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

This study investigates the potential release of PO 4 3 - from carbonate aquifers exposed to seawater intrusion. Adsorption and desorption of PO 4 3 - 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 PO 4 3 - in DIW, while adsorption was significantly less in the presence of seawater. Dissolution of CaCO3 was found to prevent PO 4 3 - adsorption at salinities less than 30 psu. Adsorption of PO 4 3 - was limited at higher salinities (30–33 psu), due to competition with HCO 3 - ions for adsorption sites. At a salinity <33, some PO 4 3 - absorption occurred as CaCO3 precipitated. Concentrations of PO 4 3 - 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 PO 4 3 - may be released into the groundwater. Consequently, adsorbed PO 4 3 - 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.