SALTWATER INTRUSION AND PHOSPHORUS DESORPTION: THE FIRST GEOCHEMICAL MODEL OF SEAWATER-INDUCED PHOSPHORUS RELEASE FROM CALCITE (Invited Presentation)

One of the most important ecological consequences of saltwater intrusion into coastal aquifers is that water-rock interactions result in a spike in phosphorus (P) in ambient groundwater, which can then be discharged to overlying estuaries. Although this phenomenon is well-documented globally, the geochemistry of the process has remained a matter of speculation. Laboratory experiments paired with geochemical modeling can provide insight into the thermodynamically favorable mechanisms behind observed phenomena. It is particularly important to understand the mechanism for seawater-induced desorption from calcite, because this mineral is the main component of limestone, a common bedrock of coastal aquifers globally.

We conducted batch experiments with calcite that had pre-adsorbed P, immersing it in freshwater, seawater, or a range of mixtures of the two, and measuring the P released to solution after equilibration. These empirical results provide input to geochemical software that was programmed to allow Ca²⁺-P ion pairs (CaPO₄^- and CaHPO₄⁰) to adsorb to the mineral surface at positively charged calcium sites in competition with common seawater ligands (CO₃^2-, SO₄^2-, and H₂O). This allowed us to compare the viability of possible surface complexation reactions in simulating our laboratory results, and calibrate association constants for plausible reactions.

Based on the surface complexation reactions that could successfully simulate our empirical observations of saltwater-induced P-desorption from calcite, we identified a “push” and “pull” mechanism. In our model, P is “pulled” from the calcite surface due to the high concentrations of dissolved Mg²⁺ in seawater, which strongly scavenges surface P to form aqueous Mg²⁺-P ion pairs (MgHPO₄⁰ and particularly MgPO₄^-). To a lesser extent, P may be “pushed” from the calcite surface due to competition by seawater CO₃^2-. Although SO₄^2- has been suggested as a possibly important competitor for P at the mineral surface, our model provided no support for this mechanism.

This study is the first to successfully model seawater-induced P desorption from any mineral. Our study provides association constants for surface complexation reactions which can be used in future modeling of phosphorus release caused by saltwater intrusion into coastal carbonate aquifers.