4.4 Surface Water Protection: Runoff, Erosion, Buffers, and Aquatic Applications

Two Different Contamination Clocks

Chapter 4.1 dealt with leaching — a slow, vertical, dilution-limited process that can take years to show up in a well and decades to clear. Surface water contamination runs on a different clock entirely. A single runoff event during a thunderstorm four hours after application can put a measurable pulse of active ingredient into a trout stream the same afternoon, kill fish overnight, and be gone from the water column before anyone samples it.

That difference explains why Wisconsin's surface water rules are written as absolute prohibitions on conduct rather than as numeric standards. You will not find a "parts per billion in a stream" trigger in ATCP 29 the way you find one in the atrazine groundwater program. You will find a rule that says: do not put it there.

ATCP 29.50(4): Contaminating Waters of the State

The core prohibition is short and broad. No person may:

1. Cause a pesticide to enter the waters of the state, either directly or through a sewer system; or
2. Use a pesticide in any manner which the user knows or should know will result in contamination of the waters of the state.

Subparagraph 2 is the one applicators underestimate. It creates a "knew or should have known" standard, which means an applicator who sprays a field with a saturated soil profile, a forecast of two inches of rain, and a stream at the bottom of the slope is exposed even if the label says nothing about that scenario.

The rule then lists five — and only five — exemptions:

Exemption 5 rewards close reading. It protects the applicator whose properly labeled application is moved by a genuinely unforeseeable event. It does not protect an applicator who sprayed into a forecast storm — that runoff was foreseeable, which puts it squarely back under the "should know" prohibition.

The Runoff and Erosion Pathways

Pesticides reach surface water by three mechanisms, and each has different controls.

+-----------------------------------------------------------------------------+
|                 THREE ROUTES TO SURFACE WATER                               |
|                                                                             |
|  [1] DISSOLVED RUNOFF   Water-soluble a.i. carried in solution over the      |
|                         soil surface. Worst for HIGH-solubility, LOW-Koc     |
|                         chemistry -- the same profile that leaches.          |
|                                                                             |
|  [2] SEDIMENT-BOUND     Strongly adsorbed a.i. carried on eroding soil       |
|      EROSION            particles. Worst for HIGH-Koc chemistry that does    |
|                         NOT leach (e.g., many pyrethroids, chlorpyrifos).    |
|                                                                             |
|  [3] DIRECT ENTRY       Drift onto water, overspray at field edges, spills,  |
|                         rinsate dumping, equipment washing near a stream.    |
+-----------------------------------------------------------------------------+

Route 2 is the counterintuitive one. A chemical with a high soil adsorption coefficient (Koc) is a poor leacher precisely because it binds tightly to soil — but that binding means it travels wherever the soil travels. On a sloping, tilled Wisconsin field, an erosion event can deliver a strongly bound insecticide to a stream even though that same product would never reach groundwater.

Controls that actually work:

• Time applications against the forecast. Most runoff loss occurs during the first significant rainfall after application, and loss declines sharply as the interval lengthens.
• Maintain untreated vegetative buffers between treated ground and surface water. Many labels specify the buffer width; where the label is silent, a buffer sized to the slope, soil, and product is still good practice and evidence of care.
• Reduce erosion. Conservation tillage, residue cover, contour operations, grassed waterways, and terraces reduce sediment-bound transport more effectively than any product substitution.
• Respect setbacks near sinkholes, tile inlets, and grassed waterways. In Wisconsin's karst regions a surface tile inlet is effectively a direct pipe to groundwater, which is why the same feature shows up in both the surface water and groundwater rules.
• Never wash equipment near water. Rinsate is a concentrated pesticide solution.

Filling and Cleaning Equipment: ATCP 29.46

ATCP 29.46(1) prohibits three things outright:

• Immersing any pesticide application equipment, mix tank, or nurse tank in the waters of the state;
• Filling any of that equipment from the waters of the state; and
• Cleaning or filling any of it near surface waters or near any well site, if pesticide spillage or overflow may contaminate waters of the state.

ATCP 29.46(2) provides the workable alternatives:

The air gap is the single most testable number here, because it is a physical dimension you can check on a fill nozzle: twice the outlet's effective opening, minimum one inch. An air gap works by making backsiphonage physically impossible — there is no continuous column of liquid for a vacuum to pull back through.

Aquatic Applications and ch. NR 107

Applying a pesticide to water to manage or control aquatic organisms — aquatic weed control, lake and pond management, mosquito larviciding in water, fish management — is a different regulatory world. It runs through the Wisconsin DNR under ch. NR 107, which requires a permit for chemical aquatic plant management, and it is the activity contemplated by the DNR-supervision exemption in ATCP 29.50(4)(b)2.

Two ATCP 29 provisions carve out space for it:

• ATCP 29.45(2)(a) exempts NR 107 aquatic applications from the spill containment requirement.
• ATCP 29.46(2)(e) permits filling a water supply tank from surface waters for an NR 107 application.

On the certification side, Wisconsin covers this work in Category 5.0 Aquatic & Mosquito and Category 5.1 Antifouling Paints. An applicator certified only in Category 1.1 or 3.0 cannot legally treat a lake, and holding an ATCP 29 certification does not substitute for the DNR permit.

Aquatic Toxicity: Why the Margins Are So Thin

Fish and aquatic invertebrates are often orders of magnitude more sensitive than mammals to the same compound. Aquatic toxicity is reported as LC50 (concentration in water killing 50 percent of test organisms, usually over 96 hours for fish or 48 hours for invertebrates), expressed in parts per million or parts per billion. Many pyrethroid insecticides carry aquatic invertebrate LC50 values in the parts-per-trillion to low parts-per-billion range, which is why a label may prohibit application within a stated distance of water even when the same product is Toxicity Category III for humans.

The practical implication: human toxicity category is a poor predictor of aquatic hazard. A CAUTION product can be devastating to a stream. Read the Environmental Hazards statement, not the signal word, when water is nearby.