『Abstract
Substantial horizontal solute transport has been demonstrated
to occur in the capillary fringe (CF) above a flowing ground water,
yet the importance of the CF for solute movement has generally
been ignored. This study was conducted to evaluate the fate and
horizontal transport of surface-applied nitrate (NO3-)
in the CF under simulated hydrologic conditions that varied flow
rates. Two soils of different organic carbon content were packed
in separate 240-cm long, 60-cm high and 25-cm thick flow cells.
A simulated water table (WT) was established at 20 cm above the
bottom of each flow cell and different pore-water velocities across
the flow cell were simulated while a solution containing NO3- and bromide (Br-) was
continuously applied over a small area on the surface of the soil
in the flow cell. Soil solution samples were collected from two
depths below the WT and two depths within the CF above the WT
at four locations along the flow cell. Subsurface horizontal transport
of surface-applied NO3- tended
to occur exclusively in the CF as the pore-water velocity was
increased. In the flow cell with soil having a small amount of
organic carbon (0.3 g kg-1), normalized concentration
of NO3- and Br- remained
very comparable at all monitoring locations above and below the
WT. Nitrate loss via denitrification in this case was not observed
as conditions were oxidizing. In flow cells with soils having
an organic carbon content of 35 g kg-1, some Br-
was detected below the WT while NO3-
was essentially absent. Conditions below the WT favored NO3- loss via denitrification as reflected
by very low redox potentials (<250 mV). These results suggest
that collection of samples from the CF should be considered when
monitoring subsurface fate and transport of surface-applied NO3- in locations with laterally moving
shallow ground water.
Keywords: Nitrate transport; Denitrification; Redox potential;
Nitrate pollution monitoring; Capillary fringe』
1. Introduction
2. Materials and methods
2.1. Experimental set-up, soil and instrumentation
2.2. Experimental simulations
2.3. Pore-water velocities
3. Results and discussion
3.1. Soil and solution properties
3.2. Nitrate, bromide and reduction potential
3.2.1. Medium sand-packed flow cell
3.2.2. Leon soil-packed flow cell
4. Summary and conclusions
References