What Is a Cross Connection? Definition, Examples, and California Rules
A cross connection is any actual or potential link between drinking water and a non-potable source. See common examples and how California controls them.
Short answer: A cross connection is any actual or potential connection between a drinking water system and a source of liquid, gas, or other substance that is not from an approved water supply. A garden hose sitting in a bucket of weed killer, an irrigation system with no backflow preventer, and a private well tied into a home’s plumbing are all cross connections. In California, the State Water Board’s Cross-Connection Control Policy Handbook (CCCPH) defines the term in Section 3.1.1 and requires every public water system to find and control cross connections through a cross-connection control program.
Last reviewed October 30, 2026, against the April 21, 2026 revision of the CCCPH. Sources are listed at the end.
What is a cross connection, in plain terms?
The CCCPH definition (Section 3.1.1) says a cross-connection is “any actual or potential connection or structural arrangement” between a public water system, including piping on a customer’s premises, and any source or system containing something that is not from an approved water supply.
Two words in that definition matter most:
- Actual means the link exists right now, such as a pipe joining a well to the house plumbing.
- Potential means the link can be made, such as a hose that can be dropped into a tank or a removable spool piece that can be swapped in.
Chapter 2.1 of the handbook adds that bypass arrangements, jumper connections, removable sections, improperly installed swivel or change-over devices, and any other temporary or permanent device through which backflow can occur are also cross connections.
A cross connection is not a problem on its own until water flows the wrong way. That reversal is called backflow.
How does a cross connection contaminate drinking water?
Water normally flows from the water main to the customer. Backflow is the reversal of that flow. The CCCPH (Chapter 2.1) describes two ways it happens:
| Type of backflow | What causes it | Common triggers named in the CCCPH |
|---|---|---|
| Backsiphonage | Pressure in the water system drops below atmospheric pressure, so the system pulls water backward | Main breaks, flushing, pump failure, emergency firefighting demand |
| Backpressure | Pressure on the customer side is higher than pressure in the water system, so water is pushed backward | Heating, cooling, waste disposal, or industrial systems connected to the potable supply |
The handbook notes that siphon action has been shown to lift water almost 34 feet in a pipe, so a supply line installed above a tank can still pull from it under the right conditions. It also states that chemical and biological contaminants have caused illness and deaths in known backflow incidents, and that reported incidents are believed to be a small share of the total.
What are common examples of cross connections?
The examples below come from the CCCPH itself (Chapter 2.1, Section 3.2.2, and Appendix D) and from the State Water Board’s April 2026 change log, plus a few everyday situations that water systems commonly find during hazard assessments.
| Setting | Cross connection example | Why it is a risk |
|---|---|---|
| Home | Garden hose submerged in a bucket, pool, or pond, or attached to a chemical sprayer | Backsiphonage can pull the contents into the home’s plumbing and then the main |
| Home | Lawn irrigation system without a backflow preventer | Irrigation water sits in contact with soil, fertilizer, and animal waste |
| Home or farm | Private well, irrigation well, rainwater collection system, or backyard pond (an “auxiliary water supply”) | The quality of the auxiliary water is usually unknown, according to the State Water Board |
| Home | Home fire sprinkler system | Water can stagnate in fire lines; some low hazard residential systems need no assembly if they meet Section 3.2.2(e)(3) |
| Commercial building | Fire protection system using wetting agents, foam, antifreeze, or corrosion inhibitor | The CCCPH treats these as high hazard and requires at least an RP (Section 3.2.2(e)(1)) |
| Commercial building | Boiler, chiller, or cooling system connected to the potable supply | A source of backpressure named in CCCPH Chapter 2.1 |
| Restroom | Siphon-jet urinal or water closet, flushing rim slop sink | Submerged inlets named in CCCPH Chapter 2.1 |
| Dental office | Dental cuspidor or other water-connected equipment | Named as a submerged inlet (Chapter 2.1); dental offices with water-connected equipment are high hazard (Appendix D) |
| Industrial plant | Chemical or process vat with a submerged inlet | Process water can be pulled back into the supply |
| Car wash, laundry, dry cleaner | Water connected to soaps, solvents, and cleaning chemicals | All three are on the CCCPH high hazard list (Appendix D) |
| Recycled water site | Dual-plumbed building or landscape that uses recycled water | Recycled water interconnected with potable piping is air gap only (Appendix D, item 12) |
| Construction site | Temporary connection to a fire hydrant | On the CCCPH high hazard list (Appendix D, item 38) |
| Water system facility | Storage tank overflow connected to a sump or storm drain | Air gap only (Appendix D, item 40) |
Cross connections are not limited to factories. The CCCPH (Chapter 2.1) says submerged inlets are found on many common plumbing fixtures, and that some are hard to detect until a water level changes or someone extends a hose below a liquid surface.
What is the difference between a cross connection and backflow?
A cross connection is the place where contamination could enter. Backflow is the event in which water actually flows the wrong way through it. Control programs work on both sides: they find and remove cross connections where possible, and they install a backflow preventer where a cross connection has to remain (CCCPH Section 3.1.3(a)(4)).
What is the difference between high hazard and low hazard cross connections?
The CCCPH sorts every cross connection into one of three categories (Section 3.2.1(b)):
- High hazard: a cross connection that threatens the potability or safety of the public water supply. Materials that enter through it are contaminants or health hazards.
- Low hazard: a cross connection that does not threaten potability or safety but may adversely affect the aesthetic quality of the water. Materials that enter through it are pollutants or non-health hazards.
- No hazard.
The degree of hazard decides which backflow preventer is required. High hazard premises need an air gap (AG) or a reduced pressure principle assembly (RP) under Section 3.2.2(c), unless the handbook provides otherwise. Our guide to high hazard vs. low hazard cross connections walks through the full Appendix D list and the assemblies allowed for each.
How are cross connections controlled?
There are two broad approaches:
- Eliminate the cross connection. Disconnect the well, remove the hose, or cut the jumper. This is the first choice under CCCPH Section 3.1.3(a)(4), which requires water systems to eliminate cross connections “when possible.”
- Control it with an approved backflow preventer. When the connection has to stay, it is protected by one of the methods the CCCPH recognizes:
| Method | Abbreviation | What it is for |
|---|---|---|
| Air gap | AG | A physical vertical separation, at least twice the effective opening and never less than 1 inch (Section 3.1.1). The highest level of protection. |
| Reduced pressure principle assembly | RP (or RPDA, RPDA-II) | High hazard premises; protects against backsiphonage and backpressure |
| Double check valve assembly | DC (or DCDA, DCDA-II) | Low hazard cross connections only (Section 3.1.1) |
| Pressure vacuum breaker | PVB | Backsiphonage only, not backpressure (Section 3.1.1) |
| Spill-resistant pressure vacuum breaker | SVB | Backsiphonage only; may not be used for premises containment (Sections 3.1.1, 3.3.2(e)) |
Protection can be placed at the property line or at the hazard inside the building. Our article on containment vs. isolation backflow protection explains when each is used in California.
Testable assemblies are field tested at least once a year and after installation, repair, relocation, or depressurization for winterizing, and air gaps are inspected at least once a year (CCCPH Section 3.3.3). Property owners can find the practical steps in our backflow testing guide for California property owners.
Who is responsible for finding cross connections in California?
The public water system is. Under CCCPH Section 3.1.3, every public water system must implement and enforce a cross-connection control program. The program has to include hazard assessments of the service area, backflow protection, certified testers and specialists, assembly testing, recordkeeping, backflow incident response, public outreach, and coordination with local entities.
The handbook also says the water system is not responsible for abating cross connections that exist inside a customer’s premises (Section 3.1.3(a)). In practice, the water system identifies the hazard and requires protection, and the customer installs, tests, and maintains the assembly. If the customer does not comply, the water system’s rules must give it authority to act, such as denying or discontinuing service (Section 3.1.3(a)(1)).
Each water system writes down how it does this in its Cross-Connection Control Plan. See our cross-connection control plan checklist, and for the full rulebook, our CCCPH guide.
What should you do if you suspect a cross connection?
- Property owners: Do not remove or bypass an existing backflow assembly. Call your water supplier and ask whether your property needs protection. Keep hoses out of buckets, pools, and chemical containers.
- Facility managers: Walk your irrigation, fire, boiler, cooling, and process connections and compare what you find with your water supplier’s records.
- Water systems: Treat it as a hazard assessment trigger. A community water system must perform a new hazard assessment when there is evidence that activities or materials on a premises have changed, or when it concludes an existing assessment may no longer be accurate (Section 3.2.1(e)). If backflow may have occurred, the system must notify the State Water Board and local health agencies within 24 hours of the determination (Section 3.5.3(a)).
How Exodus helps
Exodus Water Services is based in Thousand Oaks and runs cross-connection control programs for public water systems across California. Certified Cross-Connection Control Specialists lead the hazard assessments, and every assembly, assessment, and test report is kept in Exodus Water Compliance, our cross-connection record system. Recent program work is listed under proven work.
Frequently asked questions
What is a cross connection in plumbing? It is any actual or potential connection between the drinking water piping and a source of non-potable liquid, gas, or other substance. California’s CCCPH also counts bypasses, jumper connections, removable sections, and improperly installed swivel or change-over devices as cross connections.
What is the most common cross connection? Garden hoses and lawn irrigation systems are among the most common cross connections found at homes. A hose left in a pool, bucket, or chemical sprayer can siphon its contents back into the plumbing when water pressure drops.
Is a cross connection illegal? Not by itself. A cross connection is allowed when it is protected by the backflow preventer the water system requires for that degree of hazard. An unprotected cross connection can lead the water system to require protection or, under its program rules, to discontinue service (CCCPH Section 3.1.3).
What causes backflow through a cross connection? Either backsiphonage, when system pressure drops from a main break, flushing, pump failure, or firefighting demand, or backpressure, when customer-side pressure from a pump, boiler, or process system exceeds system pressure (CCCPH Chapter 2.1).
Is a well a cross connection? A private well is an auxiliary water supply. The CCCPH lists sites with an auxiliary water supply as high hazard (Appendix D, item 8), whether or not the well is piped into the public system. The water system’s hazard assessment decides the protection required.
Who checks for cross connections? Your public water system, through hazard assessments that a certified cross-connection control specialist must review or conduct (CCCPH Section 3.2.1(h)).
What is the best protection against a cross connection? Eliminating it. When it must stay, an air gap is the highest level of protection, followed by a reduced pressure principle assembly. The required method depends on the degree of hazard.
Last reviewed: October 30, 2026.
Sources
- State Water Board, Cross-Connection Control Policy Handbook page
- State Water Board, Cross-Connection Control Policy Handbook, April 2026 revision (PDF): Chapter 2.1 and Sections 3.1.1, 3.1.3, 3.2.1, 3.2.2, 3.3.2, 3.3.3, 3.5.3, and Appendix D
- State Water Board, Changes to the CCCPH, April 2026 change log (PDF): auxiliary water supply examples
Not sure whether your system’s cross-connection program would hold up to a State Water Board review? Request a program review or call (805) 991-7333.