Fig. 5. Nitrate concentrations and apparent chlorofluorocarbon (CFC) recharge age-dates from the 27 June 2000 synoptic survey of ground water seepage. Best fit lines obtain by weighted (w_i, Eq. [9]) linear regression were used to approximate the historical progression of the basin-wide mean annual concentration of a hypothetical chemical in ground water recharge (Eq. [1]).