Evaluation of recharge in a small temperate catchment using natural and applied delta O-18 profiles in the unsaturated zone

A water balance study was used for determining recharge rate and mechanisms in the Enler Catchment, Northern Ireland. Here spatially limited data for the water balance resulted in varied calculation of the annual and monthly net infiltration rate. This paper outlines a method whereby high-resolution soil profiles (1 to 2 cm) were obtained from field cores in the upper 2 m of the unsaturated zone using delta O-18 of water. These profiles show chan ges in isotopic composition that range from individual rainfall events to a nnually integrated cycles of rainfall. Recharge rates were calculated from stable isotope profiles for each of the four main soil types in the study c atchment and summed over each area resulting in an average recharge in the range 55 to 70 mm/a, which is comparable with previous findings. Applied is otopic tracer tests were also conducted to evaluate the extent of preferent ial flow through the two main soil types in the catchment. Rates of water m ovement found from these experiments show good agreement with natural isoto pic profiles; however, evidence suggests that preferential flow is not the dominant process controlling water movement in this catchment. This type of data provides valuable information about recharge rates and mechanisms and may facilitate better prediction of contaminant transport pathways in the vadose zone.