2.1 DRINKING WATER INGESTION
Use of contaminated
water as a source of domestic drinking water is evaluated for groundwater
and surface water transport pathways. For each of these transport pathways,
consideration is given to reductions of pollutant concentration during
processing in the water supply treatment plant (if present) and in transport
through the water distribution system to the exposed individuals. The surface
water pathway also includes estimation of losses of volatile chemicals
in transport between the point of entry to the surface water and the water
intake plant.
- Transport Medium:
- water concentration at water treatment plant, Cdwi, pCi/L or mg/L, expressed as
a 70-year average value
- Special Process:
- removal of pollutants during water treatment loss of pollutants (environmental degradation or radioactive decay)
during transport from the treatment plant to the exposure location (households) loss of pollutants during transport in the surface water body by volatilization.
- Exposure Factors:
- water ingestion rate, exposure duration, and averaging time
The average daily dose for the drinking water pathway for groundwater transport
is evaluated as follows for chemical pollutants:
(1)
where
Ddwi = average daily ingestion dose for the drinking water pathway for chemical pollutant i (mg/kg/d)
Cdwi = concentration of pollutant i at the pumping station or well for domes-tic water supply (mg/L or pCi/L)
TFi = water treatment purification factor giving the fraction of pollutant, i, remaining after treatment (dimensionless)
lgi = loss rate constant for pollutant i in confined water systems (d-1)
THdw = holdup time in transfer of water from the pumping station or well to the consumer (d)
Fdw = fraction of days per year that water is consumed (dimensionless)
Udw = drinking water ingestion rate (L/d)
EDdw = exposure duration for the drinking water pathway (yr)
BWdw = body weight of individuals exposed via the drinking water path-way (kg).
ATdwi = averaging time for drinking water exposure to pollutant i (yr).
The averaging time for noncarcinogenic chemicals is set to the exposure
duration, and the averaging time for carcinogenic chemicals is fixed at
70 years.
For radionuclide pollutants, the total dose
is evaluated, as follows, using the dose conversion factor to convert from
intake to dose (rem):
Ddwi = Cdwi TFi e (-lgi
THdw) Udw DFgi
Fdw EDdw 365.25
(2)
where
Ddwi = total lifetime dose for the drinking water pathway for radio-nuclide i (rem)
DFgi = dose conversion factor for ingestion of radionuclide i (rem/pCi ingested)
365.25 = units conversion factor (d/yr)
and other terms are as previously defined. Note that body weight
(BWdw) is not included in the equation for radionuclides and
that the averaging time (ADdwi) is fixed at 70 years.
A value of 70 kg is always used for radio-nu-clides because the radiation
dose conversion factors are based on the standard man parameters (70 kg).
For the surface water transport pathway, the average
daily intake is evalu-ated as for Equation (1) with the water concentration
being corrected for loss during transport between the entry point to the
surface water body and the pumping station. The water concentration
at the pumping station is defined as follows:
Cdwi = Cswi (0) e(-lsiTsw)
(3)
where
Cdwi= concentration of pollutant i at the pumping for domestic water supply (mg/L)
Cswi(0) = concentration of pollutant at the pumping station for domes-tic water, uncorrected for loss during transit in the surface water system (mg/L or pCi/L)
lsi = loss rate constant for pollutant i in surface waters (d-1)
Tsw = transit time in the surface water system between the point of entry and the pumping station (d).
The domestic water supply may come from either groundwater (Cgwi)
or surface water (Cswi) transport pathways.