Adsorption and desorption of phosphate on limestone in experiments simulating seawater intrusion

This study investigates the potential release of PO43- from carbonate aquifers exposed to seawater intrusion. Adsorption and desorption of PO43- in the presence of deionized water (DIW) and seawater were conducted on a large block of Pleistocene age limestone to simulate the effects of seawater intrusion into a coastal carbonate aquifer at the laboratory scale. The limestone showed strong adsorption of PO43- in DIW, while adsorption was significantly less in the presence of seawater. Dissolution of CaCO3 was found to prevent PO43- adsorption at salinities less than 30 psu. Adsorption of PO43- was limited at higher salinities (30–33 psu), due to competition with HCO3- ions for adsorption sites. At a salinity <33, some PO43- absorption occurred as CaCO3 precipitated. Concentrations of PO43- between 2 and 5 μmol/L were released by desorption when the limestone was exposed to seawater. The results of this study suggest that as seawater intrudes into an originally freshwater coastal aquifer, adsorbed PO43- may be released into the groundwater. Consequently, adsorbed PO43- is expected to be released from coastal carbonate aquifers world-wide as sea level continues to rise exposing more of the freshwater aquifer to seawater.