Modeling Influence of Sediment Heterogeneity on Nutrient Cycling in Streambeds

Rivers and their hyporheic zones play an important role in nutrient cycling. The fate of dissolved inorganic nitrogen is governed by reactions that occur in the water column and streambed sediments. Sediments are heterogeneous both in term of physical (e.g., hydraulic conductivity) and chemical (e.g., organic carbon content) properties, which influence water residence times and biogeochemical reactions. Yet few modeling studies have explored the effects of both physical and chemical heterogeneity on nutrient transport in the hyporheic zone. In this study, we simulated hyporheic exchange in physically and chemically heterogeneous sediments with binary distributions of sand and silt in a low-gradient meandering river. We analyzed the impact of different silt/sand patterns on dissolved organic carbon, oxygen, nitrate, and ammonium. Our results show that streambeds with a higher volume proportion of silt exhibit lower hyporheic exchange rates but more efficient nitrate removal along flow paths compared to predominantly sandy streambeds. The implication is that hyporheic zones with a mixture of inorganic sands and organic silts have a high capacity to remove nitrate, despite their moderate permeabilities. Plain Language Summary With the advent of intensive cultivation and livestock farms, nutrient contamination has increased in streams. Microbial communities in streambeds reduce nitrate, effectively removing it from streams. However, quantifying this removal is difficult because it is influenced by many spatially variable factors, among which sediment type plays an important role. Streambeds can contain both sandy and silty sediments distributed in complex patterns. The present study employs computer simulations to analyze how nitrate is removed in a mixture of different streambed sediments.