Applicability of d-excess and 17O-excess as groundwater tracers for determination of recharge area

Abstract

Methods to determine the recharge elevation of groundwater using the altitude effect of δD and δ18O have been extensively applied in hydrogeological investigations. The secondary parameter d-excess has also been used as a groundwater tracer. In this study, to examine the usefulness of these tracers along with 17O-excess, ~160 groundwater samples were collected from a humid region at the foot of Mt. Fuji, Japan. The sampling area covered 40 × 45 km, with most sampling sites located below 1,000 m above sea level. The relatively low elevation and small scale of the basin allowed for examination of the elevation-dependence of groundwater isotopes. Using high-precision isotope analyses, a low lapse rate but clear elevation-dependence in spring waters was observed for d-excess (0.18‰·100 m−1). The recharge elevation determined by d-excess correlated with those by δD and δ18O, indicating that d-excess has high potential as a groundwater tracer for the determination of recharge elevation. On the other hand, 17O-excess in groundwater held small spatial variation, with an average of 25 per meg in the horizontal direction and a lapse rate of 0.6 per meg·100 m−1. The low lapse rate compared to the analysis error inhibits its usefulness as a tracer. The fact that the recharge elevation determined by δD, δ18O, and d-excess were similar indicates that the combination of these tracers could increase the reliability of the results.