Water - A Branch of Public Utilities

Water Quality Data

Detected Disinfection By-Products, Disinfectant Residual and Disinfection By-Product Precursors

All drinking water must be disinfected during treatment to ensure that potentially dangerous microbes are killed. There are a variety of disinfection strategies used throughout the United States. San Diego utilizes some of the more advanced disinfection technologies available. Our Miramar and Alvarado treatment plants use ozone for disinfection, in addition to chloramines. Ozone produces less organic disinfection byproducts than chlorine or chloramines alone and thus is considered a superior disinfection method. All disinfectants have some sort of byproduct, however. When ozone is used, bromate is monitored as a potential disinfection byproduct. Our Otay Treatment Plant uses chlorine dioxide for disinfection, in addition to chloramines. When chlorine dioxide is used, chlorite is monitored as a potential disinfection byproduct. All of our results for bromate and chlorite are below the MCL established by CDPH.

In order to prevent growth of potentially dangerous microbes as drinking water travels from our treatment plant through our distribution system to your home or business, a disinfectant residual must be maintained. San Diego uses chloramines as the residual disinfectant in the distribution system. We analyzed 7,039 samples for chloramines throughout the distribution system in 2013; the average residual was 2.2 ppm and the maximum was 4.1 ppm.

Another category of disinfection byproducts that CDPH and the EPA regulate are Trihalomethanes (THMs) and Haloacetic Acids (HAAs). In 2012 San Diego began monitoring these under EPA's Stage 2 Disinfection ByProduct (DBP) rule. Compliance with the MCL under the Stage 2 rule is based on the running annual average at each location in the distribution system, rather than the system-wide running annual average previously used. The MCL for THMs is 80 ppb, and the MCL for HAAs is 60 ppb. San Diego has had no violations of the Stage 2 DBP MCLs to date. In 2013 our highest LRAA for Total Trihalomethanes was 66, and the range of individual measurements was 27.6 - 86.2 ppb. For Haloacetic Acids, our highest LRAA was 15, and the range of individual measurements was 4.3 - 20.4 ppb.

	UNITS	MCL [MRDL]	PHG (MCLG) [MRDLG]	CDPH DLR	CITY OF SAN DIEGO TREATMENT PLANTS						MWD SKINNER TREATMENT PLANT		MAJOR SOURCES IN DRINKING WATER
					ALVARADO		MIRAMAR		OTAY		MWD SKINNER TREATMENT PLANT
					AVERAGE	RANGE	AVERAGE	RANGE	AVERAGE	RANGE	AVERAGE	RANGE
Bromate*	ppb	10	0.1	5	ND	ND - 7.0	ND	ND - ND	NA	NA	5.9***	1.0 - 11	By-product of drinking water disinfection
Chlorate**	ppb	NL=800 PPB		20	n/a	n/a	n/a	n/a	135	63.1- 239	51	28-72	By-product of drinking water disinfection
Chlorite**	ppm	1	0.05	0.02	n/a	n/a	n/a	n/a	0.28	ND - 0.53	NA	NA	By-product of drinking water disinfection
Total Organic Carbon [TOC]	ppm	TT	n/a	0.3	2.7	2.0 - 3.7	2.4	2.1 - 2.7	4.2	2.5 - 5.3	2.2***	2.1 - 2.4	Various natural and manmade sources
Required for Alvarado, Miramar, and Skinner Required for Otay ** Highest Running Annual Average

	UNITS	MCL [MRDL]	PHG [MRDLG]	DLR	CITY OF SAN DIEGO DISTRIBUTION SYSTEM				MAJOR SOURCES IN DRINKING WATER
Disinfectant Residual [Chloramines as Cl2]	ppm	[4]^A	[4]	----	Distribution system average³ = 2.2		Range³ =ND - 4.1	----	Drinking water disinfectant added for treatment
Chlorite¹	ppm	1	0.05	0.02	Distribution system average³ = 0.24		Range³ = ND - 0.44	----	By-product of drinking water disinfection
Haloacetic acids [HAA5]	ppb	60^B	n/a	----	Maximum LRAA = 15		Range³ = 4.3 - 20.4	LRAA Violation - NO	By-product of drinking water disinfection
Total Trihalomethanes [TTHMs]	ppb	80^B	n/a	----	Maximum LRAA = 66		Range³ = 27.6 - 86.2	LRAA Violation - NO	By-product of drinking water chlorination
¹ Chlorite monitoring required only in the Southern section of the distribution system. ³ Range and average are based upon individual 2013 sample results.						^B Total Trihalomethane and HAA5 compliance is based on quarterly Locational Running Annual Averages (LRAA) ^A Compliance is determined by the distribution system average.