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Contaminants on or in soils may move into the wider
environment through a number of routes. The soils themselves retain
contaminants to a greater or lesser extent. The movement of
contaminants from soil occurs through evaporation and dust generation,
intake by plants through their roots, and flushing by or dissolution
into water seeping through the soil.
Water transport of contaminants will usually result in
contamination of surface water bodies through surface water drainage
and by way of groundwater aquifers. The relevance of specific
hydrogeological factors to groundwater contamination and the subsequent
transport of contaminants to distant ecosystems have been assessed (see below).
The factors specified in this extension are the nature
of the aquifer material, the type of aquifer (confined and unconfined)
and the physical nature of the soil and vadose zone connections between
the surface and subsurface waters.
The role of typical New Zealand subsoils in the vadose
zone on the transport of contaminants has been assessed by measuring
the relative strength of the binding of arsenic, cadmium, chromium,
copper, lead, mercury, nickel, zinc and the components of petrol known
as BTEX (benzene, toluene, ethyl benzene and xylene). The subsoils
selected were:
- a basalt-derived soil from Auckland city;
- a loess from near Balclutha;
- a fine silt from Raupara, Marlborough;
- a peat from near Hamilton;
- volcanic ash from Wairakei;
- coarse river sand from an overbank deposit in the
Hutt Valley;
- fine dune sand from Paekakariki;
- weathered greywacke subsoil from the eastern Hutt
Valley hills.
Several methods for measuring the distribution
coefficients for metals were trialed, using columns of various sizes and
batch experiments. Small batch experiments were used to determine the Kd
values for the metals with the different sub-soils. The results were
used to rank the strength of retention of different
metals by different soils. An additional observation was that the
bonding strength between the soil and the metal increases with time.
The BTEX retention measurements were measured
similarly, although because the components are so volatile, care was
taken in experiment design and procedure to minimise any loss by
evaporation. The results showed that the soil type had little effect on
the concentration of BTEX in the water solution, except the peat, which
removed most of the BTEX from solution.
Two reports are available that detail the contaminant
mobility study.
They are in pdf format and downloadable by clicking the appropriate links
below:
An implementation of the proposed model in Part 2 is
also available:
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