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Groundwater quality and contaminant hydrogeology 203

Fig. 6.2 Diagram for classifying irrigation water based on percentage sodium content (sodium hazard) and electrical conductivity (salinity
hazard). The sodium adsorption index (SAR) is also shown. The six classes are described as follows: Class 1 (Excellent), suitable for use on
all crop types; Class 2 (Good), suitable for use on most crops under most conditions but limiting conditions can develop on poorly draining
clayey soils; Class 3 (Fair), suitable for most crops if care is taken to prevent accumulation of soluble salts, including sodium, in the soil;
Class 4 (Poor), suitable only in situations having very well-drained soils for production of salt-tolerant crops; Class 5 (Very poor), restricted
to irrigation of sandy, well-drained soils in areas receiving at least 750 mm of rainfall; Class 6 (Unsuitable), not recommended for crop
irrigation. After Oklahoma State University Extension Facts F-2401 Classification of Irrigation Water Quality.

gypsum and lime content of the soil and the irrigation 6.3.1 Transport of non-reactive dissolved
method and management. Further information can contaminants
be found in UNESCO/FAO (1973).
Non-reactive contaminants, such as saline wastes
containing chloride, are principally affected by the
6.3 Transport of contaminants in groundwater major processes of advection and hydrodynamic
dispersion. Advection is the component of solute
The type of soil, sediment or rock in which a pollu- movement attributed to transport by the flowing
tion event has occurred and the physicochemical groundwater. The advective velocity of the contamin-
properties of individual or mixtures of contaminants ant is the average linear velocity of the groundwater
influence the spread and attenuation of groundwater and can be calculated from a consideration of Darcy’s
contaminants. The fundamental physical processes law (see eq. 2.10). Hydrodynamic dispersion of con-
controlling the transport of non-reactive contamin- taminants in porous material occurs as a result of
ants are advection and hydrodynamic dispersion mechanical mixing and molecular diffusion as illus-
which create a spreading pollution plume and cause a trated in Fig. 6.3. The significance of the dispersive
dilution in the pollutant concentration. For reactive processes is to decrease the contaminant concentra-
contaminant species, attenuation of the pollutant tion with distance from the source. As shown in
transport occurs by various processes including Fig. 6.4, a continuous pollution source will produce
chemical precipitation, sorption, microbially medi- an elongate plume, whereas a single point source will
ated redox reactions and radioactive decay. For the produce a slug that grows with time while becoming
class of contaminants known as non-aqueous phase less concentrated as a result of dispersion as the
liquids (NAPLs) both immiscible and dissolved plume moves in the direction of groundwater flow.
phases of the contaminant need to be considered. To Molecular diffusion of contaminants is not norm-
explain these processes, it is convenient to divide the ally of practical consideration where advection and
following sections into general contaminant classes, mechanical dispersion are dominant. This is typically
namely non-reactive and reactive dissolved contamin- the case for shallow groundwater environments but
ants and non-aqueous phase liquids. in situations such as the very long term deep disposal