In this case a water sample was taken each week (4 samples per month, n ¼ 12) in the 5 sampling points listed in Fig. 1, in triplicate (n ¼ 15 each week). Field blanks, consisting of purified water with 20 mg/L of ascorbic acid, were included with each sampling trip and handled like real samples, except the bottles were not opened in the field.