Seasonal and spatial variation in the location and reactivity of a nitrate-contaminated groundwater discharge zone in a lakebed
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.
Citation Information
| Publication Year | 2019 |
|---|---|
| Title | Seasonal and spatial variation in the location and reactivity of a nitrate-contaminated groundwater discharge zone in a lakebed |
| DOI | 10.1029/2018JG004635 |
| Authors | Richard L. Smith, Deborah A. Repert, Deborah Stoliker, Douglas B. Kent, Bongkeun Song, Denis R. LeBlanc, Timothy McCobb, John K. Böhlke, Sung Pil Hyun, Hee Sun Moon |
| Publication Type | Article |
| Publication Subtype | Journal Article |
| Series Title | Journal of Geophysical Research: Biogeosciences |
| Index ID | 70206016 |
| Record Source | USGS Publications Warehouse |
| USGS Organization | New England Water Science Center; WMA - Earth System Processes Division |
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Emeritus ScientistEmeritus ScientistEmailPhone