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National Drinking Water Database


National Drinking Water Database - Chemical Contaminants

The Most Polluted Communities

25,794 water utilities reported detecting Total trihalomethanes (TTHMs) in tap water since 2004, according to EWG's analysis of water quality data supplied by state water agencies

Ranked by population

RankSystem Population Served Positive test results of total reported tests Average Level
(Range)
1Metropolitan Water District of Southern California
Los Angeles, CA
17,000,0003 of 334.89 ppb
(30.38 to 43 ppb)
2New York City Department of Environmental Protection- Catskill/Delaware
Corona, NY
6,552,71825 of 2537.33 ppb
(11.6 to 57.89 ppb)
3Los Angeles Department of Water and Power
Los Angeles, CA
3,828,700268 of 30334.44 ppb
(0 to 88.6 ppb)
4Chicago Department of Water Management
Chicago, IL
2,896,01628 of 2816.11 ppb
(9.4 to 27.05 ppb)
5City of Houston Public Works
Houston, TX
2,700,00047 of 5120.11 ppb
(0 to 69.3 ppb)
6San Francisco Regional Water
San Francisco, CA
2,500,0001 of 130.31 ppb
(30.31 ppb)
7Miami-Dade Water & Sewer Department
Miami, FL
2,100,00039 of 3918.66 ppb
(2.61 to 42.99 ppb)
8Massachusetts Water Resources Authority
Boston, MA
2,000,000153 of 15446.83 ppb
(0 to 132 ppb)
9Washington Suburban Sanitary Commission
Silver Spring, MD
1,800,000102 of 10231.8 ppb
(0.53 to 65.15 ppb)
10Philadelphia Water Department
Philadelphia, PA
1,600,00044 of 4444.62 ppb
(13.5 to 90 ppb)
11Baltimore City Department of Public Works
Baltimore, MD
1,600,000109 of 10944.5 ppb
(19.5 to 90.2 ppb)
12Cleveland Division of Water
Cleveland, OH
1,500,000121 of 12122.47 ppb
(5.2 to 45.8 ppb)
13East Bay Municipal Utility District
Oakland, CA
1,300,0001 of 128 ppb
(28 ppb)
14San Diego Water Department
San Diego, CA
1,281,40066 of 6655.37 ppb
(32.97 to 81.43 ppb)
15San Antonio Water System
San Antonio, TX
1,219,11331 of 3515.38 ppb
(0 to 110.5 ppb)
16City of Phoenix Water Services Department
Phoenix, AZ
1,200,00010 of 1638.47 ppb
(0 to 95.88 ppb)
17Dallas Water Utilities
Sunnyvale, TX
1,188,58016 of 1632.11 ppb
(15.23 to 67.4 ppb)
18Las Vegas Valley Water District
Las Vegas, NV
1,181,2632 of 262 ppb
(62 ppb)
19Missouri American Water- St. Louis County/ St. Charles County
St Louis, MO
1,049,6002 of 212 ppb
(<0.01 to 24 ppb)
20Suffolk County Water Authority
Bay Shore, NY
1,009,26468 of 744.38 ppb
(0 to 14.5 ppb)