Seasonal and spatial variation in the location and reactivity of a nitrate-contaminated groundwater discharge zone in a lakebed

Journal of Geophysical Research: Biogeosciences
By: , and 

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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 groundwater-lake interface are complex and difficult to resolve. In this study, hydrograph variations in a groundwater flow-through lake altered discharge patterns of a wastewater-derived, groundwater contaminant plume, thereby affecting biogeochemical processes controlling N transport. Groundwater geochemistry 15 cm under the lakebed along transects perpendicular to shore varied from oxic to anoxic with increasing nitrate concentrations (10-75 M) and corresponding gradients in nitrite and nitrous oxide. Porewater depth profiles of nitrate concentrations and stable isotope compositions largely reflected upgradient groundwater N sources and N-cycle processes, with minor additional nitrate reduction in the shallowest lakebed sediments. Potential denitrification rates determined in laboratory microcosms were 10-100 fold higher in near-surface sediments (0-5 cm) than in deeper sediments (5-30 cm) and were correlated with sediment carbon content and abundance of denitrification genes (nirS, nosZI, and nosZII). Potential anammox-driven N2 production was highest in deeper anoxic sediments. Injection of bromide and nitrite in the lake sediments indicated a vertical porewater velocity of 4-5 cm hr-1, with highest nitrite consumption rates above 10 cm. However, short residence times in the shallow sediments allowed only a small fraction of the contaminant nitrate to be removed before discharging into the lake. Results demonstrate the importance of resolving local versus upgradient biogeochemical processes affecting contaminant distribution in discharge areas, and transient migration of local gradients and processes in response to changing lake levels and groundwater flow paths.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Seasonal and spatial variation in the location and reactivity of a nitrate-contaminated groundwater discharge zone in a lakebed
Series title Journal of Geophysical Research: Biogeosciences
DOI 10.1029/2018JG004635
Volume 124
Issue 7
Year Published 2019
Language English
Publisher American Geophysical Union
Contributing office(s) New England Water Science Center, WMA - Earth System Processes Division
Description 22 p.
First page 2186
Last page 2207