Page 241 - Hydrogeology Principles and Practice

HYDC06 12/5/05 5:33 PM Page 224

224 Chapter Six

organic compounds produced by the petrochemical
and pharmaceutical industries. Accompanying urban-
ization is the need to dispose of domestic municipal
and septic wastes leading to the risk of contamination
from toxic materials and sewage. Other sources of
pollution in the urban environment include salt and
urea used in de-icing roads, paths and airport run-
ways (Howard & Beck 1993), highway runoff poten-
tially directed to soakaways (Price et al. 1989), the
application of fertilizers and pesticides in parks and
gardens, and the presence of chlorinated compounds
such as trihalomethanes caused by leakage of chlor-
inated mains water that has reacted with organic
carbon either in the distribution system or in the sub-
surface. Atmospheric emissions of sulphur dioxide
and nitrogen oxides from urban areas contribute to
wet and dry deposition of sulphur and nitrogen in
adjacent regions that can impact soils, vegetation and
freshwaters as a result of acid deposition and eutroph-
ication (NEGTAP 2001).
The regulated control of waste disposal in urban
areas is now practised in many developed countries
but because of the slow transmission time of con-
taminants in the unsaturated zone, the legacy of
historical, uncontrolled disposal of wastes may pre-
sent a potential for groundwater pollution. A rise in
groundwater levels, caused by a reduction in ground-
water abstraction in postindustrial urban centres,
may lead to remobilization of this pollution. In the
Fig. 6.16 The effect of aquifer heterogeneity on contaminant
Birmingham Triassic sandstone aquifer in the English
zones influenced by hydrodynamic dispersion. In (a) dilution
Midlands, a region of metal manufacturing and pro-
occurs in the direction of advancing contaminant in a
homogeneous intergranular material. In (b) the presence of cessing and mechanical engineering, samples from
greater hydraulic conductivity beds and lenses causes fingering of shallow piezometers, tunnels and basements show
the contaminant transport (K > K ). In (c) contaminant spreading
1 2 that groundwater concentrations at shallow depths
is created by the presence of irregular lenses of lesser hydraulic
are often heterogeneous in distribution and much
conductivity (K > K ). In (d) contaminant migration is dispersed
1 2
throughout the network of secondary openings developed in a higher than in groundwater pumped from greater
fractured limestone with molecular diffusion into the porous depth (Ford & Tellam 1994).
rock matrix. After Freeze and Cherry (1979). Generalizations as to the likely contaminants to be
found in urban areas are not always possible except
that the dominant inorganic contaminants are likely
6.4.1 Urban and industrial contaminants to be chloride and nitrate associated with a long
history of supply and a wide range of multiple point
Urban expansion and industrial activity, in some cases sources. Other than these two, contamination is
in the United Kingdom since the industrial revolution normally correlated to land use. In the Birmingham
in the 1700s, is accompanied by continual disposal and Triassic sandstone aquifer, the highest major ion con-
spillage of potentially polluting wastes. The types of centrations (Fig. 6.18) and levels of boron and total
wastes are diverse, ranging from inorganic contamin- heavy metal concentrations are associated with metal
ants associated with mining and foundry wastes to working sites (Ford & Tellam 1994). In other areas,