Hydrology and geochemistry of the freshwater lens in coastal karst

Abstract

Interactions of freshwater and saltwater dominate the hydrogeology and hydrochemistry of coastal karst aquifers. The distribution of freshwater and saltwater in coastal aquifers is influenced by the nature and rates of recharge, size and boundary conditions of the system, distribution of hydraulic conductivities, and spatial and temporal variations in these parameters. The freshwater-saltwater mixing zone can be very thick and may intrude upward or inland due to pumping or decreased recharge. Dissolutional potential for karst development in coastal carbonates derives partially from carbonic acid from atmospheric and soil CO2, and from mixing of freshwater and saltwater. Solubility of carbonate minerals in freshwater is affected by variables including PCO2, temperature, pressure, and pH, a relationship complicated by the presence of other ions in natural waters. Freshwater/saltwater mixing induces several chemical effects that influence carbonate solubility, the strongest of which tend to cause undersaturation. Processes that complicate simple mixing dissolution include dissolution or outgassing of CO2, dissolution or precipitation of carbonate minerals, reduction of sulfur compounds, and kinetics effects. The rate of coastal karst development depends on discharge rates along the coast, degree of mixing, kinetic effects along the lens margin, and stability of sea level.