To protect public health, the U.S. Environmental Protection Agency and the State Water Resource Control Board (DDW) will commonly use the following definitions to standardize water quality information.

  • Maximum Contaminant Level (MCL): The highest level of a contaminant that is allowed in drinking water. Primary MCLs are set as close to the PHGs (or MCLGs) as is economically and technologically feasible. Secondary MCLs are set to protect the odor, taste and appearance of drinking water.
  • Maximum Contaminant Level Goal (MCLG): This level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs are set by the U.S. Environmental Protection Agency.
  • Public Health Goal or PHG: The level of a contaminant in drinking water below, which there is no known or expected risk to health. PHGs are set by the California Environmental Protection Agency.
  • Maximum Residual Disinfectant Level (MRDL): 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.
  • Maximum Residual Disinfectant Level Goal (MRDLG): The level of a drinking water disinfectant below which there is no known or expected risk to health. MRDLGs do not reflect the benefits of the use of disinfectants to control microbial contaminants.
  • Primary Drinking Water Standard or PDWS: MCLs and MRDLs for contaminants that affect health along with their monitoring and reporting requirements, and water treatment requirements.
  • Treatment Technique (TT): A required process intended to reduce the level of a contaminant in drinking water.
  • Picocuries per Liter (pCi/L): Measurement commonly used to measure radionuclides in water.
  • Nephelometric Turbidity Unit (NTU): A measure of clarity of water. Turbidity in excess of 5 NTU is just noticeable to the average person.
  • Milligrams per Liter (mg/L): Or parts per million (ppm) corresponds to 1 penny out of $10,000.
  • Micrograms per Liter (µg/L): Or parts per billion (ppb) corresponds to 1 penny out of $10,000,000.
  • Nanograms per liter (ng/L): Or parts per trillion (ppt) corresponds to 1 penny of $10,000,000,000.
  • microSiemens per centimeter (μS/cm): A measure of conductivity.
  • Threshold Odor Number (TON): A measure of odor.
  • State Regulatory Action Level (AL): Concentration of a contaminant which, when exceeded, triggers treatment or other requirements that a water system must follow.
  • N/A: not applicable.
  • ND: not detected.
  • NL: notification level.
  • DLR: Detection Level for Purposes of Reporting.
  • Drinking Water Source Assessment and Protection (DWSAP): Source assessment program for all District water sources.
  • IDSE: Initial Distribution System Evaluation
  • Running Annual Average (RAA): The yearly average which is calculated every 3 months using the previous 12 months’ data. 
  • Local Running Annual Average (LRAA): The RAA at one sample location. 
  • Disinfection By-Product: Compounds which are formed from mixing of organic or mineral precursors in the water with ozone, chlorine, or chloramine. Total Trihalomethanes and Haloacetic Acids are disinfection by-products. 
  • Secondary Drinking Water Standard (Secondary Standard): MCLs for contaminants that do not affect health but are used to monitor the aesthetics of the water. 
  • Notification Level (NL): Health-based advisory levels established by the State Board for chemicals in drinking water that lack MCLs.
  • 90th Percentile: The value in a data set in which 90 percent of the set is less than or equal to this value. The Lead and Copper Rule uses the 90th percentile to comply with the Action Level.

(Significance of Results)

The District has listed the following as a health risk informational guide only. Health risk assessments are based upon exceeding a Maximum Contaminant Level (MCL). The State Board allows us to monitor for some contaminants less than once per year because the concentrations of these contaminants do not change frequently. Some of our data, though representative, are more than one year old. Nitrate is routinely sampled within District wells annually. None of these routine nitrate samples exceeded the MCL. Perchlorate was detected in seven (7) groundwater sources. Six of these sources have treatment systems installed for Perchlorate removal and the remaining well has values below the MCL. 

Arsenic: While your drinking water meets the current EPA standard for arsenic, it does contain low levels of arsenic. The standard balances the current understanding of arsenic’s possible health effects against the costs of removing arsenic from drinking water. The U.S. Environmental Protection Agency continues to research the health effects of low levels of arsenic, which is a mineral known to cause cancer in humans at high concentrations and is linked to other health effects such as skin damage and circulatory problems. 

Nitrate: Nitrate (as nitrogen) in drinking water at levels above 10 mg/L is a health risk for infants of less than six months of age. Such nitrate levels in drinking water can interfere with the capacity of the infant’s blood to carry oxygen, resulting in a serious illness; symptoms include shortness of breath and blueness of the skin. Nitrate levels above 10 mg/L may also affect the ability of the blood to carry oxygen in other individuals, such as pregnant women and those with certain specific enzyme deficiencies. If you are caring for an infant, or you are pregnant, you should ask advice from your health care provider.



In 2002, the District, in partnership with the San Bernardino Valley Water Conservation District, conducted Source Water Assessments of all our drinking water wells. No contaminants have been detected above the Maximum Contaminant Levels (MCL) set by State Water Resources Control Board; however sources are considered most vulnerable to the following:

Fecal Coliform and E. Coli Bacteria in our Source Water Supply. Heavy recreational activities in both Lytle Creek and Lake Silverwood during warm summer months increase the vulnerability.

Methyl Tertiary Butyl Ether (MTBE) sources located near gasoline service stations and underground gas storage tanks are vulnerable. A MTBE plume is leaching from the Colton Gasoline Storage Terminal. Two (2) District Wells are located south of the Terminal. Well Nos. 40 and 41 are sampled monthly. No MTBE has ever been detected in these wells or any other District Well.

VOC & SOC Chemicals tested in all District groundwater wells were determined to be vulnerable to both Volatile Organic Chemicals (VOC’s) and Synthetic Organic Chemicals (SOC’s).

Perchlorate has been detected at low levels in seven (7) groundwater wells (Nos. 11, 16, 17, 18A, 41, 42 and Rialto Well No. 6). Six of these wells are primary water sources and have treatment systems installed. It is believed that the likely sources for Perchlorate originate from former manufactures of rocket fuel/fireworks and fertilizer. Well Nos. 16, 17, 18A, and 42 have Ion Exchange Systems installed for Perchlorate removal. The Fluidized Bed Biological Reactors (FBR) remove Perchlorate from Well No. 11 and Rialto Well No. 6.

Nitrate in some groundwater wells are vulnerable. Nitrate contamination is the result of leaching septic systems and past citrus farming.

Cryptosporidium is a microbial pathogen found in surface water throughout the U.S. Although filtration removes Cryptosporidium, the most commonly-used filtration methods cannot guarantee 100 percent removal. Our monitoring indicates the presence of these organisms in our source water and/or finished water. Current test methods do not allow us to determine if the organisms are dead or if they are capable of causing disease. Ingestion of Cryptosporidium may cause Cryptosporidiosis, an abdominal infection. Symptoms of infection include nausea, diarrhea, and abdominal cramps. Most healthy individuals can overcome the disease within a few weeks. However, immune-compromised people are at greater risk of developing life-threatening illnesses. We encourage immune-compromised individuals to consult their doctor regarding appropriate precautions to avoid infection. Cryptosporidium must be ingested to cause disease, and it may be spread through means other than drinking water.

Completed Source Water Assessments may be viewed at the District Office located at: 
855 West Base Line, Rialto, California 92376. 



Drinking water, including bottled, may reasonably be expected to contain at least 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 USEPA’s Safe Drinking Water Hotline (1-800-426-4791).

Some people may be more vulnerable to contaminants in drinking water than the general population. Immunocompromised persons such as people with cancer undergoing chemotherapy, people who have undergone organ transplants, people with HIV/AIDS or other immune system disorders, some elderly, and infants can be particularly at risk from infections. These people should seek advice about drinking water from their health care providers. USEPA/Centers for Disease Control (CDC) guidelines on appropriate means to lessen the risk of infection by Cryptosporidium and other microbial contaminants are available from the Safe Drinking Water Hotline (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 in some cases, radioactive material and can pick up substances resulting from the presence of animals or from human activity.

Contaminants that may be present in source water include:

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

In order to ensure that tap water is safe to drink, US EPA and the State Water Resources Control Board (DDW) prescribe regulations that limit the amount of certain contaminants in water provided by public water systems. The State Water Resources Control Board (DDW) also establishes limits for contaminants in bottled water that must provide the same protection for public health.