REVUE WATER MODELLING PROCEDURE

CHAPTER FIVE

5. DISTRIBUTION AND FATE OF POLLUTANTS

5.2 REVUE WATER MODELLING PROCEDURE

In order to describe the water quality in the Revue basin, the study area was divided in three parts ; upstream of, within and downstream of the mining area. The same sub divisions have been used to model water quality. Site 1 was selected to represent the water quality upstream of the mining area; site 9 to represent water quality within the mining area; and site 14 located in the Chicamba Dam to represent water quality downstream of the mining area. The composition of the water at these sites is shown in the Table 4.1 (p. 21).

In order to model the water quality was necessary to make some simplifying assumptions:

1 - A chemical equilibrium model is a suitable approximation. The simplest process that might regulate the concentration of a trace element in solution is equilibrium with respect to a solid phase containing the element as a major component (Drever, 1997).

58

Distribution andfate of pollutants

2 - The river water will be in equilibrium with the atmospheric oxygen and carbon dioxide. The turbulence of water facilitates the oxygen dissolution in the water.

3 - The oxidising environment and the introduction of redox pair can lead to precipitation of some trace metals.

4 - F or the adsorption, the iron and manganese oxyhydroxides are the adsorbing phases and the FEO-DLM.DBS database will be used as contains surface complexation reactions applicable to diffuse layer model.

The mobility of trace metal pollutants depends on a more or less complex network of interactions between aqueous and heterogeneous chemical reactions as well as particle coagulation and flocculation phenomena. Hydrolysis and dissolved complexation tend to increase the solubility of trace metals while precipitation and adsorption will delay metal availability and transport (Fig. 5.1, p. 61). The distribution of a metal between its various possible species is a result ofa series of heterogeneous and dissolved chemical reactions.

For each site four modelling runs were undertaken to allow the program to run without a phase rule violation.

In the fust run MINTEQA2 was used as a speciation-saturation program only, with the total concentrations of the water quality variables as the input. This has two purposes: (1) to see which phases are supersaturated and hence are likely to precipitate, and (2) to convert the alkalinity input into total concentration of carbonate species and the concentration of the component H+ (Drever, 1997). In this initial run the oxidation state of the Fe, Mn and Cr in the analysis was also decided upon. These elements were entered into the program as Fe 2 +, Mn2+

and Cr(OHh (reduced species) which are probably their actual form despite that in air saturated fresh waters, all trace elements should be in their maximum oxidation states, because the redox potential of the 021H20 complex is higher than that of any other complex (Lumsdon & Evans, 1995). This water should be in equilibrium with atmospheric oxygen and contain close to the maximum dissolved oxygen possible due to the turbulent flow. The reason for entering these elements in their reduced form was that the water samples were filtered using 0.45 l-lm Millipore membrane filter . Thus, any oxides or oxyhydroxides of these elements which could have been present in the water as colloidals or suspended solids have been removed. After filtering the samples were also acidified, thus any dissolved Fe, Mg and Cr reported in the chemical analysis are likely to be in reduced state. No post precipitation was noted in the sample bottles. In the initial run the water was specified to be in equilibrium with CO2 gas phase at partial pressure of 3.10"" atm and O

2

at fixed partial pressure of 0,21 atm.

59

Distribution and/ate a/pollutants

In the second run the components of the first run were called as a seed file and the total carbonate concentration obtained in the output file of the first run was entered as a component.

Saturated solids were allowed to precipitate as it is known that the precipitation limits metal solubility.

In the third run, the redox pairs for Fe and Mn were imposed on the model. With fixed redox ratio imposed, MINTEQA2 calculates the concentration of species present in the reduced and in the oxidised state.

In many aqueous environments the distribution of metals, particularly the trace metals, is regulated by adsorptionldesorption reactions occurring at the interface between the aqueous solution and particulate matter such as mineral particles, organic matter or living cells (Fig. 5.2).

Thus, in the fourth run the effect of solid surface adsorption was considered, assuming that for natural water systems the binding capacity of sediments is controlled largely by a surface layer of amorphous ferric hydroxide which forms a coating on inorganic support particles (Luoma &

Davies, 1983; Kerr 1994».

Adsorption by iron and manganese oxyhydroxides in particular, is probably the most important process in maintaining the concentrations of trace elements at levels far below those predicted by equilibrium solubility calculations. They have extremely high adsorption capacities and high adsorption affinities for many metals. MINTEQA2 provides a separate database file, FEO- DLM.DBS, which contains surface complexation reactions that are applicable to the diffuse layer model for adsorption of some metals and ligands onto the iron oxide surface (Kerr, 1994).

The properties of the iron hydroxide layer for which the FEO-DLM.DBS database can be attached to the diffuse layer adsorption model are:

Solids concentration of 3,422 g/I and with an amorphous iron concentration of 0,72 I mg/g.

Specific surface area

=

600 m

²

/g.

High energy site density

=

1,922. 10-4 moles/I.

Low energy site density

=

7,690.10-

³

moles/I.

60

NON-LIVING PARTICLES

I I

Q I

~; h

g 6J1 l

~ i

T

SORPTION PRECIPITATION

DESORPTION DISSOLUTION

"//··i··~ii·-,r;-.iii;·-r·~'r---' _ ... -... .

TRANSPORT BY MOLECULAR DIFFUSION,

BIOTURBATION, BIOIRRIGATION, ETC ...

Distribution and/ate o/pollutants

UPTAKE

RELEASE DEGRADATION

LIVING PARTICLES

PLANKTON, LIVING PARTICLES

Fig. 5.2 - Schematic representation of internal processes that influence the concentrations of

trace elements in a water system (From Tessier et al., 1994).