Page 325 - Materials Chemistry, Second Edition

CAT3525_C10.qxd 1/31/2005 12:00 PM Page 296
296 Waste Management Practices: Municipal, Hazardous, and Industrial

TABLE 10.1
Maximum Contaminant Levels for MSW Constituents
a
Chemical MCL (mg/L)
Arsenic 0.01
Barium 2.0
Benzene 0.005
Cadmium 0.005
Carbon tetrachloride 0.005
Chromium (hexavalent) 0.05
2,4-Dichlorophenoxy acetic acid 0.07
1,4-Dichlorobenzene 0.075
1,2-Dichloroethane 0.005
1,1-Dichloroethylene 0.007
Endrin 0.002
Fluoride 4.0
Lindane 0.0002
Lead 0.015
Mercury 0.002
Methoxychlor 0.04
Nitrate 10
Selenium 0.05
Silver 0.05
Toxaphene 0.003
1,1,1-Trichloroethane 0.2
Trichloroethylene 0.005
2,4,5-Trichlorophenoxy acetic acid 0.05
Vinyl chloride 0.002
a Not to be exceeded in the uppermost aquifer under a MSWLF
Source: 40 CFR §§258.40.

The clays of importance are the so-called silicate clays, those possessing a crystalline structure
composed of two relatively simple constituents, i.e., a silica tetrahedron (SiO ) and an aluminum
4
octahedron (Al [OH] ). Different clay minerals result as these basic units are stacked upon each
6
2
other. In many cases the central metal (Si or Al) is replaced by other metals of similar diameters, thus
imparting a significant electrical charge to the clay units. Also, different ions may bind the clay units
together. Some important clay properties are listed in Table 10.2. The smectite group is known for
substantial swelling upon wetting; water molecules are easily inserted between the layers, which
results in expansion. As a result, smectites (in particular bentonite clays) have been popular for land-
fill liners and caps and also for the installation of slurry walls, i.e., vertical barriers that restrict hor-
izontal liquid migration.

10.4.6 HYDRAULIC CONDUCTIVITY

Vertical seepage of leachate and consequent contamination of groundwater is an important consid-
eration in design of a Subtitle D landfill. During routine landfill operations, leachate will collect at
the base of a landfill, typically from inputs of natural precipitation and the presence of moisture
within the waste. It is of great practical importance, therefore, to appreciate the behavior of liquids
such as water or leachate in a saturated soil (or clay) column.