Effects of organic carbon and subsurface dams on saltwater intrusion and nitrate pollution in sandy coastal aquifers

This study explores the impact of a novel approach on the levels of SWI (saltwater intrusion) and NO₃⁻ (nitrate) contamination. Some numerical simulations were conducted utilizing a coupled model that incorporates variably saturation and density, as well as convection diffusion reaction within a sandy coastal aquifer. We verified the reliability of the model for SWI based on comparison lab experiments and for chemical reactions based on a comparison of previous in situ observations. Cutoff walls and subsurface dams cannot simultaneously control SWI and reduce NO₃⁻ contamination. A novel approach that combines subsurface dams and permeable CH₂O (organic carbon) walls (PC-Wall) is proposed. Subsurface dams are utilized to prevent SWI, while PC-Walls are employed to mitigate NO₃⁻ pollution. Results demonstrate that the construction of a PC-Wall with a concentration of 1.0 mM facilitated a transition from nitrification (Ni)-dominated to denitrification (Dn)-dominated. An increase in CH₂O concentration to 1.0 mM caused a significant 1942.5 % rise in m_Dn (the mass of NO₃⁻ removed through Dn). Increment of the distance between the PC-Wall and the ocean from 35 to 45 m could result in a 103.7 % m_Dn increase and reduce m_N (the compound mass of NO₃⁻ remaining in the aquifer) by 11.7 %. The study offers a detailed comprehension of the intricate hydrodynamics of SWI and NO₃⁻ pollution. In addition, it provides design guidance for engineering to mitigate contamination by NO₃⁻ and controlling SWI, thus fostering the management of groundwater quality.