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Nitrogen dynamics at the groundwater-surface water interface of a degraded urban stream (journal)
Citation:
MAYER, P. M., P. M. Groffman, E. A. Striz, AND S. S. Kaushal. Nitrogen dynamics at the groundwater-surface water interface of a degraded urban stream (journal). JOURNAL OF ENVIRONMENTAL QUALITY. American Society of Agronomy, MADISON, WI, 39(3):810-823, (2010).
Impact/Purpose:
improving the nitrogen removal process in ground water
Description:
Urbanization degrades stream ecosystems by altering hydrology and nutrient dynamics, yet relatively little effort has been devoted to understanding biogeochemistry of urban streams at the ground water-surface water interface. This zone may be especially important for nitrogen removal processes. We investigated the ground water ecosystem of Minebank Run, a geomorphically degraded urban stream near Baltimore, Maryland, USA in the Chesapeake Bay watershed. Our objectives were to identify the spatial and temporal extent of chemical, microbial, and hydrological factors that are known to influence denitrification rates. We observed predictable patterns among these factors at Minebank Run consistent with the conditions favorable for sustaining denitrification. First, nitrate (NO3¯) was lower in ground water where more dissolved organic carbon (DOC) was available. "Measurements of denitrification enzyme activity (DEA) confirmed that subsurface sediments at Minebank Run were actively dentrifying NO3¯ and that denitrification activity was highest where more organic carbon was available. Therefore, dentrification and subsequent removal of NO3¯ in ground water appeared to be limited by DOC availability. We also observed that ground water hydrology and biogeochemical parameters responded to stream discharge. When discharge decreased, water table elevations declined and so did NO3¯. Nitrate to chloride ratios suggested increased nitrogen processing in the subsurface related to the drop in ground water levels during drought. Conversely, high concentrations of ground water NO3¯ corresponded to high ground-water levels over time. Furthermore, spatial patterns of high ground water NO3¯ corresponded to regions of the stream with high oxidation-reduction potential (ORP) indicative of high ground water-surface water exchange. Our results indicated that stream flow patterns controlled stream bank and bed infiltration which dictated ground water levels. Declines in water levels likely increased subsurface mixing which led to favorable ORP conditions and increased DOC consumption that sustained NO3¯ removal via denitrification. Degraded urban streams are candidates for nutrient management because they retain the capacity to transform nitrogen. The ground water-surface water interface is an important target for nutrient management because this zone strongly influences nitrogen dynamics. Management efforts that increase DOC availability to denitrifiers, reduce stream flow velocities, and increase ground water residence time will likely improve the nitrogen removal capacity of urban stream channels.
URLs/Downloads:
NITROGEN DYNAMICS AT THE GROUNDWATER–SURFACE WATER INTERFACE OF A DEGRADED URBAN STREAM
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