Since you are able to carefully remove the cover and expose the nest I see two options. Both would require that you be wearing the appropriate protective clothing of "bee" hood, bee gloves, bee suit or some other thick clothing that prohibits the wasps from getting under it and to your skin. Regardless of whether you treat the nest chemically or non-chemically, getting that close to a nest of stinging wasps calls for protecting yourself.
Polistes dominulus is the European Paper Wasp, and these imported problems really like making their nests in just such a location - small cavities. We find them in bird houses and bird feeders, mail boxes, porch lights, piping outdoors, and here in California under the edges of Spanish tiles on roofs. Their nests may often be smaller than those of the native paper wasps that they seem to be displacing in the U.S. and they are not terribly aggressive, but having them around structures still poses that threat of stings.
If you can approach this at dusk there is a good chance that all of the adult wasps are going to be on or near the nest, and while wearing your protective clothing you could physically remove the nest for disposal. The problem I see with this is that just removing the nest does not do anything to kill all the adult wasps, so the problem could easily start again, so on to option two.
I think insecticides are called for, but how to ensure nothing ends up in the water below. Since I cannot properly picture the situation is it possible for you to remove this cover and move it slightly away from over the well tube? If so you then could treat the nest with a directed aerosol wasp "freeze" that would instantly kill all adult wasps. Again, do this late in the day when the wasps are most likely to be back on that area. You could use a product like EcoExempt Jet Wasp and Hornet which contains plant-based active ingredients of rosemary oil and hexa-hydroxyl, which you still wouldn't want dripping down into the well but which could dry quickly before replacing the cover.
If this well sits adjacent to the structure you might also look at the label of Termidor SC to see if you could use this product at that point. It is labeled now for paper wasps and can be applied to points where the wasps "enter" the structure including up to 1 foot away from the foundation. Termidor is working very well to kill social wasps that have hidden nests, but removing the nest physically is always a good idea. If the cover cannot be moved away for treatment is it possible for you to cover the tube leading down into the well first so that you then can spray the nest but prohibit any insecticide from dripping down into the well? If you can and then use one of the very short-lived plant derived products like pyrethrum or the EcoExempt product the active ingredients should break down very rapidly to prevent any chance of their being present later.
And , always discuss these options with the customer so they understand the nature of the materials you intend to use.
Mr. Pest Control
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Please note, Mr. Pest Control is answering questions supplied by PMP customers across North America. His answers are generated from industry and manufacturer-provided information. The answer may not be specific to the laws and regulations for your State, Province, Territory or Country. In addition, products mentioned may not be registered and or available in all areas. Always check with your local Univar office for specific information to your area. Always read and follow label directions.