『Summary
Nutrient loss from altered wetlands to waterways may influence
the water quality of downstream water resources. To test potential
nutrient transport from a highly fractured agricultural field
of altered wetland to waterways, we computed water and solute
budgets in a large field (320 ha) experiment. The water level
was raised in a drainage canal for 28 days (raising stage) to
generate water flow to the field and then lowered (drainage stage)
to generate back flow to the canal. Differential sampling stations
(DISS) equipped with an Eh-pH measuring system were installed
to monitor and sample crack and matrix water separately. Water
flowing in cracks exhibited Eh＞250 mV and EC＜1.5 dS m-1
while in the matrix, the Eh was ＜100 mV and EC exceeded 3.0 dS
m-1. High similarity was found between crack water
measured in the DISS and water that drained directly into the
drainage canal. We proposed a conceptual model for water and solute
transport by a dual-domain system. Most of the water flow occurs
in the cracks and most of the solute reaches the cracks by advection
and diffusive flow mechanisms in the soil matrix. The conceptual
model is based on the assumption that the crack and matrix domains
are in chemical disequilibrium because of low transport rate from
the matrix to the cracks relative to water flow rate in the cracks.
Using this conceptual model, we calculated that during the drainage
stage, only 0.64 kg P, compared with 3700 kg of the non-sorptive
sulfate ion, were transported from the field to the canal. The
oxidized conditions in the crack walls lead to Fe-hydroxide precipitation.
These fresh precipitate amorphous iron hydroxides serve as an
adsorption sink to P and other potential adsorbates.
Keywords: Altered wetland; Phosphorus; Groundwater differentiation;
Preferential flow; Dual porosity; Nutrient transport』
Introduction
Materials and methods
Study area
Field experiment
Differential sampling stations
pH and Eh measuring system
Water sampling and analysis
Water and solute balance
Results
Water balance
Chemistry of waters in the drainage canal
Water differentiation across the experimental field
Crack-matrix water differentiation
Discussion
Concluding remarks
Acknowledgements
References