『Abstract
In order to separate the effects of reaction from those of transport
on vertical porewater concentration profiles of nitrate at an
intertidal groundwater seepage site (Ria Formosa, Portugal), a
free-boundary solution of an Advection-Dispersion-Reaction (ADR)
model was used to describe the shape of NO3-
concentration profiles collected in situ. The model includes three
sequential reaction layers, postulated with basis on the local
distribution of the benthic organic C:N ratio and major identifiable
changes in concentration gradients with depth. The advective nature
of the system was used to propose a mass balance simplification
to the constitutive equations permitting a free-boundary solution,
which in turn allowed prediction of sediment-water fluxes of NO3-. Sensitivity analysis confirmed
that in similarly advective-dominated environments, both the porewater
concentration distribution and the interfacial fluxes are strongly
dependant on seepage rate and benthic reactivity. The model fitted
the measured profiles with high correlation (usually higher than
90％), and model-derived sediment-water NO3-
fluxes were in good agreement to fluxes measured in situ with
Lee-type seepage meters (0.9948 slope, R2 = 0.8672,
n = 8). Nitrate oxidation and reduction rates extracted from model
fits to the data (10-2-100 mmol m-2
h-1) agreed with literature values. Because dispersive
effects are not included in direct mass balances based on the
porewater concentrations, the model presented here increases the
accuracy of apparent reaction rate estimates and geochemical zonation
at Submarine Groundwater Discharge (SGD) sites. The results establish
the importance of sandy sediments as reactive interfaces, able
to modulate mass transfer of continental-derived contaminants
into coastal ecosystems. We suggest that tools such as the one
described here might be used to advantage in preparing further
experimental studies to elucidate how benthic reactivity affects
nitrate distribution and fluxes in sediments affected by SGD.
Keywords: Beach; Diagenesis; Discharge; Groundwater; Nitrate;
Modeling; Sand; SGD』
List of symbols
Introduction
Methods
Site description and sample collection
Modeling approach
Conceptual model
Model formulation
General formulation and solution
Free-boundary solution
Results
Range of applicability of the free-boundary solution
Sensitivity analysis
Model description on field data
Model input parameters
Fit results
Discussion
Conclusions
Acknowledgments
References