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In the Piston-Flow Model (PFM), all the water in the sample has a single age (by definition equal to the mean age or residence time t PFM) (Chapter 3, this volume), and the time it entered the system (ti) is equal to ts-t.
In this case, the concentration of a tracer in a sample collected at time ts is given by:  Cs,ts = Cin,(ts-t PFM).
For example, the selection of atmospheric environmental tracers can be changed to reflect analytes of interest, the input tracer data can be modified to reflect local conditions or different time scales, and the analytes of interest can include other types of non-point-source contaminants such as nitrate (Böhlke, 2002).
Previous versions of this workbook have been used to evaluate field data in studies of groundwater residence time and agricultural contamination (Böhlke and Denver, 1995; Focazio and others, 1998; Katz and others, 1999; Katz and others, 2001; Plummer and others, 2001; Böhlke and Krantz, 2003; Lindsey and others, 2003).
The models worksheet is arranged as follows: Column A: Times (in years) for tracer input values.