Estimating groundwater residence time and recharge patterns in a saline coastal aquifer

A detailed study using environmental tracers such as chloride (Cl) and tritium (3H), deuterium (2H) and oxygen (18O) isotopeswas performed in an alluvial coastal aquifer in two contrasting environments (urban and agricultural). These environmentaltracers combined with a high-resolution multi-level sampling approach were used to estimate groundwater residence time andrecharge patterns and to validate the hydrogeochemical conceptual model already proposed in previous studies.δ18O andδ2Hcombined with Cldata proved that the hypersaline groundwater present in the deepest part of the aquifer was sourced from theunderlying hypersaline aquitard via an upwardflux. Both chemical and isotopic data were employed to calibrate a density-dependent numerical model based on SEAWAT 4.0, where3H and Clwere helped quantifying solutes transport within themodelled aquifer. Model results highlighted the differences on estimated recharge in the two contrasting environments, with theurban one exhibiting concentrated recharge because of preferential infiltration associated to the storm water drains network, whilescarce local recharge characterized the agriculture setting. In the urbanfield site, is still possible to recognize at 9 m b.g.l. theinput of the atmospheric anthropogenic3H generated by testing of thermonuclear weapons, while in the agriculturalfield site, the3H peak has been washed out at 6 m b.g.l. because the groundwater circulation is restricted only to the upper fresh part of theaquifer, drained by the reclamation system. The presented approach that combined high-resolutionfield monitoring,environmental tracers and numerical modelling, resulted effective in validating the conceptual model of the aquifer salinization