Aquatic Sciences Meeting, Albuquerque 2001

SS05 Groundwater at Aquatic Margins (Environmental Connections)

Date: Wednesday, February 14, 2001, Time: 12:15:00 PM

Location: Cimarron

Pabich, W, J, Environmental Defense, Boston, USA, wjpabich@mit.edu

Hemond, H, F, MIT, Cambridge, USA, hfhemond@mit.edu

Valiela, I, , Boston University Marine Program, Woods Hole, USA, valiela@bio.bu.edu

DENITRIFICATION RATES IN GROUNDWATER, WAQUOIT BAY WATERSHED, CAPE COD, MA: CONTROL BY NITRATE AND DOC CONCENTRATIONS

We used a stable isotope approach to account for nitrification and to estimate denitrification rates along groundwater flowpaths at two forested sites. The sites provided a range of groundwater nitrate concentrations (0 to 91 uM) derived from soil and from fertilizer, and two discrete dissolved organic carbon (DOC) regimes (less than 2 mg C/l, and 1 to 24 mg C/l). Denitrification rates (9.1 x 10-9 to 5.5 x 10-3 uM N/hr) increased with both increasing initial nitrate and DOC concentrations and could be described with a Michaelis-Menten relationship. We also assessed mass balance losses of N in a groundwater septic system plume; denitrification rates in this system ranged from 0.01 to 2.2 uM N/hr, consistent with the higher nitrate concentrations in the wastewater (137 to 4,396 uM). The magnitude of the nitrate source, its travel distance to shore, and the DOC concentration in groundwater determine the resulting N contribution downgradient. These relationships should be considered in designing strategies to control anthropogenic nitrogen loading.

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Aquatic Sciences Meeting, Albuquerque 2001

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