J. F. Beraud et af. : New Tools for Diagnosing Underground PoIIution
47
Although daily infiltration of contaminants can be small, an accumulation over a
long period of time can be quite significant. Moreover the subsurface environment
is conservative, and flow speeds of groundwater are very low, so that pollution can
be present for a long time in the subsoil.
4
BEHAVIOR OF POLLUTANTS IN THE SUBSURFACE
4.1 The underground environment
Rocks and soils are made up of small fragments and grains separated by pore
spaces, creating routes in which fluids and gases can circulate (Fig. 1).
Moving downwards from the soil surface, the following structures can be
distinguished:
• the non-saturated zone, where the pores are only partially filled with
water, which has a more or less vertical transport characteristic under the
force of gravity. Pore spaces which do not contain water permit the
circulation of gases.
• the saturated-zone, where all pore spaces are filled with water, and the
horizontal flow component is dominant. The boundary at which this
pressure is equal to the atmospheric pressure forms the free surface of
the aquifer. Above this level is the capillary fringe, with a height ranging
from some 12 cm. for coarse sands, to 2.50 m. for silts.
4.2 Pollutant formation
4.2.1 Petroleum products lighter than water
During its vertical migration through the unsaturated zone, the pollutant liquid
leaves in its wake soil which is permeated almost to the point of saturation (Fig. 2).
If the spill is big enough, the product as a phase will reach the capillary fringe,
then the watertable, and spread over it in a lense form.
4.2.2 Dense chlorinated solvents
The behavior in the unsaturated zone is almost the same as that in the petroleum
product zone, with development of vapor (Fig. 3).
A main difference is that, if the product reaches the water-table, it sinks down to
the bottom of the aquifer. The most important difficulty is that, contrary to the
watertable, the bottom of an aquifer is a very irregular surface. The solvents can
then be trapped in holes and be difficult to investigate.
47
Although daily infiltration of contaminants can be small, an accumulation over a
long period of time can be quite significant. Moreover the subsurface environment
is conservative, and flow speeds of groundwater are very low, so that pollution can
be present for a long time in the subsoil.
4
BEHAVIOR OF POLLUTANTS IN THE SUBSURFACE
4.1 The underground environment
Rocks and soils are made up of small fragments and grains separated by pore
spaces, creating routes in which fluids and gases can circulate (Fig. 1).
Moving downwards from the soil surface, the following structures can be
distinguished:
• the non-saturated zone, where the pores are only partially filled with
water, which has a more or less vertical transport characteristic under the
force of gravity. Pore spaces which do not contain water permit the
circulation of gases.
• the saturated-zone, where all pore spaces are filled with water, and the
horizontal flow component is dominant. The boundary at which this
pressure is equal to the atmospheric pressure forms the free surface of
the aquifer. Above this level is the capillary fringe, with a height ranging
from some 12 cm. for coarse sands, to 2.50 m. for silts.
4.2 Pollutant formation
4.2.1 Petroleum products lighter than water
During its vertical migration through the unsaturated zone, the pollutant liquid
leaves in its wake soil which is permeated almost to the point of saturation (Fig. 2).
If the spill is big enough, the product as a phase will reach the capillary fringe,
then the watertable, and spread over it in a lense form.
4.2.2 Dense chlorinated solvents
The behavior in the unsaturated zone is almost the same as that in the petroleum
product zone, with development of vapor (Fig. 3).
A main difference is that, if the product reaches the water-table, it sinks down to
the bottom of the aquifer. The most important difficulty is that, contrary to the
watertable, the bottom of an aquifer is a very irregular surface. The solvents can
then be trapped in holes and be difficult to investigate.