Drinking Water

Is My Water Safe?

The City of Rockville’s drinking water is safe, as set forth in the Environmental Protection Agency (EPA) regulations and adopted and enforced by the Maryland Department of the Environment (MDE). For the 2020 calendar year, the city’s water met or exceeded all water quality requirements.

The Water Quality Data Table lists all the drinking water contaminants that were detected. None of these contaminants exceeded the drinking water standards. This report will help to inform you about the quality of your water and includes details about where your water comes from, what it contains and how it compares to standards set by state and federal regulatory agencies.

Why are contaminants in my drinking water?

Drinking water, including bottled water, may reasonably be expected to contain small amounts of some contaminants. The presence of contaminants does not necessarily indicate that water poses a health risk. More information about contaminants and potential health effects can be obtained by calling the EPA Safe Drinking Water hotline at 1-800-426-4791.

The sources of drinking water (both tap water and bottled water) include rivers, lakes, streams, ponds, reservoirs, springs and wells. As water travels over the surface of the land or through the ground, it dissolves naturally occurring minerals, and can pick up substances resulting from the presence of animals or from human activity, including:

  • Inorganic contaminants, such as salts and metals, which can be naturally occurring or result from urban stormwater runoff, industrial or domestic wastewater discharges, oil and gas production, mining or farming.
  • Radioactive contaminants, which can be naturally occurring or the result of oil and gas production and mining activities. 
  • Microbial contaminants, such as viruses and bacteria that may come from wastewater treatment plants, septic systems, agricultural livestock operations and wildlife.
  • Pesticides and herbicides, which may come from a variety of sources such as agriculture, urban stormwater runoff and residential uses.
  • Organic chemical contaminants, including synthetic and volatile organic chemicals, which are byproducts of industrial processes and petroleum production, and can also come from gas stations, urban stormwater runoff and septic systems.

In order to ensure that tap water is safe to drink, the EPA prescribes regulations that limit the amount of certain contaminants in water provided by public water systems. Food and Drug Administration (FDA) regulations establish limits for contaminants in bottled water, which must provide the same protection for public health.

Do I need to take special precautions?

Some people may be more vulnerable than the general population to contaminants in drinking water. Immuno-compromised persons, such as persons with cancer undergoing chemotherapy, persons who have undergone organ transplants, people with HIV/AIDS or other immune system disorders, and some elderly and infants can be at risk from infections. These people should seek advice from their health care providers about drinking water. EPA and the Centers for Disease Control (CDC) issue guidelines on appropriate measures to reduce the risk of infection by cryptosporidium and other microbial contaminants. Call the EPA Safe Drinking Water hotline at 1-800-426-4791 for more information.

Additional information for lead

If present, elevated levels of lead can cause serious health problems, especially for pregnant women and young children. Lead in drinking water is primarily from materials and components associated with service lines and home plumbing. The City of Rockville is responsible for providing high-quality drinking water, but cannot control the variety of materials used in plumbing components. When your water has been sitting for several hours, you can minimize the potential for lead exposure by flushing your tap for 30 seconds to 2 minutes before using water for drinking or cooking. If you are concerned about lead in your drinking water, you may wish to have your water tested. Information on lead in drinking water, testing methods and steps you can take to minimize exposure is available from the EPA Safe Drinking Water Hotline at 1-800-426-4791 or at EPA website.

Where does my water come from?

Our primary source of water is the Potomac River. When Rockville’s water plant is not operating because of necessary improvements or maintenance activities, or in cases of regional drought, Rockville purchases water from the Washington Suburban Sanitary Commission (WSSC). In 2020, Rockville purchased about about 27 million gallons of water (approximately 1.6% of our annual production) from WSSC, which also receives its water from the Potomac River.

Source water assessment and its availability

MDE performed a source water assessment of the Potomac River as it applies to the Rockville water plant. The 2002 report may be obtained online or by contacting the Water Supply Program at MDE, 1800 Washington Blvd., Baltimore, MD 21230. You can also call 410-537-3589.

For more information visit the Maryland Source Water Protection Program.

For more information, please contact: 

Glenn Maggard, Water Plant Superintendent
Phone: 240-314-8556 • Email Glenn Maggard

Paper copies are also available in City of Rockville facilities, including City Hall and recreation centers.  If you would prefer a paper copy of the Drinking Water Quality Report mailed to your home, please call 240-314-8500.  Please share this information with all other people who drink City of Rockville water, especially those who may not have received this notice directly, (e.g. in apartments, nursing homes, schools and businesses).  You can do this by printing and posting this report in a public place and/or by distributing copies or the web address. Visit the Agenda Center for upcoming meetings of the Mayor and Council. The city provides numerous opportunities for public participation. For more details, visit Mayor and Council.

This report is required by the United States Environmental Protection Agency and the Maryland Department of the Environment.

Water Quality Data Table 

Water Treatment Plant Performance

Detected Regulated ContaminantsMCLG or MRDLGMCL, TT or MRDLHighest Level DetectedRange
Low / High
Is this a Violation?Likely Source of Contamination
Turbidity (NTU)NATT= / 0.17NoSoil runoff.
Lowest monthly % meeting limitNATT=0.3NA100% / NANo
Residual Chlorine (ppm)4TT>0.2NAMet all TT requirements.NoWater additive to control microbes.
Total Organic CarbonNATTMeasured MonthlyMet all TT requirements.NoNaturally present in the environment.

Detected Unregulated ContaminantsMCLGMCLAverageRange
Low / High
Is this a Violation?Likely Source of Contamination
HAA6Br (ppb)NANA
10.43.7 / 17.6NoByproduct of drinking water disinfection
HAA5 (ppb)
18.73.2 / 42.0NoByproduct of drinking water disinfection
HAA9 (ppb)

27.66.3 / 56.4NoByproduct of drinking water disinfection
Manganese (ppb)NANA
6.01.1 / 10.8NoNaturally present in the environment.

*Unregulated contaminants are those for which EPA has not established drinking water standards. The purpose of unregulated contaminant monitoring is to assist EPA in determining the occurrence of unregulated contaminants in drinking water and whether future regulation is warranted. Rockville’s testing detected only four of the 30 compounds included in the fourth round of unregulated contaminant monitoring. The detections were one metal and three haloacetic acid disinfection byproduct groups.

Inorganic ContaminantsMCLGMCLHighest Level Detected or AverageRange
Low / High
Is this a Violation?Likely Source of Contamination
Barium (ppm)220.040.04 / 0.04NoDischarge of drilling wastes; discharge from metal refineries; erosion of natural deposits.
Fluoride (ppm)
440.40.38 / 0.38NoErosion of natural deposits; water additive, which promotes strong teeth; discharge from fertilizer and aluminum factories.
Nitrate (ppm), measured as nitrogen101021.63 / 1.63NoRunoff from fertilizer use; leaching from septic measured as nitrogen tanks, sewage; erosion of natural deposits.
Synthetic Organic Contaminants Including Pesticiedes and Herbicides
Atrazine (ppb)33NDNA / NANoRunoff from herbicide used on row crops.

Water Distribution System

Total ColiformMCLGMCLHighest No. of Positive Total ColiformMCL Fecal Coliform or E. ColiTotal No. of Positive Fecal Coliform or E. Coli SamplesIs this a Violation?Likely Source of Contamination
% positive samples per month*05%000NoNaturally present in the environment.

*Minimum sampling frequency of 60 samples per month. 752 total samples tested. Met TT requirements.

Disinfectants and Disinfection ByproductsMCLG or MRDLGMCLAverage**Range
Low / High
Is this a Violation?Likely Source of Contamination
Residual Chlorine (ppm), measured as free chlorine / 1.1NoWater additive to control microbes.
Total Trihalomethanes (ppb)
NA80569.7 / 85.8NoByproduct of drinking water disinfection.
Haloacetic Acids (ppb)NA60317.7 / 42.3NoByproduct of drinking water disinfection.

**Regulatory compliance with some MCLs are based on running annual average of quarterly samples.

Metals at Consumer TapsMCLGAction Level90th PercentileNo. of Sites of Action LevelUnitsIs this a Violation?Likely Source of Contamination
Copper (ppm)

Corrosion of household plumbing systems; erosion of natural deposits.

Lead (ppb)015ND0ppbNoCorrosion of household plumbing systems; erosion of natural deposits.

Definitions Used in this Report

Unit Descriptions are as follows:






Nephelometric Turbidity Unit
Parts per million, or milligrams per liter (mg/L).  
1 ppm is similar to 1 ounce in 7,350 gallons of water.
Parts per billion, or micrograms per liter (µg/L).  
1 ppb is similar to 1 ounce in 7,350,000 gallons of water.
Not Applicable
Not Detected (by a test procedure)

Important Drinking Water Definitions

MCL - Maximum Contaminant Level: The highest level of a contaminant that is allowed in drinking water. MCLs are set as close to the MCLGs as feasible, using the best available treatment technology.

MCLG - Maximum Contaminant Level Goal: The level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs allow for a margin of safety for sensitive individuals.

MRDL - Maximum Residual Disinfectant Level: The highest level of a disinfectant allowed in drinking water. There is convincing evidence that addition of a disinfectant is necessary for control of microbial contaminants.

MRDLG - Maximum Residual Disinfection Level Goal: The level of a drinking water disinfectant below which no health risk is known or expected. MRDLGs do not reflect the benefits of using disinfectants to control  - Action Level: The concentration of a contaminant which, if exceeded, triggers treatment or other requirements that a water system must follow.

AL - Action Level: The concentration of a contaminant which, if exceeded, triggers treatment or other requirements that a water system must follow.

TT - Treatment Technique: A required process intended to reduce the level of a contaminant in drinking water.

How is my water treated? 

The City of Rockville’s Water Treatment Plant was put into service in 1958 and, at that time, was capable of producing 4 million gallons per day (MGD) of treated water. The plant was upgraded in 1967 to increase production to 8 million gallons per day. In the mid-1990s, and in 2017, additional upgrades to the plant were made to meet EPA and MDE regulations. Since then, an average of 5 million gallons per day of raw (untreated) water is withdrawn from the Potomac River, treated at the plant and distributed to the city’s water customers. Once at the plant, the water is put through a six-step treatment process to ensure it meets Safe Drinking Water Act standards. Once treated, the water is sent through a series of underground water lines and water storage tanks and to your faucet.

Water Quality Filter Diagram

The river water is treated to remove suspended sediments, algae, parasites, bacteria, metals and other contaminants through the following processes:


Water from the Potomac River is pumped through a screen to remove large debris such as sticks, leaves and rocks. If algae blooms are present in the raw water withdrawn from the river, it is treated with potassium permanganate.


Water is treated with chemical compounds that make small suspended particles stick together and settle out of the water. This particle conglomerate is removed from the water prior to filtration.


Water is passed through a settling basin or clarifier, allowing time for mud, sand, metals and other sediment to settle out.


Water is passed through a dual media (sand and anthracite) filter, which removes many remaining contaminants.


Chlorine is added to the water to kill and/or inactivate any remaining pathogens. Fluoride is added to prevent tooth decay and a corrosion inhibitor is added to preserve the pipes that deliver the water to homes and businesses.

To Homes and Businesses

The treated water is stored in two storage tanks and is gravity-fed to houses and businesses when needed. The water is sampled at the plant, in the distribution system and at the tap in homes and businesses for lead, copper, other potentially harmful contaminants, bacteria and residual chlorine.

Past Annual Drinking Quality Reports (PDF)