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Seasonal and Spatial Variation in the Location and Reactivity of a Nitrateā€Contaminated Groundwater Discharge Zone in a Lakebed

Smith, RL
; 
Repert, DA
; 
Stoliker, DL
; 
Kent, DB
; 
Song, BK
; 
al, et
Abstract
Groundwater discharge delivering anthropogenic N from surrounding watersheds can impact lake nutrient budgets. However, upgradient groundwater processes and changing dynamics in N biogeochemistry at the groundwaterā€lake interface are complex. In this study, seasonal waterā€level variations in a groundwater flowā€through lake altered discharge patterns of a wastewaterā€derived groundwater contaminant plume, thereby affecting biogeochemical processes controlling N transport. Pore water collected 15 cm under the lakebed along transects perpendicular to shore varied from oxic to anoxic with increasing nitrate concentrations (10ā€“75 Ī¼M) and corresponding gradients in nitrite and nitrous oxide. Pore water depth profiles of nitrate concentrations and stable isotopic compositions largely reflected upgradient groundwater N sources and N cycle processes, with minor additional nitrate reduction in the nearā€surface lakebed sediments. Potential denitrification rates determined in laboratory microcosms were 10ā€“100 times higher in nearā€surface sediments (0ā€“5 cm) than in deeper sediments (5ā€“30 cm) and were correlated with sediment carbon content and abundance of denitrification genes (nirS, nosZI, and nosZII). Potential anammoxā€driven N2 production was detectable in deeper anoxic sediments. Injection of bromide and nitrite in the lake sediments showed that the highest net nitrite consumption rates were within the top 10 cm. However, short transit times owing to rapid upward pore water velocities (4ā€“5 cm hrāˆ’1) limited removal of the contaminant nitrate transiting through the sediments. Results demonstrate that local hydrologic and biogeochemical processes at the point of discharge affect the distribution and discharge rate of N through lakebed sediments, but processes in the upgradient groundwater can be more important for affecting N speciation and concentration.
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2019-07-01
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Biological Sciences Peer-Reviewed Articles, groundwater, denitrification, lakes
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Virginia Institute of Marine Science
DOI
https://doi.org/10.1029/2018JG004635
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https://scholarworks.wm.edu/handle/internal/17629
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