Hot Spots and Hot Moments of Nitrogen in a Riparian Corridor. Issue 1 (16th January 2018)

• Record Type: Journal Article
• Authors: Dwivedi, Dipankar; Arora, Bhavna; Steefel, Carl I.; Dafflon, Baptiste; Versteeg, Roelof
• Is Part Of: Water resources research. Volume 54:Issue 1(2018)
• Page Start: 205
• Page End: 222
• Publication Date: 2018-01-16
• Subjects: hot spots -- nitrogen -- nitrate -- PFLOTRAN -- surface water‐groundwater interactions; Hydrology -- Periodicals; 333.91
• DOI: 10.1002/2017WR022346
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: We use 3‐D high‐resolution reactive transport modeling to investigate whether the spatial distribution of organic‐carbon‐rich and chemically reduced sediments located in the riparian zone and temporal variability in groundwater flow direction impact the formation and distribution of nitrogen hot spots (regions that exhibit higher reaction rates when compared to other locations nearby) and hot moments (times that exhibit high reaction rates as compared to longer intervening time periods) within the Rifle floodplain in Colorado. Groundwater flows primarily toward the Colorado River from the floodplain but changes direction at times of high river stage. The result is that oxic river water infiltrates the Rifle floodplain during these relatively short‐term events. Simulation results indicate that episodic rainfall in the summer season leads to the formation of nitrogen hot moments associated with Colorado River rise and resulting river infiltration into the floodplain. The results further demonstrate that the naturally reduced zones (NRZs) present in sediments of the Rifle floodplain have a higher potential for nitrate removal, approximately 70% greater than non‐NRZs for typical hydrological conditions. During river water infiltration, nitrate reduction capacity remains the same within the NRZs, however, these conditions impact non‐NRZs to a greater extent (approximately 95% less nitrate removal). Model simulations indicate chemolithoautotrophs are primarily responsible for the removal of nitrate in the Rifle floodplain. These nitrogen hot spots and hot moments are sustained by microbial respiration and the chemolithoautotrophic oxidation of reduced minerals in the riparian zone. Plain Language Summary: Riverine floodplains play a significant role in the cycling of nitrogen. Reliable predictions of nitrogen dynamics in the floodplain and its export to the river depend on accurate representation of hydrologic flow paths and biogeochemical processes in the subsurface. The objective of this study was to quantify the transport and distribution of nitrogen at a floodplain site in Rifle, CO, using a high‐resolution, 3‐D flow and reactive transport model. Groundwater flows primarily toward the Colorado River from the floodplain but changes direction occasionally. The simulations demonstrate that nitrogen hot spots are both flow‐related and microbially driven in the Rifle floodplain. Overall, 3‐D simulations were able to capture the significant spatial and temporal variability associated with nitrogen fluxes in the floodplain environment. Key Points: We investigate the formation and distribution of nitrogen hot spots and hot moments using a high‐resolution, 3‐D reactive transport model Results indicate that nitrogen hot spots occur because of complex fluid mixing, localized reduced zones, and biogeochemical variability Chemically reduced sediments of the Rifle floodplain have 70% greater potential for nitrate removal than nonreduced zones