Role of the aeration soil zone on the groundwater flow contamination

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Role of the aeration soil zone on the groundwater
flow contamination

• Zdravko Diankov, Olga Nitcheva
• Institute of water problems to the Bulgarian
  academy of sciences
• Sofia, BULGARIA

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Introduction

• The investigation consists in a registration of
  the soil water distribution in the unsaturated
  soil zone
• under development of
• grass vegetation and
• in applying numerical simulation of the processes
  for the same conditions as in the field.
• Soil samples were collected periodically by hand
  drilling at each 10 cm to the groundwater level
  from two points (EAST and WEST) of the
  investigated area (situated in Sofia kettle).

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Field investigations
The soil water content ? cm3/g ( ?man0,24
cm3/g) was obtained by gravimetric method. The
measured soil moisture data for a given date
(example for 20 March 2003) could be present in
graphical way it is the profiles of the
moisture distribution.
The volumetric soil moisture ? (cm3/ cm3) in the
soil layers was obtained according to the
relation ? ?.? where ? (g/cm3) is the soil dry
bulk density (?1,5 g/cm3, ?max0,36)
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During the investigation 08.05.2002 28.08.2004
were created profiles of the gravimetric soil
moisture for 12 dates from the test plots WEST
and EAST. The data have been analyzed together
with the having influence factors on soil
moisture status.
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The groundwater level changes observed in EAST
and WEST plots are presented on this figure.
Comparing the two lines it is remarked that the
depths of the groundwater levels are in
synchronous and away each other with 0.75 m..
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The profiles of the soil moisture ? for
observations in 2003 are seen in the following
figures Similar are the graphs for the
observations in 2002 and 2004

• Two essential factors are pointed out
• The capillary zone (capillary fringe - CF)
  phenomenon is very clearly manifested in the soil
  layers over the zone of saturation and
• 2. The volumetric soil moisture ? on the depth
  of groundwater exhibits lower values than the
  maximal ones (?max). This indicates that under
  the zone of the measured groundwater level a zone
  of incomplete saturation is formed too.

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On the direction of soil water movement

• Let consider the soil moisture status in two
  layers (1) and (2), with the difference in the
  ordinates ?zz 2-z 1, they have different soil
  water content (?2) and(?1). The velocity of water
  flow from point (1) to point (2) related to
  equation of Richards is derived from the
  differential expression
• where ?(?) is soil water retention values.
• On the scheme are presented tree possible values
  of the hydraulic potential in point (2) related
  to point (1). It is shown
• a) for ?2 ?1?z, v0 the velocity is zero
• b) for ?2gt ?1?z, vgt0, the velocity has the
  direction of increasing elevation of soil layers.
• c) for ?2 lt ?1?z, vlt0, the velocity has the
  direction of decreasing elevation of soil
  layers).

The conclusion may be drawn that the direction of
soil water flow in the vadoze zone can be
determined from the type of the graphs ?(z), or
?(H-t).
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Summarily about the soil moisture regime in the
observed area and period

• They were registered time periods when the moving
  of the water in aeration zone is in ascending
  direction. In such case the feeding of the
  capillary fringe comes from the saturated zone.
• They were registered time periods, after
  rainfalls, when the soil moisture reached to the
  maximal values, then the water motion went down
  and leaded to a water recharge of the saturated
  zone

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Numerical modeling
The well-known WAVE mathematical model and
software product was used for representing the
processes of water exchange in the vadoze zone A
part of the input parameters were those from the
field investigations precipitation, temperature
of the air, groundwater level kinetics, data for
soil mechanical composition and , moisture
retention ?(?) -ig.8. The other part of the input
data were mainly the daily development of
evapotranspiration (kind of plant), filtration
coefficients of schematized soil layers,
conductivity k(?) depending on volumetric
moisture content (fig. 9).
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The graphs in the following two figures show the
volumetric water content ? obtained by the
simulation procedure with the WAVE software for
the dates 16.05.2003 and 10.06.2003. The data
for the determined soil moisture profiles by the
field investigation, for the same days, are shown
as well.
The analogy between the experimentally obtained
and calculated profiles gives a good reason for
further iterative procedures by varying the input
parameter values to satisfactory correspondence
between the experimental (tested) and computation
results
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The graph in the following figure shows the water
exchange (recharge) between the unsaturated and
the saturated zone with the simulated intervals
of the upward and downward water flow formation
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Conclusion
The exhibited investigations show that the soil,
hydraulic, atmospheric and other conditions have
a marked influence on the processes of the water
and salt (fertilizer) moving in the aeration soil
zone. The intensity and the direction of the
water and fertilizer exchange between the
unsaturated soil zone and the groundwater flow
can be different in the different time periods of
the year. The prognostication for a groundwater
pollution as a result of fertilizing of intensive
land use calls for methodical experimental
(field) and simulation investigations. In this
area of problems are in developing now
investigations in the Institute of Water Problems
to the Bulgarian Academy of Sciences - Sofia