Deep water circulation, residence time, and chemistry in a karst complex

Area of Science:

• Hydrogeology
• Geochemistry
• Environmental Science

Background:

• Investigated a complex karst hydrosystem comprising two carbonate units adjacent to a coastal lagoon.
• Observed groundwater discharge from a submarine spring and mixing with circulating thermal waters.
• Noted variations in water chemistry linked to marine influences and anthropogenic activities in different carbonate units.

Purpose of the Study:

• To elucidate the hydrochemistry and characterize groundwater circulation within a dual-unit karst system.
• To differentiate water sources and flow paths influenced by geological structures, marine intrusion, and thermal activity.
• To establish a hydrogeological framework integrating geochemical data and water residence times.

Main Methods:

• Analyzed hydrochemistry of groundwater from two distinct carbonate units.
• Utilized isotopic tracers (36Cl, 14C, 3H) to determine water residence times.
• Interpreted geochemical data to understand dissolution processes and fluid mixing.

Main Results:

• Identified incongruent dolomite dissolution in one carbonate unit and saline fluid contamination in the other, leading to elevated Ca and Mg.
• Characterized four distinct groundwater circulation types with residence times ranging from <20 years to ~100,000 years.
• Established a hydrogeological model based on geochemical constraints of the deep thermal system and surface-to-deep flow.

Conclusions:

• The karst hydrosystem exhibits complex hydrogeochemical processes and diverse circulation patterns.
• Geochemical signatures and isotopic data are crucial for understanding water origin, flow paths, and residence times.
• The study provides a comprehensive hydrogeological framework for managing this coastal karst environment.