Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
natural waters exist in both dissolved and sorbed phases. In rapidly
moving water systems, advection controls mass transport and dissolved
substances move at essentially the same velocity as the bulk of the
water in the water system. Contaminants that are sorbed to colloidal
materials and fine suspended solids can also be entrained in the
current, but they may undergo additional transport processes that
increase their effective residence time in surface waters. Such
processes include sedimentation, deposition, scour and resuspension.
Thus, determining the transport of contaminants in surface waters
requires that we follow both water movement and sediment movement.
A water balance is the first step in assessing surface water
transport. A water balance is established by equating gains and losses
in a water system with storage. Water can be stored within estuaries,
lakes, rivers and wetlands by change in elevation or stage. Water
gains include inflows (both runoff and stream input) and direct
precipitation. Water losses include outflows and evaporation.
6.4.4 Groundwater
In groundwater, the dilution of contaminants occurs much more
slowly than it does in surface water. After precipitation, water
infiltrates the ground surface where it travels vertically down
through the unsaturated zone until it contacts the water table, and
then flows approximately horizontally. This horizontal movement is
driven by the hydraulic gradient, which is the difference in hydraulic
head at two points divided by the distance (along the flow path)
between the points. Bear & Verruijt (1987) and Freeze & Cherry (1979)
have compiled extensive reviews on the theory and modelling of
groundwater flow and on transport of contaminants in groundwater. The
movement of contaminants in groundwater is described by two principal
mechanisms: gross fluid movement (advective flow), and dispersion.
Dispersion depends on both fluid mixing and molecular diffusion. The
transport of many chemical species in groundwater is often slowed or
"retarded" relative to the flow of the bulk fluid by sorption of the
contaminant material to soil particles or rock. As is pointed out by
Bear & Verruijt (1987), many groundwater models are available for
assessing the transport of contaminants in the subsurface environment,
ranging from simple one-dimensional hand calculations to large
three-dimensional computer programmes. The choice of an appropriate
model for any situation depends to a large extent on the information
available, the type of information needed to carry out an exposure
assessment and the tolerance of the analyst for large, complex
computer programmes.
6.4.5 Soil
Soil, the thin outer zone of the earth's crust that supports
rooted plants, is the product of climate and living organisms acting
on rock. A true soil is a mixture of air, water, mineral and organic
components (Horne, 1978). The relative mix of these components
determines to a large extent how a chemical will be transported and/or
transformed within the soil. The movement of water and contaminants in
soil is typically vertical as compared to horizontal transport in the
groundwater (i.e., saturated) zone. A chemical contaminant in soil is
partitioned between soil water, soil solids, and soil air. For
example, the rate of volatilization of an organic compound from the
soil solids or from soil water depends on the partitioning of the
compound into the soil air and on the porosity and permeability of the
soil.
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