Environmental Engineering Reference
In-Depth Information
can still occur as a result of episodes of poor water quality, caused mainly by
runoff from urban surfaces or storm-water outflows during heavy rainfall.
Relevant events and trends affecting ecological recovery
Over the past 40 years, many changes in the law, technologies, infrastructure
and the economy have occurred, some or all of which have had repercussions
for river pollution. For example, the Rivers (Prevention of Pollution) Act 1961,
fully implemented in 1963, introduced retrospective control over effluents of
all types. In 1966 natural gas began to replace coal gas. The coal-based gas
works, with their grossly polluting effluents containing phenols, ammonia,
coal tars and cyanides, closed, and the discharges ceased over a few years after
about 1964. Also, in the mid 1960s, heavy industries, particularly blast fur-
naces, coke ovens and metal finishing industries began to decline, many
moving operations overseas. Regional GDP (RGDP) expressed as a per capita
percentage relative to the national GDP declined from over 100% to around
90% of its long-term average between 1965 and 1981. In the same period,
unemployment rose relative to the national average from 48% to almost
120%. There is a highly significant direct relationship between the RGDP and
ammonia values in the river (
Fig. 13.8
) which can be expressed by the equation
(y
86.3 (R
2
0.001)) where y is the average annual ammo-
nia concentration and x is the RGDP (expressed as a percentage of the national
GDP per capita). By about 1970, old sewage works in the Black Country were
being closed and the sewage directed to the main treatment works at Min-
worth. Finally, in 1980 large artificial lakes through which the river was partly
directed to improve settlement of solids and oxidation became fully oper-
ational following trials beginning in 1968. However, in a long-established
industrial conurbation with a complex history of change, numerous localised
events and actions will also have occurred, many of which may be unrecorded.
Thus, the presence and location of hidden or culverted outfalls, contaminated
areas of land and the sources of diffuse pollution may not all be known even
at the present time. Further, some untraced pollution sources may have ceased
because of industrial and economic changes.
The separate effects of the 1961 Act, the recession and gas works closures are
difficult to distinguish at this stage. By the time the treatment lakes were fully
opened, water quality was already greatly improved in the river downstream.
There was little immediate improvement at Chetwynd Bridge following
the opening of the lakes though ammonia values declined from around
2.5 to less than 1 mg L
1
over 15 years following the opening. However, Heng
(
2000
) recorded a longitudinal reduction in ammonia concentrations of from
2.5 mg L
1
to 0.8 mg L
1
from the inlet to the outlet of the lakes. This change
does not appear to have been detectable as far down as Chetwynd Bridge,
probably as a result of dilution and effects of other tributaries.
¼
9.32 Ln x
þ
¼
0.93, p
¼<
