Geochemical and multi-isotopic (δ18O, δD, 87Sr/86Sr, 143Nd/144Nd) approaches on sources, water-rock interactions, and mixing processes of geothermal waters from a complex intertidal region of the Gulf of California

The hydrogeochemical processes governing intertidal spring waters remain poorly understood because the potential sources and factors influencing their chemical and isotopic compositions are rarely documented. The northwestern coast of the Gulf of California hosts numerous geothermal intertidal springs. Hydrothermal systems are commonly associated with major fault zones. However, the occurrence of all springs near the shoreline is not entirely reported. Our study primarily focuses on four intertidal discharge zones: (i) San Felipe, (ii) Punta Estrella, (iii) El Coloradito, and (iv) Puertecitos. Using geochemical (major species and Rare Earth Elements REE) and isotopic data (δ 18 O, δD, 87 Sr/ 86 Sr, 143 Nd/ 144 Nd), we infer the sources of the intertidal springs, characterize water-rock interactions, and determine mixing processes among water types. Arid environmental conditions, evaporite dissolution, and reverse ion exchange primarily control the hydrogeochemical facies of the intertidal springs. Stable isotope data (δ 18 O and δD) demonstrate either mixing between meteoric and seawater components or influence from a major brine source originating in the southernmost basin, associated with inputs from the Colorado River. Solute inputs derived from bedrock weathering and lixiviation processes on the continent dominate the geochemistry of seawater from the upper Gulf of California. Mixing calculations using 1/Sr and 87 Sr/ 86 Sr ratios confirm that the intertidal spring waters are a mixture of seawater, meteoric water, and deep brines, with notable variations in the proportions and isotopic signatures of these end-members across the sites. Neodymium isotopes elucidate a more radiogenic input in waters from San Felipe, suggesting a connection to fluids leached from the Colorado River. Finally, we present hydrogeological models to integrate structural and geophysical information from the literature. • The occurrence of intertidal hot springs is controlled by several hydrogeological factors. • Various chemical and isotopic end-members associated with intertidal hot springs are presented. • The composition of coastal seawater is altered by inputs from the Colorado River • The hydrogeochemical facies are controlled by arid environmental conditions, evaporite dissolution, and reverse ion exchange. • The isotopic data reveal water-rock interactions and mixing processes among seawater, local freshwater, and deep sources from the Colorado River.