GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 250-5
Presentation Time: 9:00 AM-6:30 PM


SMITH, Richard L.1, REPERT, Deborah A.2, STOLIKER, Deborah L.3, KENT, Douglas B.3, HYUN, Sung Pil4, SONG, Bonkeun5, LEBLANC, Denis R.6, MCCOBB, Timothy D.7 and BOHLKE, J.K.8, (1)U.S. Geological Survey, 3215 Marine St, Boulder, CO 80303, (2)U.S. Geological Survey, 3215 Marine St., Boulder, CO 80303, (3)U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, (4)Korea Institute of Geoscience and Mineral Resources, 124 Gwahang-no, Yuseong-gu, Daejeon, 34132, Korea, Republic of (South), (5)Virginia Institute of Marine Science, College of William and Mary, P.O. Box 1346, Gloucester Point, VA 23062, (6)U.S. Geological Survey, 10 Bearfoot Rd., Northborough, MA 01532, (7)U.S. Geological Survey, 10 Bearfoot Road, Northborough, MA 01532, (8)U.S. Geological Survey, 431 National Center, 12201 Sunrise Valley Dr, Reston, VA 20192,

Groundwater discharge delivering anthropogenic N can have a profound impact on whole-lake nutrient budgets. A field study was conducted in a groundwater flow-through lake on Cape Cod which receives discharge of N-contaminated groundwater. Biogeochemical processes influencing the fate and transport of contaminant N species were examined seasonally with changes in temperature, lake level, and groundwater geochemical gradients. Sediment incubations were conducted to quantify potential rates of denitrification, anaerobic ammonium oxidation (anammox), and nitrate uptake on a relative basis at sites along transects where groundwater chemistry below the lake progressed from oxic to suboxic to anoxic with increasing nitrate concentrations (10-75 µM) and corresponding gradients in nitrite and nitrous oxide. Temporal shifts in lake level and groundwater geochemical gradients suggest that sediments from the same location experience many, if not all, of these chemical conditions throughout the year. Denitrification and nitrate uptake potential were 10-100 fold higher in surface sediments (0-5 cm) than in deeper sediments (5-30 cm) and correlated with sediment carbon content, but not necessarily with nitrate discharge. Functional genes coding for N2O reductase (nosZ) in denitrification and hydrazine oxidoreductase (hzo) in anammox were present in sediments at all sites, though incubations spiked with 15N labeled tracer indicated that N2 production was dominated by anammox in deeper sediments while denitrification dominated in shallow sediments. Porewater depth profiles of nitrate concentration and stable isotope fractionation indicated active nitrate consumption and dilution in the shallower sediments. Vertical porewater velocity, measured with seepage meters and conservative tracers injected into the sediment porewater, was 4-5 cm hr-1. Thus, while nitrate removal rates in the more carbon-rich shallow sediments were relatively high, the short residence time allowed only a small fraction of the contaminant nitrate to be removed before discharge into the lake.