Environmental Engineering Reference
In-Depth Information
Sewage Inputs into Lake Washington
a
TABLE 17.4
Sewage P input
(1000 kg year
1
)
Years
Sewage input
1891-1936
Sewage from Seattle dumped directly into Lake Washington
?
1936-1958
Suburbs dump sewage directly into the lake
20-40
1958-1963
Population of suburbs grows as individual communities work
100-0
to divert sewage outfalls from lake
1963
All major sewage inputs into lake halted; sewage diverted to
0
Puget Sound with large mixing zone
a
After Edmondson (1991).
Lake Trummen
This is the best documented case of the long-term effects of sediment
dredging to mitigate internal loading problems. The 100-ha Swedish lake
began receiving sewage in the late 1800s and this continued until 1959,
leading to poor lake quality and frequent winter fish kills. Even after sewage
input was stopped, the lake quality was so bad that citizens considered fill-
ing the lake. Rich phosphorus deposits in the top layers of the sediments of
the shallow lake (mean depth 1.1 m) caused high rates of internal loading.
In 1970 and 1971, dredging was used to remove sediments, increasing the
mean depth from 1.1 to 1.75 m. The dredged sediments were drained and
sold as topsoil, partially offsetting the dredging costs (Cooke
et al.,
1993).
250
A
200
150
100
50
0
70
B
56
42
28
14
0
100
C
80
60
40
20
0
61
63
65
67
69
71
73
75
77
79
Year
FIGURE 17.15
Changes over time in P loading (A), epilimnetic chlorophyll (B), and pro-
portion of cyanobacteria (bluegreen algae; C) in Lake Washington (redrawn from Edmond-
son and Lehman, 1981).
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