Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): Field data and reactive transport modeling

Abstract

A two-dimensional (2D) reactive transport model is used to investigate the controls on nutrient ( NO 3 - , NH 4 + , PO 4) dynamics in a coastal aquifer. The model couples density-dependent flow to a reaction network which includes oxic degradation of organic matter, denitrification, iron oxide reduction, nitrification, Fe 2+ oxidation and sorption of PO 4 onto iron oxides. Porewater measurements from a well transect at Waquoit Bay, MA, USA indicate the presence of a reducing plume with high Fe 2+, NH 4 + , DOC (dissolved organic carbon) and PO 4 concentrations overlying a more oxidizing NO 3 - -rich plume. These two plumes travel nearly conservatively until they start to overlap in the intertidal coastal sediments prior to discharge into the bay. In this zone, the aeration of the surface beach sediments drives nitrification and allows the precipitation of iron oxide, which leads to the removal of PO 4 through sorption. Model simulations suggest that removal of NO 3 - through denitrification is inhibited by the limited overlap between the two freshwater plumes, as well as by the refractory nature of terrestrial DOC. Submarine groundwater discharge is a significant source of NO 3 - to the bay.