Reactive Transport Simulations of Groundwater-derived Nutrients in a Sandy Beach in Daya Bay, China

Submarine groundwater discharge (SGD) can be a significant terrestrial input of nutrients to the coastal ocean. The mixing between nearshore groundwater and seawater in coastal aquifers modifies the chemical composition of the water prior to discharge. Agricultural, aquaculture and leaky urban sewers may elevate the land-derived contaminates in the near-shore areas. The nutrient structure of Daya Bay has been strongly changed with the economic and urban development. In this study, the spatial distribution of nutrients (e.g. NO₃^-, NH₄⁺, PO₄^3-, SO₄^2-, S^2-), groundwater salinity and level were systematically investigated along an intertidal beach transect. Two-dimensional variable density and saturation, and nutrient reactive transport simulations were developed using the finite-element model MARUN. Tidal and meteorological data were also collected from local weather station to correct the model boundary.  Besides, surface air evaporation and precipitation were considered in this model to better match the field observations. The results showed that the distribution pattern of nutrients both of field observation and simulation was similar to that of salinity. For example, the concentration of NO₃^--N from the landward side towards the seaward side of the beach decreased and then increased, presenting an upper NO₃^--N plume, lower location NO₃^--N saltwater wedge and NO₃^--N discharge tube. It can approximately correspond to the upper salt plume, classical salt wedge and freshwater discharge tube. In stead of using flow velocity or simple attenuation constant to calculate the nutrient fluxes, this study used complex coupled solute reaction transport to improve the computational accuracy.