Atmospheric and Geogenic Controls of Rainwater and Stream-Water Chemistry in an Aleppo-Pine-Dominated Mediterranean Karst Catchment

This study investigated the hydrochemical linkage between rainwater and stream water in an Aleppo pine-dominated Mediterranean karst catchment in Croatia during 2020–2022, focusing on ion sources, seasonal patterns, and processes governing the transformation of atmospheric inputs into stream-water chemistry. Rainwater reflected mixed atmospheric inputs, with sea-salt fractions accounting for 94.8% of Cl− and 97.9% of Mg2+, whereas 97.5% of Ca2+ and 79.2% of K+ originated from non-sea-salt sources. Stream water was substantially more alkaline and mineralised, with a higher pH (8.23 vs. 6.53), electrical conductivity (327 vs. 19.63 µS cm−1) and Ca2+ concentrations (65.08 vs. 6.22 mg L−1). These differences reflect carbonate weathering during subsurface flow, whereby the dissolution of calcite and dolomite releases Ca2+ and Mg2+, increases alkalinity and buffering capacity, and results in approximately tenfold-higher stream-water Ca2+ and Mg2+ concentrations. Seasonal variability was evident, with ion enrichment during low-flow conditions and dilution during wetter periods. Limited synchronous hydrochemical correspondence between rainwater and stream water indicates that atmospheric inputs are modified during catchment transport. Overall, atmospheric deposition provides the initial hydrochemical signal, whereas carbonate weathering, hydrological flow paths, and seasonal biogeochemical processes are the predominant controls on stream-water chemistry.