Seawater Intrusion Inhibits Nitrate Removal in Tidal Marsh Aquifers.

Tidal freshwater marshes are threatened by seawater intrusion globally due to freshwater discharge reduction and sea‐level rise. However, terrestrial nitrate (NO3−) transport responding to seawater intrusion remains poorly understood in tidal marshes. After validation against laboratory experiments, numerical simulations were conducted to analyze seawater intrusion effects on terrestrial NO3− transport and transformation in tidal marsh aquifers. Results reveal that seawater intrusion noticeably affects NO3− transport from the marsh aquifer to the tidal creek. Seawater intrusion results in an upper saline plume and a saltwater wedge within the aquifer, which markedly narrows the discharge outlet width of the NO3− plume and intensifies the peak NO3− flux across the creek bank. Consequently, both the NO3− removal efficiency and total nitrogen gas load to the creek decrease substantially after seawater intrusion. This is because the reduction of the transit time and the mixing zone width of the NO3− plume after seawater intrusion weakens denitrification. Sensitivity analyses indicate that the difference of the NO3− removal efficiency before and after seawater intrusion depends on soil properties. A larger unsaturated flow effect, saturated hydraulic conductivity or effective porosity leads to a greater difference of the NO3− removal efficiency before and after seawater intrusion. The predicted decrease of the NO3− removal efficiency after seawater intrusion is consistent with existing field data. Plain Language Summary: Tidal marshes, including salt marshes and freshwater marshes, are the last barrier to discharging land‐sourced solutes to the ocean. As a result of sea‐level rise and anthropogenic activities, seawater intrusion is a widespread threat to tidal freshwater marshes. Conversely, salt marshes can be freshened by inland freshwater during the flood season. Based on a combination of laboratory experiments and computer simulations, the present study explores how terrestrial nitrate (NO3−) transport responds to seawater intrusion in tidal marsh aquifers. The NO3− removal efficiency and total nitrogen gas load to the tidal creek are calculated to quantify seawater intrusion effects on terrestrial NO3− transport. Both the NO3− removal efficiency and total nitrogen gas load to the tidal creek decrease substantially after seawater intrusion. This study has wide applications while designing strategies for coastal water and marsh management. Key Points: Seawater intrusion plays an important role in terrestrial nitrate transportNitrate removal efficiency and total nitrogen gas load to the tidal creek decrease substantially after seawater intrusionThe difference of the nitrate removal efficiency before and after seawater intrusion depends on soil properties [ABSTRACT FROM AUTHOR]