Environmental Engineering Reference
In-Depth Information
11.4.1 Conceptual Model
Contaminated domestic water can lead to the exposure of humans in several ways:
consumption of drinking water;
inhalation of volatilised domestic water;
dermal contact during showering or bathing.
Normally, consumption of drinking water is the most important of these three
pathways. The exposure is directly calculated from the contaminant concentration in
the domestic water and the consumption rate of drinking water, either as beverages
or in food. Treatment of groundwater may reduce contaminant concentrations before
its use as drinking water, but this is normally not taken into consideration in Risk
Assessment models.
Volatile contaminants in domestic water will evaporate during tapping or when
water is held in open containers, such as water heaters, dishwashers, toilets, bath
tubs and washbasins. This exposure pathway has been shown to be of importance
for radon present in groundwater. Radon has a very high Henry's Law constant
and a relationship has been found between the radon content in domestic water
and the radon concentration in indoor air (Gesell and Pichard 1978 ;Hessetal.
1982 ). Exposure occurring during showering is often chosen as the critical pathway
for volatilization from contaminated domestic water. The relative source strength
from showers, considering overall volatilisation efficiency and water consumption,
is estimated to be higher for showers than other domestic sources (Howard-Reed
et al. 1999 ). During showering the volatile contaminants can evaporate from the tap
water and be inhaled with the water vapour or as fine aerosols. The evaporation rate
in a shower is high, since the water droplets have a high surface-volume ratio. Also
the elevated temperature of a normal shower will increase the evaporation rate. For
contaminants with a dimensionless Henry's Law constant greater than 1, more than
70% of the contaminant in the water is expected to evaporate (Moya et al. 1999 ).
Human exposure is calculated from the concentration of contaminant in the shower
stall and the exposure time. Showering may also lead to an increased contaminant
concentration in other rooms in the house.
Dermal uptake during showering or bathing has little influence on total human
exposure, but is considered in several models, e.g., the Dutch and Flemish mod-
els. Dermal exposure is determined by the concentration of the contaminant in the
water, the dermal absorption rate, the surface area of the skin that is exposed and the
exposure time.
11.4.2 Mathematical Equations
11.4.2.1 Consumption of Drinking Water
The modelling approach for consumption of drinking water is straightforward,
where the exposure is given by:
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