Regulation and Risk Assessment of 1,4-Dioxane - Environmental Investigation and Remediation - page 271

in question because data from mechanistic studies are largely inconsistent. Similarly,
information regarding tumor susceptibility following early-life exposures and exposures
later in life is not yet available for 1,4-dioxane. Therefore, CDPHE concludes that the pos-
sibility of having different modes of action during different periods of life cannot be ruled
out for 1,4-dioxane. This uncertainty for 1,4-dioxane also justii es using the linear low-
dose extrapolation method, because a nonlinear model could underestimate risk by dispro-
portionately reducing risk with dose reductions.

Finally, CDPHE supports its standard by noting that available evidence does not support the
assumption that tumors will only develop if the exposure is high enough to saturate metabolic
detoxii cation (CDPHE, 2004).

6.1.2.2 California's Notifi cation Level
The California Environmental Protection Agency's Ofi ce of Environmental Health Hazard Assess-
ment (OEHHA) publishes notii cation levels (NLs) to provide water utility operators with a thresh-
old for announcing detections of contaminants in drinking water for which MCLs have not yet been
established. Prior to 2004, California NLs were called action levels. When chemicals are found at
concentrations greater than these levels, water utilities are encouraged to notify consumers.
The NL is usually set at the theoretical lifetime risk of up to one excess case of cancer in a popu-
lation of 1,000,000 people—the 10 −6 risk level. If the chemical is not detectable as low as the NL by
available laboratory analytical methods, detectability prevails.
The California Department of Public Health (CDPH) regulates drinking water utilities. It recom-
mends that wells in which unregulated contaminants have been detected be taken off-line when the
chemical's concentration is 10 times the NL, if based on noncancer endpoints, or 100 times the NL,
if based on cancer risk established at the 10 −6 risk level (CDPH, 2007a). A level 100 times the NL
corresponds to a theoretical lifetime risk of up to one excess case of cancer in 10,000 people.
A California water utility may continue to operate a well or other water source following detec-
tion of an unregulated contaminant. To continue using a well with detections of 1,4-dioxane or other
unregulated contaminants, the utility must notify the local governing body (i.e., the city council) to
advise that water exceeding the chemical's NL is being served to customers. CDPH also recom-
mends that water system operators directly notify all the water system's customers and issue a press
release advising that the contaminant is present in drinking water at a concentration greater than its
NL. Following notii cation, CDPH recommends that water system operators proceed with (1)
monthly sampling and analysis of the drinking water supply for as long as the contaminant exceeds
its NL and (2) quarterly sampling for a year after the concentration drops below the NL (CDPH,
2007a). Although the guidance for notii cation of water utility customers following detection of an
unregulated contaminant is in the form of recommendations, in practice, most California water
utilities are inclined to place wells with detections on standby to protect their customers.
California NLs are calculated by using standard risk-assessment methods for noncancer and
cancer endpoints and typical exposure assumptions, including a 2 L/d ingestion rate, a 70 kg adult
body weight, and a 70-year lifetime. For noncarcinogenic chemicals, the NL is derived from the
No Observed Adverse Effect Level (NOAEL—the highest dose at which none of the animals in a
test population displays a toxic effect), adjusted by uncertainty factors (UFs). An estimate of drink-
ing water's contribution to total exposure to the contaminant, the RSC, is also included. The com-
monly used RSC is 20% of the total exposure originating from the ingestion of contaminated
drinking water.
The California Action Level or NL for carcinogens is calculated as

R
qV

BW ¥

(6.1)

NL = ,

* ¥

1 day