Geoscience Reference
In-Depth Information
that form the major drainage conduit for the Greater Manchester conurbation, and in the
past combined sewer overflows have supplied domestic and industrial sewage, and road runoff
to the canal. The Manchester Ship Canal is up to 8 m deep, steep sided and up to 50 m wide.
Sediment is carried into the canal in shallow, fast moving rivers. As this river water meets the
deep, slow flowing canal, sediment is deposited at the bottom of the canal, as flow strengths
are not high enough to keep it in suspension. Owing to the urban nature of the catchment
this sediment is organic-rich and highly polluted by sewage and metals. These contaminated
sediments have caused a range of water quality problems (White et al. 1993). Aerobic bacteria
oxidize organic matter using free oxygen in the water. This process, therefore, uses up oxygen
from the water column, giving rise to what is termed anoxia - oxygen-depletion in the water
column. Methanogenic bacteria also break down organic matter in anoxic conditions, and release
methane gas (CH
4
). This methane, a flammable and noxious gas, bubbles up to the water column,
lifting up mats of sediment and sewage to the water surface (a process termed
sediment rafting
).
Nitrate (mg L
−
1
)
Sulphate (mg L
−
1
)
Ammonium (mg L
−
1
)
1
2
3
5
0
40
8
0
Sed - water
interface
(SWI)
0
0
SWI
SWI
0
10
10
10
20
20
20
30
30
30
40
40
40
50
50
50
Fe (mg L
−
1
)
Mn (mg L
−
1
)
Zn (mg L
−
1
)
20
4
0
0.1
0.2
1.5
3
0
SWI
0
SWI
0
SWI
10
10
10
20
20
20
30
30
30
40
40
40
50
50
50
Case Fig. 6.3(c)
Porewater profiles for the Salford Quays sediments showing the post-depositional consumption of chemical
species, and the release of metals into sediment porewater. Note the release of Fe and Mn, probably via a combination of bacterial
iron and manganese reduction and glass dissolution. (Modified from Taylor et al. 2003.)
