wAbstract
@It is commonly assumed that nitrate (NO3-)
and other anions entering the soil move downward through the vadose
zone, and then move horizontally in the groundwater. Recent laboratory
studies, however, indicate that water movement and transport of
pollutants can also take place in the capillary fringe (CF) above
the water table (WT). This field study evaluated the fate of NO3- in the CF and shallow groundwater
SGW) for a sandy soil (Aeric Alaquod) with shallow water table.
Ten L of a solution containing approximately 18 mmol L-1
nitrate [2.66 g L-1 Mg(NO3)2] and 77 mmol L-1 bromide (9.12 g
L-1 KBr) were applied to the soil above the CF. The
movement of both NO3- and Br-
was monitored for 84 days by using tension lysimeters installed
at depths between 45 and 105 cm at radial distances of 20, 60,
120, 220 and 320 cm from the application point. Nitrate and Br-
plumes that entered the CF from the unsaturated zone moved horizontally
in the CF until both species were partially carried into the groundwater
by the fluctuating WT following rain events. Normalized concentrations
of NO3-N and Br- remained
comparable as they moved horizontally in the CF up to 320 cm from
the tracer application spot. However, below the WT the detected
normalized concentration of Br- was higher than that
for NO3- indicating nitrate loss,
perhaps due to denitrification. When monitoring subsurface NO3-, solely relying on collection of
groundwater samples may lead to an underestimation of the extent
of NO3- contamination and transport
in the subsurface.
Keywords: Capillary fringe; Nitrate transport; Denitrification;
Groundwater monitoringx
1.Introduction
2. Materials and methods
@2.1. Installation of samplers and wells
@2.2. Monitoring of solute transport
3. Results and discussion
@3.1. Soil and site characteristics
@3.2. Persistence of nitrate in the CF and SGW
@3.3. Transport of nitrate in the CF
4. Summary and conclusions
Acknowledgements
References