Herbicide Near a Brook, a Well, and a Rain Forecast
You advise a small vegetable farm on a 6-acre field that slopes 5% toward a brook. The soil is loamy sand with 1.5% organic matter and a seasonal high water table at 3 feet; a farm well is 60 feet downslope. You plan a preemergence herbicide with water solubility 300 mg/L, Koc 80 mL/g, and soil half-life 30 days. The forecast calls for 1.5 inches of rain within 24 hours. The label requires a 25-foot vegetative buffer from surface water and prohibits application when runoff is likely. A granular alternative has lower solubility but must be incorporated, and the field is already seeded.
1. Assess the relative risk of runoff versus leaching and justify your conclusion with the product and site properties.
Read the worked answer
Both risks are real, with runoff and dissolved transport especially high right now.
- Solubility 300 mg/L: high enough that a substantial fraction can dissolve in runoff water.
- Koc 80 mL/g: low sorption, so the herbicide is weakly bound to soil and moves readily with water.
- Loamy sand with 1.5% organic matter: low sorption capacity and high infiltration, which increases both downward movement and the chance that rain will drive the product toward the water table.
- 5% slope and a brook downslope: runoff energy and a short travel path to surface water.
- 1.5 inches of rain in 24 hours: a runoff-producing event likely to occur before the herbicide has dissipated; half-life 30 days means it will still be active.
- Well 60 feet downslope and water table at 3 feet: the same low-Koc product can move toward groundwater.
The 25-foot buffer is only a label minimum; with this slope, soil, and forecast it is not enough to call the application safe.
2. Recommend whether to apply today and identify the most defensible mitigation if the grower insists on using this product.
Read the worked answer
Do not apply today. The label prohibits application when runoff is likely, and the 1.5-inch rain forecast plus slope makes runoff likely. Applying before that rain would also increase leaching risk.
If the grower insists on this active ingredient, the defensible options are:
- Wait until the forecast clears and the soil is not saturated, then apply with a wider buffer (for example, 50–100 feet) and a vegetative strip.
- Reduce the rate to the minimum labeled effective rate and use banding rather than broadcast where practical.
- Avoid application on the downslope edge nearest the brook and the well.
- Consider the granular alternative only if incorporation can be done without increasing downward movement; incorporation reduces surface runoff but may increase soil contact and leaching on this sandy site.
- Split applications or use a cover crop to reduce erosion and runoff.
Record the decision. The label minimum is not a safe-harbor number when site conditions point the other way.
3. What would you monitor and document after the application?
Read the worked answer
Monitor: the brook edge for injured vegetation, the farm well for the herbicide or its degradates if testing is available, soil moisture and rainfall, and any ponding or eroding channels. Watch for phytotoxicity in sensitive crops and off-site plants.
Document: product and EPA registration number, rate, date and time, soil temperature and moisture, wind, forecast, buffer width, application method, equipment calibration, and any visible runoff. Keep a map showing the brook, well, buffer, and treated area.
Judgment point: if runoff occurs, stop further application, notify the state if required, and investigate before blaming the product. The records should show whether the label's runoff prohibition was honored.