9.3 Chemigation & Backflow Prevention
Key Takeaways
- Chemigation applies a pesticide through an irrigation system, and its defining hazard is backflow - contaminated water siphoning or flowing back into the water source, which can contaminate a well or a surface water body with concentrated product.
- The federal safety package is an interlock system: a functional irrigation-line check valve with a vacuum relief and a low-pressure drain, an injection-line check valve, an injection-line normally closed solenoid, and interlocking controls that shut the injection pump off when the water pump stops.
- Only products whose labels expressly permit chemigation may be applied that way, and the label directions for chemigation are mandatory - applying a product through irrigation contrary to the label is a use inconsistent with the labelling.
- A physical air gap - a discharge pipe ending at least twice its diameter above the maximum water level of the receiving tank - is the most reliable backflow prevention and is also the standard for filling a spray tank from a well or hydrant.
- Chemigation applications in Arkansas are pesticide applications like any other - the applicator must be licensed and the record obligations apply - and the Worker Protection Standard Application Exclusion Zone is sized by the spray quality the emitters produce and the release height, not by a chemigation-specific rule.
9.3 Chemigation & Backflow Prevention
Core Concept: Chemigation - applying a pesticide, fertilizer or other chemical through an irrigation system - is efficient on large irrigated acreage and eliminates a separate equipment pass. Its defining hazard is not drift or exposure. It is backflow: if the water pump stops while the injection pump keeps running, or if a vacuum forms in the mainline, the concentrated chemical solution can be drawn backward into the water source. A single backflow event into an irrigation well can contaminate an aquifer with a slug of concentrate.
Arkansas irrigates heavily - rice, soybeans, cotton and corn across the Delta run on wells drawing from a shallow alluvial aquifer. That is the setting where backflow is least forgiving.
Which Products May Be Chemigated
Only products whose labels expressly permit chemigation. A label that is silent on chemigation is not permission; many labels carry the explicit statement "Do not apply this product through any type of irrigation system." Where chemigation is allowed, the label carries a mandatory chemigation section specifying the system type - sprinkler, drip, flood or furrow - and the required safety equipment. Those directions are enforceable label language, not guidance.
The Safety Equipment Package
Federal labelling requires an interlocked set of devices. Learn them as a system, because the exam tests what each one prevents.
| Device | Where it goes | What it prevents |
|---|---|---|
| Functional, automatic, quick-closing check valve | Irrigation main line, between the pump and the point of pesticide injection | Water and chemical flowing back toward the water source |
| Automatic, low-pressure drain | On the main line, on the water-source side of the check valve | Chemical pooling against the check valve; it drains any leakage away from the water source |
| Functional vacuum relief valve | Main line, water-source side of the check valve | A vacuum forming in the line and siphoning solution backward |
| Injection line check valve | On the injection line | Chemical flowing from the supply tank into the irrigation line by gravity or siphon when the system is off |
| Normally closed solenoid-operated valve | Injection line, on the intake side of the injection pump, interlocked to the water pump | Chemical being drawn into the system when the water pump stops or loses power |
| Interlocking controls | System controls | The injection pump continuing to run when the water pump shuts off - the interlock stops both together |
| Metering pump | Injection point | Uncontrolled injection rate; the pump must be positive-displacement and constructed of materials compatible with the pesticide |
The logic chain is worth memorizing as a sentence: the check valve stops backward flow, the vacuum relief stops siphoning, the low-pressure drain keeps leakage away from the source, the injection check valve and solenoid stop gravity feed, and the interlock stops the injection pump when the water pump stops.
The Air Gap
The single most reliable backflow prevention device has no moving parts. An air gap is a physical vertical separation between the discharge end of the supply pipe and the maximum water level of the receiving vessel. The standard is that the gap must be at least twice the diameter of the discharge pipe, and never less than one inch.
An air gap cannot fail mechanically, cannot be defeated by a stuck valve, and requires no maintenance. It is the correct answer whenever a question asks for the most reliable protection.
This applies to routine spray tank filling, not just chemigation. Filling a spray tank by putting the hose end down into the tank creates a direct path for tank contents to siphon back into the well or hydrant if pressure drops. Either keep the hose end above the tank's maximum liquid level by twice the hose diameter, or fit an approved anti-siphon device. Better still, fill from a nurse tank rather than directly from the water source, and never leave a filling tank unattended - an overflow at a wellhead is a groundwater contamination event.
Operating a Chemigation Application
- Read the label's chemigation section and confirm the system type matches.
- Inspect every safety device before injection begins - check valve, vacuum relief, low-pressure drain, injection check valve, solenoid, and the interlock. A failed interlock is a stop-work condition.
- Bring the irrigation system to full, uniform operating pressure and confirm distribution uniformity before injecting.
- Calibrate the injection rate. The product must be metered so that the labelled rate per acre is delivered over the area the system covers in the time it runs. In practice: determine the acres the system covers per hour, multiply by the per-acre rate to get product per hour, and set the metering pump to that output. Verify by timed catch from the injection line.
- Inject only while water is flowing, and flush the system with clean water after the chemical is exhausted while irrigation continues, so no concentrate is left standing in lines or emitters.
- Stay with the system. An unattended chemigation run is how a failed device becomes a contamination event.
- Post the field as the label and WPS require.
Worker Protection and Records
- Application Exclusion Zone: 40 C.F.R. Sec. 170.405(a)(2) does not name chemigation as its own trigger. Classify a chemigation application the way you would any other spray - by the spray quality the emitters produce and the release height. A sprinkler system producing a spray quality smaller than medium falls in the 100-foot AEZ; a medium or larger spray released more than 12 inches above the planting medium falls in the 25-foot AEZ. Either way the applicator must suspend the application if anyone other than a trained and equipped handler enters the zone.
- REI applies to the treated field exactly as it would for a conventional application.
- Records: a chemigation application is a pesticide application. The Arkansas record obligations under Ark. Code Ann. Sec. 20-20-215 and any applicable class-specific record fields apply, and the applicator must hold the appropriate licence.
[!WARNING] Exam Trap: The Interlock Is the Answer to "The Water Pump Stopped" A very common item describes a power failure or a water pump shutting down while the injection pump keeps running. The device that prevents disaster is the interlock between the injection pump and the water pump, backed by the normally closed solenoid on the injection line intake. The main-line check valve stops flow toward the water source, but it is the interlock that stops the injection pump from continuing to push concentrate into a static line.
During a center pivot chemigation run, the electric water pump loses power while the chemical injection pump continues to operate. Which device or arrangement is specifically designed to prevent this scenario from becoming a contamination event?
An applicator fills a spray tank by running a hose from an irrigation well and dropping the hose end into the tank so it will not fall out. What is the hazard and the correct practice?
A product label states: "Do not apply this product through any type of irrigation system." A grower wants to chemigate it anyway because the field is already under a pivot and the ground is too wet to drive. What is the correct analysis?