Page 382 - Soil and water contamination, 2nd edition

Patterns in surface water 369

product of calcium carbonate (calcite ) to be exceeded. As a result, the calcium carbonate
precipitates and starts settling out of the water column to the lake bed sediments . This
process is illustrated by Figure 18.23, which shows the seasonal variation in the composition
of settling particles collected at 40 m depth in Lake Zurich, Switzerland, from April to
October 1989 (Wieland et al., 2001). Lake Zurich is a pre-alpine hard-water lake along the
northern rim of the Alps. The lake is approximately 29 km long and 2.5 km wide and its
maximum depth is 137 m. Most of the suspended particulate matter at the sampling location
is autochthonously produced, since the riverine input of allochthonous sediment is effectively
trapped in the upstream sections of the lake. The particles are mainly comprised of calcium
carbonate, organic matter , iron oxyhydroxides , and residual material, which contains silica
and aluminosilicates. In June, the calcium carbonate content of the suspended particles
increases dramatically from less than 30 percent to approximately 95 percent (Figure 18.23).
The precipitation of calcium carbonate also increases the total mass flux towards the lake
bottom by a factor of two to four. In September and October, the calcium carbonate content
gradually declines to the spring levels.

18.5 FURTHER READING ON CONTAMINANTS IN SURFACE WATER

• Miller, J.R. and Orbock Miller, S.M., 2007, Contaminated rivers: A geomorphological-
geochemical approach to site assessment and remediation, (Dordrecht NL: Springer).
• Westrich, B. and Förstner, U. (Eds.), 2007, Sediment dynamics and pollutant mobility in
rivers: An interdisciplinary approach, (Berlin: Springer).
• Plate, E.J. and Zehe, E. (Eds.), 2008, Hydrologie und Stoffdynamik kleiner Einzugsgebiete:
Prozesse und Modelle, (in German), (Stuttgart: Schweizerbart).

EXERCISES

1. Explain briefly:
a. why the nitrate concentration in river water increases downstream of a wastewater
discharge;
b. why the chloride concentration in streams decreases during a storm event;
c. why relatively large amounts of groundwater are entrained during runoff events;
d. why hysteresis occurs in the relationship between river discharge and suspended solids
concentrations during single flood events;
e. why the phosphate concentration in rivers decreases during summer;
f. why calcium carbonate precipitation occurs in hard water lakes during summer.

2. Explain the role of hydrological events for the export of contaminants from catchments.

3. Describe the timing of the peak or trough concentration relative to peak discharge for:
a. positive, clockwise Q–C response;
b. negative, clockwise Q–C response;
c. positive, anti-clockwise Q–C response;
d. negative, anti clockwise Q–C response.

4. a. Give a definition of concentration rating curves.
b. Describe how the coefficient a in Equation (18.1) differs between catchments on
intensively weathered rocks and catchments on bare unweathered rock.

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