Groundwater-borne nitrate removal within heterogeneous sediments under interactions between hydraulics, nutrients, and biofilms

Abstract

Excessive anthropogenic nitrate loads around coasts and rivers threaten human and ecosystem health. In this study, we established new numerical models to investigate how dynamic interactions between hydraulics, nutrients, and biofilms (HNB) drive the removal process of nitrate from upwelling groundwater within heterogeneous sediments. The results showed nitrate can be removed as much as 60 times more efficiently in heterogeneous sediments under interactions between HNB than in the typical heterogeneous and equivalent homogeneous conceptualizations with assuming constant microbial biomass. Driven by the interactions between HNB, aquatic sediments are more microbially active to mediate biogeochemical cycling. Specifically, progressively increasing biomass with biofilm growth promotes oxygen consumption, resulting in enhanced denitrification in heterogeneous sediments. Meanwhile, not only can the hotspots of denitrification develop in low-permeability silt structures with rich organic matter (OM), but they can also form in high-permeability and low-OM sand structures as well. In contrast, by assuming constant microbial biomass, the hotspots can only form in silt structures for heterogeneous sediments and cannot exist for equivalent homogeneous sediments. These findings are significantly different from the classical understanding of heterogeneous aquatic sediments, in which coarse structures inhibit efficient nitrate removal and mainly function as delivery channels of nutrients, whereas only fine structures host hotspots of denitrification. Significant underestimation of groundwater-borne nitrate removal can be induced by assuming constant microbial biomass. Overall, this study is useful for understanding nitrate fate within aquatic sediments, and benefits for coastal and riverine restorations and watershed nitrogen management.