Civil Engineering Reference
In-Depth Information
relative risk range of 1 in 10,000 to 1 in 1,000,000. An MCL for a Category II or III
contaminant is often identical to the contaminant's MCLG.
EPA may require the use of a treatment technique instead of establishing an MCL
if it is not technically or economically feasible to monitor for the contaminant, such
as for viruses and parasites.
CURRENT DRINKING WATER STANDARDS
Inorganics
Table 1-5 summarizes the MCLs, health effects, and monitoring requirements for
inorganic contaminants. Refer to Chapter 2, ''Inorganic and Radionuclide Contami-
nants,'' for further discussion of health effects and international standards for these
same contaminants.
The action levels for lead and copper shown in Table 1-5 are measured at the tap
and must be met in 90 percent of the samples. All systems that exceed the lead or
copper action limits must provide corrosion control and public education. See Chapter
21, ''Distribution System Corrosion: Monitoring and Control,'' for a discussion of
corrosion control.
Organics
Table 1-6 summarizes the MCLs, health effects, and monitoring requirements for
organic contaminants in the two categories of Synthetic Organic Compounds (SOCs)
and Volatile Organic Compounds (VOCs). Chapter 3, ''Organic Compounds in Drink-
ing Water,'' provides a more detailed discussion of these compounds. At the time of
this writing, the MCLs for aldicarb and metabolites were pending reproposal.
Microbial Contaminants
Table 1-7 summarizes the MCLs, health effects, and monitoring requirements for
microbial contaminants. Chapter 4, ''Microbiology of Drinking Water,'' presents a
more detailed discussion of these contaminants.
EPA does allow variances to the coliform rule for nonpathogenic biofilms in the
distribution system. Biofilms are discussed further in Chapter 22, ''Water Quality Con-
trol in Distribution Systems.''
Under the 1989 Surface Water Treatment Rule, filtered water turbidities must be
maintained at less than or equal to 0.5 NTU in 95 percent of the samples collected
each month. The 1998 Interim Enhanced Surface Water Treatment Rule (IESWTR)
requires that surface water or groundwater systems under the influence of surface
waters that serve 10,000 or more must produce a turbidity of less than or equal to 0.3
NTU in 95 percent of the measurements taken each month. The Long Term 1, En-
hanced Surface Water Treatment Rule was proposed by EPA in April 2000 and applies
the same turbidity requirement to smaller systems (serving less than 10,000) as the
IESWTR (i.e., 0.3 NTU—95% of samples). This rule is expected to be promulgated
in early 2001. If the water system meets these turbidity criteria, it is assumed to achieve
the required two-log
Cryptosporidium
removal. Also, all systems are required to main-
tain a minimum disinfectant residual above 0.2 mg / L entering the distribution system.
