Water Supply and Quality - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 159

compounds. While association with the
organic matter within soil aggregates reduces
the mobility and toxicity of xenobiotic com-
pounds (Luthy et al ., 1997), sorption may
also reduce the xenobiotic compound's ac-
cessibility to pollutant-degrading micro-
organisms. Experimental evidence shows
that organic soil amendments reduce the
aerobic mineralization of xenobiotic com-
pounds by reducing their readily bioacces-
sible concentrations in soil (Marchal et al .,
2013), but soil organic matter bound com-
pounds apparently remain to some extent
susceptible to microbial breakdown (Yang
et al ., 2012). If soil organic matter bound
compounds remain susceptible to biodegrad-
ation, the benefits of the pollutant retention
and subsequent breakdown of immobilized
pollutants would be lasting.

facilitates the transport of metals and hydro-
phobic organic pollutants in surface water
and groundwater, and may thus enable
entry of these pollutants into reservoirs and
aquifers used for water supply. High con-
tents of naturally dissolved organic carbon
give colour to water (see Plate 9), which is
aesthetically undesirable in water used for
consumption and may also add unpleasant
tastes and odours. Dissolved organic carbon
is a growth substrate for microorganisms,
and it may cause high cell counts and bio-
fouling problems in drinking water treat-
ment facilities, for instance the clogging of
filters and membranes. Natural dissolved or-
ganic matter is also an important precursor
for the formation of undesirable by-products
from water disinfection; for instance, the
formation of trihalomethanes if chlorination
is used. Dissolved organic matter is not re-
leased continuously into seepage water, but
depends on the 'pre-history' of the soil.
Freezing/thawing events, snow melts/floods
in winter have been observed to release peak
concentrations of dissolved organic matter
that is often associated with the transport
of pollutants. Furthermore, the digging of
trenches for laying cables to wind farms
through organic carbon-rich soil is specu-
lated to have led to increased dissolved or-
ganic matter leaching into surface waters
and drinking water reservoirs.

Generation of Colloidal and
Dissolved Organic Carbon

Soil organic carbon has many benefits for
water filtration, as outlined above, but the
dissolution of smaller and more polar natural
organic molecules into water, for instance
the release of fulvic and humic acids by organic
carbon-rich soils, can also create problems
for water supply. Dissolved organic carbon

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