9.5 Chemigation, Backflow Prevention & Safe Mixing/Loading Sites
Key Takeaways
- South Carolina's Chemigation and Backflow Prevention laws (SC Regulations 27-1090 through 27-1092) require four anti-siphon safeguards on every chemigation system: a positive-closing check valve, a vacuum breaker, a low pressure drain, and a mechanical or electrical interlock between the injection pump and the irrigation pump.
- The vacuum breaker and the low pressure drain must both be installed upstream of the check valve — that is, between the check valve and the water source — and their position relative to each other does not matter.
- The low pressure drain outlet must discharge at least 20 feet from the water source, sit at least two inches above grade, and be built so the top of the drain is flush with the bottom of the irrigation pipe so all contaminated water drains out.
- Chemigation records must be kept for two years and must show the type of chemical applied, the date, the rate applied, the site, and the water source; Clemson Regulatory Programs may assess a civil penalty of up to $500 per offense per day of noncompliance.
- A properly designed mixing and loading site keeps the operation at least 100 feet from wells and surface water or on an impervious pad with a sump, maintains an air gap of at least twice the fill-hose diameter above the tank rim, and never leaves a filling tank unattended.
Chemigation, Backflow Prevention & Safe Mixing/Loading Sites
Almost every documented case of a pesticide reaching a drinking-water well traces back to one of two moments: a chemigation system that let treated water flow backward into the source, or a fill hose left in a spray tank while the operator walked away. Both are engineering failures, and both are fully preventable with equipment South Carolina specifically requires.
Chemigation is defined in SC Regulation 27-1090 D as any process by which chemicals are applied to land or agricultural commodities — including crops, nursery stock, turf, golf courses, and greenhouse sites — through an irrigation system. It is regulated under South Carolina's Chemigation Act and Backflow Prevention Act, enforced by Clemson Regulatory Programs through the Department of Pesticide Regulation.
[!NOTE] One important carve-out: irrigation systems connected to public water supplies are regulated under the State Safe Drinking Water Act and the State Primary Drinking Water Regulations, and must meet those requirements in lieu of the chemigation rules. Only EPA or the Director may approve, in writing, alternative chemigation safety equipment.
1. The Four Required Anti-Siphon Safeguards
An anti-siphon device is any equipment designed to prevent backflow of chemicals into a water supply. South Carolina requires four working components on a chemigation system, and 27-1092 A states that none of them may be altered in any way that negates its effectiveness. All four must be built of chemical-resistant materials, kept free of corrosion and build-up, and remain operative at all times when chemigating.
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| SC REQUIRED CHEMIGATION SAFETY EQUIPMENT (27-1092) |
| |
| 1. CHECK VALVE ---> Positive-closing. Provides absolute closure |
| so that when the irrigation pump stops, no |
| material can flow back past the valve. |
| |
| 2. VACUUM BREAKER ---> Automatically relieves vacuum in the |
| irrigation pipeline so the line cannot |
| siphon backward. |
| |
| 3. LOW PRESSURE DRAIN ---> Self-activated. Automatically drains the |
| pipe section when water pressure drops due |
| to a check-valve malfunction. |
| |
| 4. INTERLOCK ---> Mechanical or electrical connection so that |
| if the IRRIGATION pump stops, the INJECTION |
| pump stops too. |
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The Installation Order Is Tested — Learn It
Regulation 27-1092 B fixes the layout, oriented by the direction of water flow:
WATER SOURCE ---> [ VACUUM BREAKER ] ---> [ CHECK VALVE ] ---> FIELD
[ LOW PRESSURE ] ^
[ DRAIN ] |
(both UPSTREAM of the chemical injection
check valve; order enters here
between them is
unimportant)
- The low pressure drain and vacuum breaker must be placed upstream from the check valve — between the check valve and the water source.
- Their position relative to each other is unimportant.
- Chemical is injected on the downstream (field) side of the check valve, so the valve stands between the injected chemical and the water source.
Detailed Component Requirements
| Component | South Carolina Requirement (27-1092) |
|---|---|
| Check valve | Must be positive closing. Must be flanged and bolted to the irrigation pipe, or otherwise removable for servicing — welded-in check valves that cannot be removed for inspection and servicing are prohibited. Seals and discs must be maintained; a Regulatory Programs agent's determination that a part needs replacement is final. |
| Low pressure drain | Should be connected to a hose or pipe that prevents contaminated water from draining back into the water source. The drain outlet must be at least 20 feet from the water source. The drain outlet on the irrigation pipeline should be at least two inches above grade to allow the drain line to be connected. The top of the drain must be flush with the bottom of the irrigation pipe so that all contaminated water in the pipe actually drains out. |
| Vacuum breaker | Automatically relieves vacuum in the irrigation pipeline; installed upstream of the check valve. |
| Interlock | Mechanical or electrical, so the injection pump cannot continue running after the irrigation pump shuts down. |
Gravity Flow and Venturi Systems
Where chemigation is accomplished by gravity flow or by a venturi, 27-1092 H requires one of three additional safeguards so that chemical cannot continue moving after the system shuts down:
- A check valve requiring positive head to open — at least 10 psi cracking pressure — installed on the chemical supply line on the discharge side of the injection pump, so it opens only when the irrigation line is adequately pressurized; or
- An automatically operated valve on the chemical supply line on the intake side of the injection pump, opening only when the irrigation line is pressurized and preventing leakage from the supply tank at shutdown (if electrically operated, loss of power at the irrigation pump must shut it down); or
- An elevated loop of chemigation pipe between the irrigation pipe and the chemigation tank, with the top of the loop higher than the top of the chemigation tank and a vacuum relief valve at the top of the loop to stop flow in either direction if the venturi draw stops.
2. Chemigation Records, Inspection & Penalties
- Records: Chemigation application records must be kept for two years and produced to a Regulatory Programs agent on request. Each record must show the type of chemical applied, the date, the rate of chemical applied, the site, and the water source. These entries may be satisfied as notations on the chemical purchase invoice or in production logs.
- Label compliance: All chemical label requirements for chemigation must be followed. If a label says "Do not apply this product through any type of irrigation system," chemigating that product is a label violation regardless of how well the equipment is plumbed.
- Contamination reporting: Regulatory Programs must be notified of any actual or suspected contamination of the water source resulting from chemigation.
- Inspection authority: Agents may enter any public or private site where chemigation equipment is used, at a reasonable time, by consent or warrant, to inspect equipment, sample water reported to be exposed to chemicals, and sample the chemicals being applied.
- Penalty: After an opportunity for a hearing, the Director may assess a civil penalty of not more than $500 for each offense, per day of noncompliance.
3. Designing a Safe Mixing and Loading Site
Mixing and loading is the phase where the applicator handles undiluted concentrate, and it accounts for the large majority of serious handler exposures and point-source water contamination events.
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| SAFE MIX/LOAD SITE CHECKLIST |
| |
| [X] At least 100 feet from wells, streams, ponds, and ditches --- OR --- |
| on an impervious containment pad with a collection sump |
| [X] Level ground; never a slope that channels a spill toward water |
| [X] Backflow prevention on the water source (anti-siphon device, or |
| fill from a nurse tank rather than directly from the well) |
| [X] AIR GAP maintained: fill hose outlet held at least 2 hose diameters |
| above the tank rim, never submerged in the spray mix |
| [X] Tank NEVER left unattended while filling |
| [X] Full PPE for concentrate handling: gauntlet gloves, chemical apron, |
| splash goggles and/or face shield, boots, long sleeves |
| [X] Spill kit, clean water, soap, and a change of clothing on site |
| [X] Rotate the mix/load location or use a pad to avoid building a |
| contaminated "hot spot" in one place |
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The Air Gap
The simplest and most reliable backflow protection at the fill point is a physical air gap: hold the discharge end of the fill hose above the tank opening so that it can never touch the liquid surface. The accepted standard is a gap of at least twice the inside diameter of the fill hose measured from the hose outlet to the rim of the tank. A 2-inch hose therefore needs at least a 4-inch gap. An air gap cannot corrode, stick, or fail — but it only works if the hose is never dropped into the tank and the tank is never left to overfill unattended.
Rinsate and Excess Spray
- Rinsate from triple-rinsing containers and from tank cleanout is a dilute pesticide, not waste water. Add it to the spray tank and apply it to a labeled site at or below the labeled rate.
- Never discharge rinsate to a storm drain, ditch, septic system, or bare soil at the fill point.
- Mix only what the job requires. The partial-tank arithmetic in Section 9.4 exists precisely so that no leftover mix has to be disposed of.
[!WARNING] The classic South Carolina scenario: an operator drops the fill hose into a half-full spray tank at the wellhead and walks off to answer a phone call. The well pump cycles off, pressure drops, and the tank's contents siphon back down the hose into the well casing. That single unattended minute can contaminate a domestic well permanently. Either maintain the air gap, or install a functioning anti-siphon device — and stay with the tank.
Under SC Regulation 27-1092, where must the vacuum breaker and the low pressure drain be installed on a chemigation system?
A chemigator installs a check valve by welding it permanently into the irrigation pipeline and routes the low pressure drain so it discharges 8 feet from the irrigation well. Which statement identifies the violations?
An applicator is filling a 300-gallon sprayer from a farm well using a 2-inch inside-diameter fill hose. What is the minimum air gap that must be maintained, and what is the second non-negotiable rule at the fill point?