Exploring tracer dynamics at different spatial scales in a pre-Alpine catchment with a temporary stream network

Hydrological studies on temporary streams are crucial for understanding their activation and response during wet and dry conditions. Additionally, the use of geochemical tracers (e.g., electrical conductivity, water stable isotopes, major ions) can help assess the impact of climate change on these ephemeral water bodies. This study aims to i) investigate the relation between discharge and tracer concentration at different spatial scales and at the seasonal and event scales; ii) analyze the effect of antecedent conditions on tracer temporal variability at different spatial scales; iii) compare the contribution of rainwater to stream runoff at three scales during selected rainfall-runoff events. The work relies on an integrated database of isotopic and geochemical compositions of water samples coupled with hydrometeorological data from the Regional Environmental Agency (ARPAV) in the 116 km2 Posina catchment in the Italian pre-Alps. The lithology consists mainly of carbonate rocks, and the typical fracturing of dolomites and limestones facilitates water infiltration, thus favoring the presence of temporary streams during dry periods. Conversely, the limited presence of volcanic rocks in some sub-catchments tends to favor perennial streams characterized by a rapid response to rainfall events. In this work, water samples were collected from the Posina river and its main tributaries between September 2015 and March 2019. Temperature and electrical conductivity were measured in the field by portable probes, whereas major ions and water stable isotopes were analyzed by ion chromatography and laser spectroscopy, respectively. Preliminary results show that relationships between discharge and tracer concentration reveal significant associations: δ18O increases with discharge, whereas electrical conductivity (EC) shows a decreasing trend with discharge, better represented by logarithmic and polynomial functions for different selected sections of the main streams and tributaries. Similar trends are observed for sulphates and sodium.  Discharge data at Ressi (a tributary flowing on volcanic rocks) and Posina at catchment outlet have also been compared with selected tracer data from water samples from these two sections. Positive correlations are found between average tracer concentration (δ2H, δ18O, and nitrates) and peak antecedent discharge, while negative correlations exist for δ2H, EC, chloride, sulphates, bicarbonate ions, sodium, magnesium, and calcium. Antecedent precipitation positively correlates with δ2H and nitrates but negatively with sulphates and sodium. EC shows positive (negative) correlations with δ2H and nitrates (sulphates and sodium), respectively, with varying patterns along different sections and tributaries. The 5-day antecedent rainfall exhibits the highest correlations with tracer compositions, particularly for EC, δ2H, and nitrates. The obtained results suggest the importance of an interdisciplinary approach in the analysis of the hydrological and geochemical connectivity of temporary stream networks.