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32 tritium analyses were performed on 13 different water sources within Soreq Cave, Israel; eight of the water sources were analyzed once each, and the other five between 3 and 6 times each, on samples collected between 1990 and 2000. The results were compared with similar analyses of a 1981–82 collection and with the tritium concentrations of the annual rainfall from 1952 to 1998. The tritium in several of the cave waters was so high (up to 110 TU) that they must have originated mainly in the peak tritium rain year 1964. Most of the other cave waters had tritium concentrations, which though somewhat lower, must have originated largely from rain that fell between 1962 and 1966. The consequences of these tritium observations are that: (1) it generally takes 26–36 years for rainwater to percolate from ground surface to Soreq Cave ceiling; and (2) the type of flow undergone during this percolation is in several cases very similar to ‘piston flow’, hence the annual packets of rainfall reach the cave without undergoing much mixing with either earlier or later rains. In about one-third of the cases, tritium was too low to determine the percolation time. Finally, the time variations of the tritium concentrations in specific cave water sources seem to indicate that the conduits which bring water to these sources may have undergone physical changes which affect the amount of time it took for rain to reach the cave ceiling. [Copyright &y& Elsevier]

An EXCEL workbook is presented for calculating the mean residence time of groundwater based on the environmental tracers, tritium, CFC-11 and CFC-12. The program TRACER is written in Visual Basic for Application language and uses piston, exponential, linear, exponential-piston and linear-piston flow types of lumped-parameter models. Input and output data are stored in worksheets and a graph of results that are best fitted to observations is drawn for visual evaluation. Recharge temperature and altitude are used to convert atmospheric partial pressures of CFC-11 and CFC-12 to dissolved concentrations to provide a direct comparison between the models’ output and observed data. The model can also be used to check whether an inferred flow type could be valid in the groundwater system being investigated. Other radioactive and gaseous environmental tracers and reactions such as, sorption and degradation can be included either as decay constant or with modifications in the program code. TRACER matches, satisfactorily, the results obtained from other softwares. [Copyright &y& Elsevier]

• A generic framework is presented for studying recharge processes at field scales. • Piston flow is dominant during soil water infiltration in loess tableland. • The tritium peak method can still be used in loess unsaturated zones. • The groundwater in Xifeng tableland is tritium-free. The corrected 14C age ranges from 220 to 19,220 years. Characterization of the rainfall infiltration mechanism, whether piston or preferential flow, through an unsaturated zone is necessary for groundwater resource assessment and for analysis of environmental geological hazards. However, several limitations and difficulties remain in determining the infiltration mechanism in arid and semiarid areas. This study combines the evaluations of soil water infiltration characteristics, recharge rate, and determination of groundwater age at a water table in a semiarid loess tableland to examine the recharge processes. The case study is the Xifeng loess tableland in the Loess Plateau of China, which has an unsaturated zone depth of 40–75 m and an obvious 1963-3H peak at a depth of 7.5 m. According to the piston flow, the soil water infiltration velocity is 0.12–0.14 m/yr, corresponding to a recharge rate of 37–41 mm/yr, based on the 1963-3H peak and chloride mass balance method. All of the groundwater from the water table and wells is old (tritium-free), with ages ranging from hundreds of years to 20,000 years, which suggests that no rapid modern recharge has occurred since the 1950s. The chloride content of soil water in the deep unsaturated zone, with an average value of 10.8 mg/L in the unsaturated zone below 30 m, overlaps with that in groundwater, at 4.0–10.9 mg/L. The comprehensive isotopic and chemical compositions for soil water and groundwater showed that piston flow was dominant during soil water infiltration at the field scale, resulting in the delay of modern water and related solutes entering the groundwater. Combined evaluations of the unsaturated zone with those of the saturated zone and the use of multiple environmental tracers, such as Cl, Br, 2H, 18O, 13C, 3H and 14C, can be regarded as a generic framework for studying the recharge processes in arid and semiarid areas with thick unsaturated zones at macroscopic and field scales. [ABSTRACT FROM AUTHOR]