Fig. 13. Two-dimensional model predictions of the tritium concentration of fluid in the vadose zone at various times. Concentration units are log10 of concentration in picocuries per liter. (a) 1 Jan. 1963 (presumed to be background, before significant releases); (b) 8 July 1973; (c) 20 July 1986; (d) 1 Aug. 1999.