Denitrification of anthropogenic nitrogen in groundwater : measurement and modeling using stable isotopic and mass balance approaches

Abstract

Denitrification is a microaerophilic, microbially-mediated process, by which nitrate is reduced to biologically-unavailable N2 gas; the reaction is generally coupled to the oxidation of organic carbon. We hypothesized that denitrification rates in groundwater in the Waquoit Bay watershed on Cape Cod, USA, were controlled by both nitrate and dissolved organic carbon (DOC) concentrations, and that groundwater DOC concentrations were inversely related to the thickness of the vadose (unsaturated) zone through which recharge occurred. We found that the deeper the vadose zone, the lower the concentration of DOC in groundwater near the water table; similarly, DOC concentrations decreased with increasing depth below the water table, suggesting quite active biogeochemical processing in these boundary environments. We used stable isotope and mass balance approaches to estimate denitrification rates in groundwater at two forested field sites and in a septic system plume. These sites provided a large range of groundwater nitrate and dissolved organic carbon (DOC) concentrations. At all sites, denitrification rates increased with increasing nitrate concentration. First order denitrification rate constants with respect to nitrate were highest where groundwater DOC concentrations were highest: k = 2.8 y-1 in the septic plume (~ 26 mf C 1-1), k = 1.6 y-1 at South Cape Beach (DOC = 0.8 to 23.4 mg C-1), and k = 0.25 y-1 at Crane Wildlife (0.1 to 1.9 mg C 1-), suggesting that, independent of nitrate, DOC concentrations exert significant control on denitrification rates. A simulation of N losses along groundwater flowpaths suggests that a saturating kinetics expression with respect to both nitrate and DOC best predicts nitrate concentrations measured at downgradient well ports (R2 = 0.96 for [NO3]model vs. [NC 3 imeas). In contrast, a saturating kinetics expression with respect to nitrate only, often overpredicts nitrate losses along groundwater flowpaths, particularly where DOC concentration are low, further confirming that DOC concentrations are an important control on groundwater denitrification rates. The magnitude of a nitrate source, its travel distance to shore, and the DOC concentration in groundwater are useful predictors of N downgradient. These relationships can help in designing strategies to control anthropogenic nitrogen loading.