Nitrogen removal and production within transient storage zones in a freshwater tidal environment, Delaware, U.S.A.

Abstract:

Groundwater and surface water flow through riparian aquifer soils and streambed sediment provides opportunities for microbial communities to transform water chemistry, particularly for nitrogen (N). The efficiency of NO₃^- removal is generally thought to decrease from headwaters to coasts but is poorly constrained near coasts, particularly within tidal freshwater zones (TFZs). High resolution geochemical sampling of reactive and conservative species in White Clay Creek, reveals hotspots of nitrogen removal and production within the stream bed and riparian aquifer. Nitrate removal was greatest in streambed sediments with removal rates decreasing with average stream depth. In addition, nitrate removal occurred within the riparian aquifer at the well 2 m from the stream bank. In contrast, nitrate production was highest in the riparian aquifer (4-13 m from the stream channel). Removal and production are related to the ratio of dissolved organic carbon to nitrate in the pore water, with production occurring when ratios are less than 7 and removal or uptake dominating at ratios greater than 7. Furthermore, nitrogen processes vary more in space than time (i.e. over the tidal cycle); suggesting that nitrogen cycling is driven in large part by substrate chemistry.