Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 589

nitrogen-fixing bacteria in the soil and by bacteria that reside in the rhizosphere in
symbiosis with plants of the Leguminosae (e.g., clover) family. These bacteria create
nodules in leguminous plants in which they form colonies.
13.4.3.5 Cleaning Function
An important process in soil ecosystems is the degradation, and hence elimination,
of organic contaminants such as pesticides and other anthropogenically introduced,
potentially toxic organic contaminants, mainly through microbial degradation pro-
cesses. This function provides natural cleaning of the upper soil layer and, hence,
reduced organic contaminants load for the groundwater and surface water. From an
anthropogenic point of view, this process is often referred to as the cleaning func-
tion (also called 'filtering function' or 'purification function') of soil, focusing on
the provision of clean groundwater. This cleaning function is often sufficient to be
able to use groundwater as drinking water without the need for any purification pro-
cess. In other cases, mainly in urban areas, it facilitates the purification process for
the waterworks.
Also in the groundwater are organisms that degrade organic contaminants. In
in situ remediation of soils, the natural cleaning capacity is used explicitly. It is
even stimulated by the addition of chemicals, enhancing the activities of the soil's
microbial community, such as electron acceptors (nitrate, ozone), nutrients (nitro-
gen, sulphate), vitamins, or even specific organisms, in order to upgrade the soil
microbial community.
The process of degradation in combination with dilution by transport processes
is often called Natural attenuation (see Chapter 22 by Peter et al., this topic, for
details on Natural Attenuation).
It must be mentioned that the process time for degradation of persistent organic
contaminants in soils might take a long time (half lives of more than half a year).
Persistency is defined as a characteristic of a contaminant which pertains to the dura-
tion of its effectiveness. However, since these contaminants usually do not migrate
fast through the soil, the time span between entering the soil and reaching the
groundwater is often relatively long, which means that there is much time avail-
able for degradation. One drawback of the degradation of organic contaminants is
that the degradation products sometimes result in the formation of compounds that
are even more toxic than the mother contaminants.
Another process that supports the reduction of contaminant leaching into ground-
water is the precipitation of heavy metals in the form of sulphides or phosphates,
produced by anaerobic bacteria. Also the increase in pH, often resulting from bio-
logical activities in lower soil layers, results in a reduction of metal mobility and,
hence, of leaching into the groundwater.
13.4.3.6 Disease Control
In most agricultural soils the incidence of plant diseases is a well-known problem.
Soil organisms may play a role as the cause of, and the remedy against, crop diseases