Seasonal and Spatial Variation in the Location and Reactivity of a Nitrate‐Contaminated Groundwater Discharge Zone in a Lakebed. Issue 7 (19th July 2019)

• Record Type: Journal Article
• Authors: Smith, Richard L.; Repert, Deborah A.; Stoliker, Deborah L.; Kent, Douglas B.; Song, Bongkeun; LeBlanc, Denis R.; McCobb, Timothy D.; Böhlke, J. K.; Hyun, Sung Pil; Moon, Hee Sun
• Is Part Of: Journal of geophysical research. Volume 124:Issue 7(2019)
• Page Start: 2186
• Page End: 2207
• Publication Date: 2019-07-19
• Subjects: groundwater -- nitrate -- lakes -- denitrification; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2018JG004635
• Languages: English
• ISSNs: 2169-8953

Abstract

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. In this study, seasonal water‐level variations in a groundwater flow‐through lake altered discharge patterns of a wastewater‐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 (10–75 μM) 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 near‐surface lakebed sediments. Potential denitrification rates determined in laboratory microcosms were 10–100 times 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 ( nir S, nos ZI, and nos ZII). Potential anammox‐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 (4–5 cm hr −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. Plain Language Summary: This study examined what happens to nitrate, a primary wastewater contaminant in groundwater, as contaminated groundwater flows upward into the bottom of a freshwater lake. Nitrate can serve as a fertilizer within the lake, causing a decrease in the quality of the lake water and potentially contributing to the growth of harmful algae. Groundwater nitrate discharged into the lake primarily near the lake shore, but the location of the discharge moved as the lake level rose and fell. Only a fraction of the nitrate passing through the sediments was removed before the groundwater exited into the lake because rapid flow through the lake‐bottom sediments limited the time for biogeochemical nitrate removal to occur. Key Points: Changing lake levels control the location of wastewater‐contaminated groundwater discharging to a freshwater glacial kettle lake Lakebed pore water geochemistry at the discharge location reflects N cycle processes in upgradient groundwater Short pore water residence time in more carbon‐rich surficial sediments limits nitrate removal prior to discharge into the lake