Water transit times and flow paths from two line injections of H-3 and Cl-36 in a microcatchment at Gardsjon, Sweden

To investigate groundwater transit times and flow paths in shallow till soi l, within an acidification study at Gardsjon, Sweden, two line injections o f Cl-36 and H-3 Were made in groundwater during 1992. The first injection i n January, when the two tracers were injected at the same depth, gave trace r transit times from injection line to outlet of some hours due to a runoff event on the first day. The subsequent recession period left a considerabl e amount of tracer in the unsaturated zone, which had transit times that ra nged from weeks to months. Tracer recovery at the outlet was 78% for H-3 an d 47% for Cl-36. Cl retention was indicated. The second injection in Novemb er, when H-3 was injected at 30 cm depth and 36C1 at 60 cm depth, gave reco veries of 96% for H-3 and 83% for Cl-36. Apart from an advective flow-depen dent tracer transport, very fast tracer pulses occurred. The velocities for those pulses were of the order of tens of metres per hour. Highest tracer concentrations were observed in the superficial soil layers, which suggest that these layers were the dominant flow paths. Copyright (C) 1999 John Wil ey & Sons, Ltd.