Premium
Coupled transport and reaction kinetics control the nitrate source‐sink function of hyporheic zones
Author(s) -
Zarnetske Jay P.,
Haggerty Roy,
Wondzell Steven M.,
Bokil Vrushali A.,
GonzálezPinzón Ricardo
Publication year - 2012
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2012wr011894
Subject(s) - biogeochemical cycle , biogeochemistry , denitrification , sink (geography) , nitrification , environmental science , environmental chemistry , hyporheic zone , nitrate , chemistry , hydrology (agriculture) , nitrogen , environmental engineering , surface water , geology , cartography , geotechnical engineering , organic chemistry , geography
The fate of biologically available nitrogen (N) and carbon (C) in stream ecosystems is controlled by the coupling of physical transport and biogeochemical reaction kinetics. However, determining the relative role of physical and biogeochemical controls at different temporal and spatial scales is difficult. The hyporheic zone (HZ), where groundwater–stream water mix, can be an important location controlling N and C transformations because it creates strong gradients in both the physical and biogeochemical conditions that control redox biogeochemistry. We evaluated the coupling of physical transport and biogeochemical redox reactions by linking an advection, dispersion, and residence time model with a multiple Monod kinetics model simulating the concentrations of oxygen (O 2 ), ammonium (NH 4 ), nitrate (NO 3 ), and dissolved organic carbon (DOC). We used global Monte Carlo sensitivity analyses with a nondimensional form of the model to examine coupled nitrification‐denitrification dynamics across many scales of transport and reaction conditions. Results demonstrated that the residence time of water in the HZ and the uptake rate of O 2 from either respiration and/or nitrification determined whether the HZ was a source or a sink of NO 3 to the stream. We further show that whether the HZ is a net NO 3 source or net NO 3 sink is determined by the ratio of the characteristic transport time to the characteristic reaction time of O 2 (i.e., the Damköhler number, Da O2 ), where HZs with Da O2 < 1 will be net nitrification environments and HZs with Da O2 ≪ 1 will be net denitrification environments. Our coupling of the hydrologic and biogeochemical limitations of N transformations across different temporal and spatial scales within the HZ allows us to explain the widely contrasting results of previous investigations of HZ N dynamics which variously identify the HZ as either a net source or sink of NO 3 . Our model results suggest that only estimates of residence times and O 2 uptake rates are necessary to predict this nitrification‐denitrification threshold and, ultimately, whether a HZ will be either a net source or sink of NO 3 .