Five test tubes on a water quality lab counter.

Water Quality

The Zone 7 Standard

Zone 7 continued to provide high quality treated drinking water to over a quarter million residents and businesses, in partnership with our retailers. Our water not only met, but often performed better than state and federal health standards to meet our own more stringent internal water quality targets.

22,451 Water Tests in 2020

5 Lab Staff

32 Different analytical methods

In the 2020-2021 fiscal year, we met major milestones in improving our infrastructure in order to maintain our commitment of not only meeting but performing better than regulatory compliance requirements to provide an additional margin of safety and address local concerns for taste, odor, and water hardness with new state-of-the-art facilities for ozonation. Ozone treatment is the technology of choice for disinfecting water, reducing chlorine-related byproducts, and killing even more pathogens than chlorine, making our water cleaner, safer and better tasting – straight from the tap.

How Ozone Treatment Works

  1. Ozone Molecules • Adding an electric spark to Oxygen (O2) creates supercharged Ozone (O3) molecules.
  2. Why? • Contaminants in water may include bacteria, viruses, and algal byproducts that impact taste and odor.
  3. Injection • Ozone is injected into the water as a gas at our new ozonation site in the Del Valle Water Treatment Plant. The Ozone destroys taste & odor causing chemicals and algal toxins in the water.
  4. Complete! • Ozonation leaves behind pure, high-quality water, with fewer disinfection byproducts than other disinfectants.

Significant Achievements in Water Quality

Startup of ozone process facility at Del Valle Water Treatment Plant

Construction of ozone addition and plant upgrades at Patterson Pass Water Treatment Plant

PFAS: Proactive monitoring and regulatory compliance

Per and Polyfluoroalkyl substances (PFAS), and more specifically Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), are chemicals that are prevalent in the environment, that were once commonly used in many consumer products. These chemicals are considered “contaminants of emerging concern” as they are being detected in ground and surface water throughout the environment. However, because their use is so common, there are multiple paths to human exposure in our everyday lives, not just water. PFAS may be found in food wrappers, carpet, rain jackets, or building materials used in the construction of your home. In short, PFAS are everywhere.

Over the past several years, the science on PFAS and its impacts to the environment and public health have prompted regulatory actions. The USEPA has a 70 nanograms per liter (ng/L) combined Lifetime Health Advisory for PFOS and PFOA and is moving forward with regulatory development for these two PFAS by 2024. The California State Water Resources Control Board has issued drinking water advisory levels for three PFAS (including PFOS and PFOA) so far and is pursuing advisory levels for six additional PFAS found throughout the state. The State Water Board is also in the process of developing Public Health Goals (PHGs) for PFOA and PFOS, which is the first step in establishing Maximum Contaminant Levels (MCLs) for these PFAS. Final PHGs are expected summer 2022 and once finalized, it will be approximately 2 to 3 years to set Maximum Contaminant Levels (MCLs).

State Regulatory Advisory Levels for PFAS (ng/L)*

Perfluorooctanesulfonic acid (PFOS)
Perfluorooctanoic acid (PFOA)
Perfluorobutanesulfonic acid (PFBS)
Notification Level
Response Level

*When a contaminant is found at concentrations greater than its advisory level, certain notification requirements and recommendations apply.

Zone 7 has been proactive in monitoring for PFAS in water supplies well before any requirements took effect because we know PFAS tend to accumulate in groundwater and are commonly found in groundwater sources throughout the developed world.  Fortunately, there have been no detections of PFAS in our imported surface water supplies, which make up the majority of the Tri-Valley region’s water.  In addition, two of our ten groundwater wells have not had any detections of PFAS either. However, eight groundwater wells have had some detection levels and one of the affected wells (Mocho 1) has been taken out of service and designated as a standby well for emergency use only.  Water from affected operating wells are blended and/or treated at existing facilities so they fall below the applicable Response Levels before distribution, ensuring that water delivered to our customers meets our high standards. 

In addition, Zone 7 completed a PFAS Treatment Feasibility Study in summer of 2020 and is in the process of planning and designing a new PFAS treatment facility at the Chain-of-Lakes (COL) wellfield to ensure compliance with MCLs for PFOA and PFOS in 2024.  

Zone 7 also completed a PFAS Potential Source Investigation Study in December 2020 to assist in characterizing the extent of PFAS across Tri-Valley’s groundwater basin and to identify potential sources of contamination.  At this time, there is no indication of a single source for this contamination, because of the widespread prevalence of PFAS in thousands of consumer products and because there are no known local manufacturing sites.  

Currently, Zone 7 is working on developing a groundwater contaminant transport model to further investigate how the PFAS plume could be moving in the groundwater basin under various operating scenarios and PFAS management tools.   

The safety of our community’s water supply is our number one priority, and our community can be assured that all water delivered to our customers is below Response Levels.

Zone In On
What You Can Do

Source water protection for surface water is everyone’s responsibility! You can help protect our surface water by disposing of trash properly and reporting spills, ensuring our waterways stay clean and free of pollutants.

Pink to yellow gradient styled button with a white arrow pointing left

Strategic Goals Update

Green to blue gradient styled button with a white arrow pointing right

Water Reliability