Alkalin- ity is measured on a filtered sample. Study Unit could have collected ANC, alkalinity, or both to date. 4Turbidity measurements are required whenever trace-element samples are collected to evaluate potential colloidal contributions to measured concentrations of iron, manganese, and other elements. 5VOCs are required at all urban Land-Use Study wells, but optional in agricultural Land-Use Studies. If VOCs are chosen as part of an agricultural Land-Use Study, then they should be measured in at least 20 of the Land-Use Study wells. 6VOCs are required at all urban flowpath wells for at least the first round of sampling. If VOCs are measured in an agricultural Land-Use Study, then they should be measured at all Flowpath-Study wells within that Land-Use Study for at least the first round of sampling. 7Radon is required at any Land-Use or Flowpath Study well if that well also is part of a Study- Unit Survey; otherwise, radon collection is recommended for Land-Use or Flowpath Study wells located in likely source areas. 8Collection of tritium, tritium-helium, chlorofluorocarbons (CFCs), and/or other samples for dating ground water is recommended, depending on hydrogeologic setting. For tritium methods, see NWQL catalog; for CFCs, see Office of Water Quality Technical Memorandum No. 95.02 (unpub- lished document located in the USGS Office of Water Quality, MS 412, Reston, VA 22092). 9For a general discussion of the use of environmental isotopes in ground-water studies, see Alley (1993).