Page 233 - Hydrogeology Principles and Practice

HYDC06 12/5/05 5:33 PM Page 216

216 Chapter Six

BO X
Continued
6.4

−1
in 1962. The WHO guide value for Cr is 0.05 mg L . Concentra- moved horizontally southwards, with an average velocity of about
−1
tions of Cd have apparently decreased in some places and increased 0.5 m day , and discharged to the Massapequa Creek.
in others, and peak concentrations do not coincide with those of Cr. Analysis of cores of aquifer material along the axis of the plume
These differences are probably due partly to changes in the chem- showed that the median concentrations of Cr and Cd per kg of
ical character of the treated effluent over the years and partly to the aquifer material were, respectively, 7.5 and 1.1 mg, and the max-
influence of hydrogeological factors such as aquifer permeability imum concentrations were, respectively, 19 and 2.3 mg. Adsorption
and sorption characteristics. A test site near the disposal basins occurs on hydrous iron oxide coatings on the aquifer sands. The
recorded as much as 10 mg L −1 of Cd in 1964. The WHO guide ability of the aquifer material to adsorb heavy metals complic-
−1
value for Cd is 0.003 mg L . ates the prediction of the movement and concentration of the
At the time of operation, the pattern of movement of the plating plume. Furthermore, metals may continue to leach from the aquifer
waste was vertically downwards from the disposal basins, through material into the groundwater long after cessation of plating
the unsaturated zone, and into the saturated zone of the Upper waste discharges, and so necessitating continued monitoring of the
glacial aquifer. From here, most of the groundwater contamination site.

where a and b are the number of moles and and leachate and the competitive effect between the
ad aq
indicate the adsorbed and aqueous phases, respect- heavy metals present.
ively. The equilibrium coefficient for this reaction is: Yong and Phadungchewit (1993) demonstrated
that a change in soil solution pH results in a corre-
a
[ A ] [ B ] b sponding change in the dominant retention mech-
ad
aq
K = K = eq. 6.20
−
AB s a b anism of heavy metals in soils. At high pH values,
A [ aq ] B [ ad ] precipitation mechanisms, for example precipita-
tion of hydroxides and carbonates, dominate. As pH
Thus, for a problem involving binary exchange, the
decreases, precipitation becomes less important and
retardation equation is now:
cation exchange becomes dominant. It was also
shown that the selectivity order governing the reten-
V ρ K CEC
w =+ bs tion of heavy metals in soils depends on the soil solu-
eq. 6.21
1
V θτ tion pH. At pH values above 4–5, when precipitation
c
prevails, the selectivity order was found to be Pb >
For some exchange systems involving electrolytes Cu > Zn > Cd, as demonstrated for illite, montmoril-
and clays, K is found from experimentation to be lonite and natural clays soils. At lower soil solution
s
constant over large ranges of concentrations of the pH, the selectivity order was Pb > Cd > Zn > Cu, as
2+
adsorbed cations and ionic strength. For the Mg - shown in the case of kaolinite and montmorillonite.
2+
Ca exchange pair, K is typically in the range
sMg-Ca
2+
0.6–0.9, meaning that Ca is adsorbed preferentially
2+
to Mg (Freeze & Cherry 1979). 6.3.3 Transport of non-aqueous phase liquids
Clay minerals, metal oxides and organic material
exhibit preferential exchange sites for ion occupation The transport of non-aqueous phase liquids (NAPLs)
and attempts have been made to establish a select- concerns the contamination of groundwater by
ivity sequence, particularly with respect to heavy organic compounds and includes dense non-aqueous
metals, where equivalent amounts of cations are phase liquids (DNAPLs) with a density greater than
arranged according to their relative affinity for an water, for example chlorinated hydrocarbon sol-
exchange site. In general, the greater the charge on a vents, and light non-aqueous phase liquids (LNAPLs)
cation, the greater the affinity for an exchange site. with a density less than water, for example hydro-
Account must also be taken of the pH of contaminant carbons (refined mineral oils). NAPLs have relatively