Joint Assessment of the Behavior of Nitrate and Saltwater Intrusion Within Negative Hydraulic Barrier Setups

Abstract

Nitrate is a common groundwater contaminant resulting from excessive agricultural fertilizer use, especially in coastal regions. Negative hydraulic barriers (NHBs) are widely used to mitigate seawater intrusion by altering groundwater behavior and pumping saline groundwater, but their impact on nitrate pollution remains unclear. This study investigated the mechanisms and impacts of NHBs on nitrate contamination using a 3D variable-density model coupled with multi-species transport simulations. We found that NHBs intensified nitrate accumulation near pollution sources, leading to the formation of high-concentration zones. Nitrate removal was mainly through submarine groundwater discharge (SGD, accounting for 62.77%) before NHBs were introduced, but SGD efficiency dropped significantly (to 29.03%) after NHBs installation, with NHBs contributing 21.17%. Increasing NHBs pumping rates enhanced salt mass reduction and shifted nitrate removal toward NHBs-based processes. Additionally, LH-type aquifers, characterized by a low hydraulic conductivity (K) layer at the top, exacerbated nitrate retention and denitrification, while HL-type aquifers, with a low K layer at the bottom, promoted horizontal diffusion and improved SGD. Furthermore, NHBs consistently intensified nitrate removal across different seawater boundary conditions. Our investigation highlights the importance of incorporating the effects of NHBs in the assessment of coastal nitrate pollution to avoid negative consequences associated with the barriers.