The Sr-isotope systematics as a groundwater tracer: an application to the Classical Karst

The preferential flow path of groundwaters through limestone aquifers by matrix flow within rock pores or networks of micro fractures and by fast conduit flows in large fissures and openings has important consequences on solute concentration, and poses critical problems in terms of contaminant transport. In particular, fast conduit flows can transmit contaminants rapidly, and pollutants may arrive earlier to pumping stations for drinkable water supply yielding peaks in concentrations. The slow draining of the resident mass water and diffusion into rock pores produce a continuous source of contamination, even if generally at lower concentration. The detection of the preferential flow paths, residence times and flow dynamics is hence of the uppermost importance in carbonate karst aquifers, with implications on the possible seasonal variations in water quality. Conduit flow and diffuse flow often result in springs with different chemical patterns, temperature and discharge rates, and anthropogenic tracers such as chlorofluorocarbons are effective for tracing groundwater flow. However, the usefulness of the Sr-isotope systematics has been in many cases demonstrated in the reconstruction of the water-carbonate rock interaction, becoming an important tracer of groundwater movement. In fact, the 87Sr/86Sr ratios in groundwaters reflect the water-rock interaction. In this study, surface, cave and spring waters have been collected during high and low-flow conditions in the Classical Karst area (northern Italy and Slovenia), a carbonate plateau that rises above the northern Adriatic Sea and consisting in a thick sequence of limestones and dolostones dated Cretaceous and Tertiary. Waters belong to the Ca-HCO3 and Ca-Mg-HCO3 hydrofacies. The 87Sr/86Sr ratio in surface, cave waters and springs ranges between 0.70781÷0.70838, 0.70787÷0.70824 and 0.70757÷0.70786, respectively, during low flow, and between 0.70760÷0.70843, 0.70749÷0.70810 and 0.70759÷0.70800 during high flow. The data indicate the relative contribution of the Isonzo and Reka rivers, with relatively high Sr isotopic composition, as inputs to the Classical Karst aquifer. Furthermore, cave waters have 87Sr/86Sr ratio higher with respect to the Sr isotopic composition measured in limestone rock samples (87Sr/86Sr=0.70743÷0.70752), while springs range from near-isotopic equilibrium with limestones towards a more radiogenic character, suggesting different water-rock contact times. These data are interpreted in terms of the role of the processes in the upper, phreatic zone and on the different flow dynamics in the area, which have a different vulnerability towards pollution. This study is supported by the Italy-Slovenia crossborder cooperation Programme 2007-2013, INTERREG Project standard 02/2009, priority line 1: HYDRO KARST”.