Chemistry, Third edition

LAND POLLUTION 419

Table 22.1River classification (Source:Understanding our Environment,

Edited by R. M. Harrison, Royal Society of Chemistry)

River quality Dissolved O 2 BOD NH 4 as N Comment
class saturation/% /ppm /ppm

Class 1A  80 3 0.4 supports high-class fisheries
(non-toxic to trout and coarse fish) high quality

Class 1B  60 5 0.9 lower quality than 1A but still
(non-toxic to trout and coarse fish) high quality

Class 2  40 9 – moderate quality

Class 3  10 17 – low grade; fish rarely present

Class 4 very low or anaerobic – – very polluted; no fish

Land pollution

Landfill

Landfill involves disposal of hazardous waste on, or within a hole in, the ground. When

full, the site is then covered with soil and planted with green cover. Problems have been

associated with landfill sites in the past – volatile waste may lead to unpleasant odours,

and the decomposition of covered garbage may release methane and other gases for

many years. Also, rain may seep into the site and dissolve (leach) buried toxic material

• the resulting solution can contaminate drinking water supplies via underground

water courses. Modern sites are constructed so that the leachate is collected and con-

tained and evolved gases are treated. The latter is particularly important because gener-

ation of methane could lead to explosions. Although landfill is a relatively inexpensive

way of containing waste, fewer suitable sites are becoming available.

Pesticides

Pesticides are substances that are used to kill, or block the reproductive processes of,

unwanted organisms. Synthetic pesticides are of concern to us, because of the pos-

sible effect upon human health of eating food, or drinking water, contaminated with

these chemicals. Most pesticides can be put into one of three categories:

1. Insecticides

Control of insects by insecticides helps to curb disease (for example malaria and

yellow fever) and protect crops. Organochlorinesare a group of compounds which

have been developed and used as insecticides since the 1950s. Organochlorines are

stable in the environment, toxic to insects in small amounts, but much less so

to humans; and, because they are organic compounds, not very soluble in water.

Probably the best-known organochlorine compound is DDT (1,4-dichloro-

diphenyltrichloroethane):

22.4

BOX 22.4

Problem with

hazardous waste

disposal – the Love

Canal

The Love Canal waste tip in

Niagara Falls (USA) was

originally a site that had

been excavated to build a

canal. During the 1930s

and 1940s, a chemical

works had used it as a site

for the disposal of many

different chemicals,

including toxic chlorinated

organic compounds.

In the 1950s, a housing

estate and school were

built on the land. During a

particularly wet winter in the

mid-1970s, leachate

containing many toxic

species flooded the

basements of homes and

drums containing chemical

waste surfaced through the

soil. Many people were

evacuated and over $100

million was spent in an

attempt to rectify the

situation.