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Groundwater quality and contaminant hydrogeology 219

Fig. 6.12 Behaviour of organic contaminants in groundwater. The chlorinated solvent contaminant shown in (a) has a density greater than
water (a dense non-aqueous phase liquid or DNAPL) and so sinks to the base of the aquifer. Here, transport of the DNAPL is controlled by
the slope of the base of the aquifer, while the dissolved aqueous phase moves in the direction of groundwater flow. The hydrocarbon
contaminant shown in (b) has a density less than water (a light non-aqueous phase liquid or LNAPL) and so floats on the water table. In this
case, transport of the LNAPL is controlled by the slope of the water table, while the dissolved aqueous phase moves in the direction of
groundwater flow.

similar manner to chlorinated solvents except, as oil table will rest directly on the water table. In the
shown in the Fig. 6.12b, by reason of their density, case of a thick zone of mobile LNAPL, the water
they float on the water table. The aromatic BTEX table may be depressed by the weight of the LNAPL
compounds, benzene, toluene, ethylbenzene and (Fig. 6.12b). The mobile LNAPL can migrate in the
xylene, are released in significant amounts by petro- vadose zone, following the slope of the water table,
leum and can be transported by groundwater in the while the dissolved components can disperse with the
aqueous phase. When spilled at the land surface, oil advecting groundwater. The residual LNAPL phase
will migrate vertically in the vadose zone under the in the vadose zone can partition into the vapour
influence of gravity and capillary forces, in an ana- phase as well as the water phase, with the degree of
logous manner to water, until it reaches the top of partitioning dependent on the relative volatility of
the capillary fringe. Much of the LNAPL will be left the hydrocarbon and its solubility in water. In frac-
trapped in the vadose zone, but on reaching the tured rocks, LNAPL will typically resist migration
capillary fringe the LNAPL will accumulate and below the water table but where there is sufficient
an ‘oil table’ will develop. As the weight of LNAPL weight, LNAPL can penetrate below the water table
increases, the capillary fringe will become thinner to a limited extent when the pressure exerted by the
until mobile or ‘free’ product accumulates. Eventually, LNAPL exceeds the displacement pressure of the
the capillary fringe may disappear completely and the water in the fractures (Fig. 6.14b).