Contamination with inorganic lead

We construct in this section a model of how inorganic lead reacts as it infiltrates and contaminates an aquifer, and then as the aquifer is flushed with fresh water during pump-and-treat remediation (Bethke, 1997 Bethke and Brady, 2000). We assume groundwater in the aquifer contacts hydrous ferric oxide [Fe(OH)3, for simplicity] which sorbs Pb++ ions according to the surface complexation model of Dzombak and Morel (1990), as discussed in Chapter 10. 

We employ the LLNL thermodynamic data for aqueous species, as before, omitting the PbC03 ion pair, which in the dataset is erroneously stable by several orders of magnitude. The reactions comprising the surface complexation model, including those for which equilibrium constants have only been estimated, are stored in dataset FeOH+.dat . 

We consider a 100-m length of an aquifer with a porosity of 30% and a nominal dispersivity of 10 cm the dispersivity reflected in the calculation results will be somewhat larger than this value, due to the effects of numerical dispersion. The domain is divided into 100 nodal blocks, each 1 m long. We assume local equilibrium, so time enters into the calculation only as a measure of the cumulative volume of fluid that has passed through the aquifer. Specifying the aquifer s pore volume be replaced 30 times over the course of the simulation, and setting the time span to 30 years, each year in the simulation corresponds to a single replacement of the aquifer s pore fluid. 

Initially, the aquifer contains a dilute Ca-HC03 groundwater, including a negligible amount of Pb++ as well as an equal quantity of Br, which serves in the calculation as a non-reactive tracer. At the onset of the simulation, water containing 1 mmolal Pb++ and Br- passes into the aquifer until half its pore volume has been displaced. At this point, the composition of water entering the aquifer changes to that of the initial fluid, uncontaminated water nearly devoid of lead and bromide. The simulation continues until water in the aquifer has been replaced 30 times. 

We assume Pb++ during imbibition of the contaminated fluid is retarded by a factor of two (R = 2 see Section 21.1). This retardation requires the metal ion to be sorbed on the ferric surface to its aqueous concentration, 1 mmolal. Accounting for the concentrations of the two forms of sorbed lead, (w)FeOPb+ and (s)FeOPb+, after reaction with the contaminated fluid, this retardation is expected when the sorbing mineral Fe(OH)3 makes up 0.0085 volume percent of the aquifer. 

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Fig. 32.1. Simulation of the contamination at 25 °C of an aquifer with inorganic lead. The 100-m long section of aquifer contains a small amount of Fe(OH)3, to which Pb++ sorbs. Aquifer is initially uncontaminated, but at t = 0 water containing 1 mmolal Pb++ and 1 mmolal Br , which serves as a non-reactive tracer, passes into the left side. Pb++ is taken to sorb according to surface complexation theory, and the amount of Fe(OH)3 is chosen so that migration of the metal is retarded by a factor of two relative to the groundwater flow. After half the groundwater has been displaced by the contaminated water (V2 p.v.), clean water is flushed through the aquifer.

Arsenic is an ancient and well-known hazard and, along with lead and mercury, is an important environmental contaminant. The inorganic form is far more toxic than organic arsenic, which is commonly found in seafood. Arsenic-contaminated drinking water is a worldwide problem that affects millions of people. Human exposure also occurs from arsenic-treated lumber. 

Acute inorganic lead poisoning is uncommon today. It usually results from industrial inhalation of large quantities of lead oxide fumes or, in small children, from ingestion of a large oral dose of lead in the form of lead-based paint chips small objects, eg, toys coated or fabricated from lead or contaminated food or drink. The onset of severe symptoms usually requires several days or weeks of recurrent exposure and manifests as signs and symptoms of encephalopathy or colic. Evidence of hemolytic anemia (or anemia with basophilic stippling if exposure has been subacute), and elevated hepatic aminotransferases may be present. 

Elevated lead contents were recorded in various species of plants from the vicinity of metal smelters, roadsides, soils heavily contaminated with lead, natural ore deposits, and lead recycling factories. Bioavailability of lead in soils to plants is limited, but is enhanced by reduced soil pH, reduced content of organic matter and inorganic colloids, reduced iron oxide and phosphorus content, and increased amounts of lead in soils. Lead, when available, becomes associated with plants by way of active transport through roots and by absorption of lead that adheres to foliage. Lead concentrations were always higher in the older parts of plants than in shoots or flowers. 

A large chemical company quartered on the East Coast spends about 400 million annually to remediate contaminated aquifiers and sites associated with past manufacturing operations. Much of this is spent on sites contaminated with lead from the manufacture of tetraethyl lead, lead-based paints, and lead cartridges. For example, the soil of a 25-acre site within a large plant located in New Jersey contains as much as 2,000 ppm of lead as inorganic salts to a depth of 2 feet. The distribution is as follows at the surface, 2,000 ppm 6 , 1,000 ppm 12 , 500 ppm 24 , 0 ppm. 

The NRC [48] also calls attention to the excavation marsh where bulldozers unearthed an old landfill that contained toxic materials. The sediment was sufficiently contaminated with lead that large quantities had to be trucked to a toxic waste dump at a large and unanticipated cost. Wetlands can both remove and transform organic and inorganic substances from inflow-... 

Purity control limits contamination with pathogenic bacteria Staphylococcus aureus, Escherichia coli. Salmonella-species, Pseudomonas aeruginosa, Clostridium-species and others), yeasts, moulds, microbial toxins (aflatoxins, endotoxins), toxic heavy metals (lead, cadmium, mercury, arsenic e.g. from industrial emission), pesticide and herbicide residues, fumigants (ethylene oxide, methyl bromide, phosphine) and radionuclides. Furthermore, impurities with other plants parts ( foreign organic matter ) are limited. Moist levels must be below a certain maximum to avoid deterioration by microorganisms. Excreta of animals and dead insects must be absent. The ash value and acid-insoluble ash limits the amount of inorganic impurities (soil, sand). 

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