『Abstract
A modeling study on fertilizer by-products fate and transport
was performed in an unconfined shallow aquifer equipped with a
grid of 13 piezometers. The field site was located in a former
agricultural field overlying a river paleochannel near Ferrara
(Northern Italy), cultivated with cereals rotation until 2004
and then converted to park. Piezometers were installed in June
2007 and were monitored until June 2009 via pressure transducer
data loggers to evaluate the temporal and spatial variation of
groundwater heads, while an onsite meteorological station provided
data for recharge rate calculations via unsaturated zone modeling.
The groundwater composition in June 2007 exhibited elevated nitrate
(NO3-) and chloride (Cl-)
concentrations due to fertilizer leaching from the top soil. The
spatial distribution of NO3- and
Cl- was heterogeneous and the concentration decreased
during the monitoring period, with NO3-
attenuation (below 10 mg/l) after 650 days. A transient groundwater
flow and contaminant transport model was calibrated versus observed
heads and NO3- and Cl-
concentrations. Cl- was used as environmental tracer
to quantify groundwater flow velocity and it was simulated as
a conservative species. NO3- was
treated as a reactive species and denitrification was simulated
with a first order degradation rate constant. Model calibration
gave a low denitrification rate (2.5 e-3 mg-NO3-/l/d) likely because of prevailing
oxic conditions and low concentration of dissolved organic carbon.
Scenario modeling was implemented with steady state and variable
flow time discretization to identify the mechanism of NO3- attenuation. It was shown that transient
piezometric conditions did not exert a strong control on NO3- clean up time, while transient recharge
rate did, because it is the main source of unpolluted water in
the domain.
Keywords: Groundwater modeling; NO3-;
Cl-; Unconfined aquifer; Groundwater pollution; Solute
transport』
1. Introduction
2. materials and methods
2.1. Field site
2.2. Analytical and field methods
2.3. Modeling
3. Results and discussion
3.1. Unsaturated zone characterization and recharge estimation
3.2. Transient groundwater flow model
3.3. Transient solute transport model
3.4. Comparison with steady state results
4. Conclusions
Acknowledgments
References