Always good to be concerned about such things, IMHO. Given the depth of your well, I don't see an issue with occasional, spot usage. We've been on well for 20 years and have our thoroughly tested every few years. We have not seen any significant changes in our water quality as a result of judicious use of fertilizers or typical lawn chemicals. But, I am always careful to use the minimum necessary to get the job done.

If you had a shallow well or were tapping off a spring, the risks would be different, of course.

The active ingredients are Imazapyr, and Glyphosate. The Glyphosate is completely harmless to ground water and breaks down to simple salts in the soil withing 72 hours.

Imazapyr short story is it does not contaminate ground water, but From the Employment Prevention Agency says:

To estimate drinking water concentrations resulting from the use of imazapyr, screening level models were used. Non-crop uses with high and low application rates, and corn uses were modeled to represent the labeled imazapyr uses (1.5, 0.9, and 0.014 lbs. a.i./acre, respectively). The highest labeled rate for imazapyr is 1.5 lbs. a.i./acre.

The Agency has determined that the residue of concern for imazapyr in drinking water is parent only. Environmental fate data suggest that imazapyr is mobile and persistent. Except for photolysis in water, imazapyr was stable under the conditions and duration of the submitted fate studies. In the photolysis study, imazapyr degraded with half-lives of approximately 3 to 5 days.

To predict concentrations of imazapyr that may be present in surface water as a result of the terrestrial uses, Tier I FQPA Index Reservoir Screening Tool (FIRST) exposure modeling was performed. The modeled estimates of drinking water concentrations (EDWCs) of imazapyr in surface water for chronic durations range from 0.34 to 79üg/L. These values were established by modeling imazapyr use on corn and non-crop uses with high and low application rates.

To predict concentrations of imazapyr in ground water as a result of terrestrial uses, Tier I Screening Concentration in Ground Water (SCI-GROW) exposure modeling was performed. The modeled concentrations of imazapyr in ground water are not expected to exceed 36üg/L. This value was established by modeling imazapyr non-crop uses at the highest maximum application (1.5 lbs a.i./A).

Exposure to imazapyr from drinking water resulting from aquatic applications is also possible. The EDWCâÂÂs for both surface and ground water from direct application to surface water are both 61 üg/L. This does not take into account the current imazapyr label requirement of a one-half mile setback from drinking water intakes because the Agency does not currently have an approved methodology for calculating EDWCs in water bodies where pesticides are applied with a setback distance from drinking water intakes. As a result, the EDWC is more conservative than had setback distances been considered. Direct applications to water were modeled assuming uniform application over an entire reservoir at the maximum labeled rate.