Seawater Intrusion Inhibits Nitrate Removal in Tidal Marsh Aquifers

Abstract

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.