Page 375 - Soil and water contamination, 2nd edition

362 Soil and Water Contamination

Table 18.3 Lake inflow and retention rates in Grand-Lieu, France, in two hydrologically contrasting seasons
(source: Marion and Brient, 1998).
1993–1994 1995–1996
6
Lake inflow (m ) 3 292 10 6 76 10
Particulate matter (%) 14 20
Total P (%) 40 18
3-
Ortho-PO 4 (%) 79 72
Total N (%) 32 60
NO 3 (%) 61 86
-
+
NH 4 (%) 72 66
lake outflow relative to inflow) of particulate matter was lower with the large flow of 1993–
94 than with the small flow in 1995–1996. The lower retention rate might be attributed
to residence time s being shorter during periods of high inflow, so less time is available for
the settling of sediment . However, the sediment exported from the lake consisted primarily
of organic, autochthonous particulate matter and resuspended particles from the bed
sediment (Marion and Brient, 1998). The resuspension and autochthonous particulate
matter production are higher during wet periods, because there is more wind and because the
volume and surface area of the lake are larger thanks to the high inflow. Thus the sediment
exported from the lake is of different origin and composition than the sediment transported
-
to the lake. During the wet season the retention rates for NO and total N were also lower.
3
This can largely be attributed to the shorter residence time during periods of high inflow.
+
3-
The retention rates for ortho-PO and NH were similar for both seasons. In contrast,
4 4
with the high outflow of 1993–94, the retention rate of total P was double. This may be
attributed to the increased inflow of particulate P during high flow, which is effectively
retained in the lake.
18.4.2 Lateral variation

The water composition of a lake, like that of rivers, is primarily controlled by the geology,
climate, and land use in the catchment . Müller et al. (1998) studied the influence of climate
and land use on the variation in water composition between 68 lakes in carbonaceous
catchments in Switzerland, Italy, and France. They found strong relationships between
water composition and altitude of the lakes: the lakes below 700 m above sea level contain
substantially higher concentrations of major dissolved constituents and nutrients. Obviously,
there is a strong relation between altitude and climate on the one hand, and agricultural
land use and the degree of urbanisation on the other hand. Although the proportion of
agricultural land does not differ much between the lake catchments studied, intensive
agriculture is more common at lower altitudes. Urban areas are also mainly located in the
lower altitude valleys. The concentrations of Na and K in the lakes investigated correlated
positively with the presence of urban areas in the catchment. The effect of intensive
agriculture was particularly evident in the increased concentrations of nutrients (N and P)
in lake waters at lower altitude. The higher Ca and Mg concentrations and alkalinity in the
lower altitude lakes can be attributed to more rapid weathering . At higher altitudes, the low
temperatures, thin or absent soils, and short contact times between rock and water result
in less chemical weathering. Atmospheric deposition of acidifying compounds contributes
to the increased weathering rates at lower altitudes. Air pollution from traffic, industry, and
agriculture can become trapped underneath an atmospheric temperature inversion, which
frequently occurs at 700–800 m above sea level in the alpine and peri-alpine regions in

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