Heterogeneity of aquatic sediment significantly increases nitrogen removal

Abstract:

In recent decades, nitrate loads to rivers and coasts have increased dramatically and contributed to eutrophication and hypoxia in coastal waters. A major sink for nitrate in the environment is denitrification in aquatic sediments. Here, I show that nitrate removal rates are as much as 100 times more efficient in heterogeneous than equivalent homogeneous aquatic sediments. Numerical experiments quantify nitrate removal from groundwater discharging through shallow columns of heterogeneous aquatic sediment. The steady groundwater flow equation was coupled to the advection-dispersion-reaction equations for dissolved oxygen, nitrate, and dissolved organic carbon (DOC). Bimodal sediments composed of sand and clay were simulated using TPROGS for a wide range of clay fractions. Small (centimeter-scale) sedimentary structures were intended to represent infilled burrows or clay rip-up clasts. Clay structures were assigned a relatively high organic carbon content (2%). The local source of DOC fuels oxygen consumption in clay structures and promotes restricted zones of denitrification in otherwise aerobic sediments. As a result, redox transformations do not strictly depend on residence times but rather on distributions of organic carbon in aquatic sediments. Furthermore, bimodal sand and clay deposits are more efficient than either clean sand or homogeneous sandy-clay at removing nitrate. These results help explain observations of efficient nitrate removal in relatively sandy, oxygenated aquatic sediments when small organic-rich structures are present. The results also suggest that models of reactive transport in homogeneous sediment underestimate biogeochemical transformation rates relative to heterogeneous sediment. Similarly, laboratory-derived denitrification rates on homogenized cores are likely underestimated relative to intact cores.