Characteristics of Natural and Urban Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 123

Fig. 2.13 Phosphorus
concentrations in suburban
and urban areas of Nanjing,
China (Yuan et al. 2007 )

200

150

100

50

0

suburban urban (new) urban (old)

increasing distance to the city centre the phopsphorus content decreased (Fig. 2.13 ).
The suburban areas indicating a built-up period with current urban extension have
lower soil P values in comparison with new (after 1947) and old (ancient times
until 1947) urban terrains, consisting mainly of anthropogenic deposits (Yuan et al.
2007 ).
Another problematical technogenic material refers to the coal mining waste ,the
coal gangue, usually deposited in large heaps aboveground. There are several points
of consideration in association with environmental damage. Firstly, dust resulting
from the weathering processes near the surface can be blown away, affecting neigh-
bouring sites. Second, slope instability can cause damage through land slides, in
particular in heaps with steep slope gradients and a huge percentage of bare soil
surfaces. The most important problems, however, are the oxidation processes after
depositing sulfidic materials leading to oxidation of pyrite (4FeS 2 + 15O 2 + 10H 2 O
→ 4FeOOH + 8H 2 SO 4 ). The resulting leachate of acids (acid mine drainage AMD),

influencing the mobility of heavy metals which are able to contaminate groundwater
and surface water, must be taken into consideration. Simultaneously, this exother-
mal chemical reaction may cause spontaneous ignition. In case of old coal mining
heaps completely vegetated with shrubs and trees based on natural succession, the
vegetation may catch fire aboveground.
The process of strong acidification is well-known in relation to the sludge
lagoons, containing dredged harbour or stream sediment as well. As long as water-
logged conditions prevail, sulphur and iron remain in reduced form. But during
drying, oxidation can take place producing sulphuric acid, and accelerating heavy
metal mobility (Brady and Weil 2008 ). Thus, sites consisting of coal gangue or
dredged harbour sludge tend to acidify. In industrialized areas this kind of acidifica-
tion and the acid deposition from the atmosphere often decreases the soil pH value
enormously.
A special kind of deposited material is sludge derived from raw material extrac-
tion like ore mine tailings. After separation processes (flotation, gravity separation,
solution mining) it is necessary to dispose of acid waste water, using tailing ponds
or lagoons. The tailing ponds may dry quickly. After drying, dust accumulated from
heavy metals develops at the surface, is subjected to wind erosion, and sometimes