Sr isotopes and U series radionuclides in the Sangemini area (Central Italy): Hydrogeology implications

Abstract

The strontium isotopic ratio (expressed as 87Sr/86Sr) of groundwater represents a useful method for studying and understanding the
groundwater circulations, also, the U and Ra isotopic compositions can vary as function of the groundwater residence time.
This paper reports an evaluation of the probable recharge area of the Sangemini mineral water springs (Terni-Umbria Central Italy)
and an estimate of the residence time of the aquifer by coupling Sr and U series isotopic systematics. For this study have been analyzed
four water sample for the isotope ratio of 87Sr/86Sr, and eleven samples, shallow waters and groundwaters, for U and Ra, furthermore
were determined the values of isotopic ratio for sample of typical rocks of the area. The results of this study allow to identify: a
recharge area in a restricted sector of the Meso-Cenozoic carbonates a longer and more effective water/rock interaction in the
Quaternary series. U and Ra recoil models allow to estimate a groundwater residence time of about 350 years and a total water
volume whose value (64*106 m3) agrees with the limited extension of the aquifer.
The extension of the aquifer was constrained by comparing Sr isotopic composition of waters and local geological formations.
Groundwaters seem mainly to circulate in clayey sandy Quaternary series characterized by low redox conditions.