Environmental Engineering Reference
In-Depth Information
where: q = specific discharge (m/d), k = hydraulic conductivity (m/d), I = hydraulic
gradient
Pollutants transported by advection travel at the same rate as the average linear
velocity of the groundwater flow (Fetter 1994). The hydraulic conductivity, âkâ, is a
function of the properties of both - the fluid and the porous media through which it is
flowing. In its general form, Darcy's equation is valid for flows at a very low velocity
and through soils with isotropic structure (sands). A schematic representation of the
transport mechanisms of pollutants in groundwater is shown in Figure 9.2. It shows that
the shape, which the polluted flow would take after leaving the source, is conical and is
known as a pollution plume. After different time intervals the plume would change its
shape, covering larger parts of the aquifer but with reduced concentrations of pollutants
along the basic direction of the advective flow.
Dispersion represents the transport of pollutants in all directions, including the main
flow direction. Dispersion is caused by heterogeneities in the aquifer that result in
groundwater pollutants flowing through different pores at different rates and various flow
paths. This results in diffusion and mixing due to velocity variations in the flow of the
groundwater or the pollutant. We could differentiate between mechanical and
hydrodynamic dispersion. Mechanical dispersion is the mixing of the pollutants resulting
from the physical movement through the porous media. When a pollutant is diluted in the
groundwater, it follows the path of the main flow, which is called lateral dispersion
(Domenico & Schwartz 1997).
Due to the soil matrix heterogeneity in terms of soil particles size and distribution,
dispersion will occur and will distribute pollutants not only in the direction of flow but in
other directions as well (Fig.9.2).
Molecular diffusion is the movement of both ionic and molecular species dissolved in
water, from a region of higher concentration to a region of lower concentration. Diffusion
could be a major transportation process in less permeable media and where the pollutant
has a higher concentration than adjacent areas (Fetter 1994). For an one-dimensional
analysis under steady state conditions, Fick's first law governs diffusion, where the mass
flux of solute is proportional to the concentration gradient. In practice, diffusion and
mechanical dispersion cannot be separated and their combined effect is termed
hydrodynamic dispersion.
Retardation of pollutants depends on the nature of the pollutant activity in the soil
matrix. Conservative pollutants, like the chloride ion, do not react with the soil or do not
undergo biological or radioactive decay. The process of absorption is the major cause of
retardation of pollutants in groundwater (Domenico & Schwartz 1997). Clays tend to be
stronger absorbers owing to their high specific area and high negative surface charge.
Although positive adsorption sites are present in clays, they are not as abundant as
negative sites. For this reason, usually negatively charged ions such as bicarbonate,
sulfate and nitrate, are not affected by retardation. Also, they are too large to be adsorbed,
hence the high mobility of these ions in groundwater. Soil adsorption capacity could be
described by the Freundlich adsorption isotherm, which presents a straight-line
relationship in logarithmic format between the mass of pollutant adsorbed by the
