Page 357 - Soil and water contamination, 2nd edition

344 Soil and Water Contamination

at 0.6 km has a particularly clear impact on the concentrations of the four substances. The
Biebrza river flows through a relatively undisturbed peatland without agricultural inputs, so
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that diffuse pollution sources are almost absent. Since Cl is a conservative substance, it is
not removed by adsorption or biochemical processes, nor is it produced. Only dilution by
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groundwater discharge into the river and inflowing tributaries affects the Cl concentration
-1
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(Figure 18.4a). The local Cl concentration in groundwater amounted to about 1 mg l .
-1
3
As the influx of groundwater was only moderate (approximately 0.035 m s per kilometre
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of river), the Cl concentration decreased rather slowly in downstream direction. The
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3-
PO concentration (Figure 18.4b) decreased much faster, since PO is removed through
4 4
adsorption onto stream bed sediments and particulate matter , and through biological
uptake by algae and aquatic macrophytes, with adsorption onto bed sediments being the
+
3-
3-
major mechanism for PO retention (Van der Perk, 1996). Similarly to PO , the NH
4 4 4
concentration (Figure 18.4c) also decreased to pre-discharge concentrations within a
relatively short distance. Although adsorption to bed sediments played a relevant role in
+
the decline in NH concentrations, nitrification was identified as being the main process
4
+
+
of NH removal. The nitrification of NH is also manifested in the gradual increase of
4
4
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NO concentrations (Figure 18.4d) in the first few kilometres downstream of the waste
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water discharge at 0.6 km. In this stretch of the river, the production of NO by nitrification
3
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exceeded the NO removal by denitrification . At about 6 km downstream of the wastewater
3
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discharge, the NH concentration was depleted to such an extent that nitrification could not
4
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compensate for the NO lost by denitrification. In the stretch of river further downstream
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from the peak NO concentration, the denitrification process prevails, so the NO
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concentration falls to pre-discharge concentration within 15 to 20 km downstream from the
wastewater discharge.
In steady state conditions, as in the above example in which a continuous inflow of
wastewater occurs in a river with constant discharge, the effect of longitudinal dispersion
on the spatial distribution of concentrations is insignificant (Thomann, 1973; Fisher

et al., 1979). The Peclet number , which expresses the ratio of mass transport by advection
to that by dispersion (see Section 11.3.3), is usually much larger than 1 for most rivers.
Moreover, the concentration gradients in longitudinal direction that result from dilution and
biochemical processes are so small that the net dispersive flux is negligible by comparison
with the advective flux.
As with diffuse sources, point sources located in the downstream parts of river
catchments contribute more to the total contaminant export from catchments than those in
the headwaters, because the shorter flow path to the catchment outlet and deeper water result
in less contaminant removal. Moreover, in many parts of the world, the population density
tends to increase in downstream direction with the largest population centres be in coastal
areas, i.e. near the catchment outlets (Vörösmarty et al., 2010) . These areas are characterised
by large contaminant inputs into surface waters from urban runoff and industrial and
domestic wastewater treatment facilities. Destouni et al. (2008) demonstrated that small,
near-coastal catchment areas may generate large nutrient and mass loads entering the sea that
are as large or larger than river loads from large river basins.
18.3 TEMPORAL VARIATION IN RIVER WATER COMPOSITION
18.3.1 Short-term dynamics
Rivers and streams are very dynamic environments. Concentrations of the substances they
carry may exhibit a typical and complex response to increased runoff events. The response
is the result of many processes that occur simultaneously during such hydrological events,
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