I personally would not worry about it unless you're pouring Roundup into your well (others may differ). It gets adsorbed by soil particles and biodegrades. If you want to find information, you'll get lots by using a Google search for "glyphosate 'half-life'". Caveat emptor -- some info is from Monsanto, but you can find stuff from The Nature Conservancy, which uses it selectively in conservation areas and others as well. And of course, be sparing in your use.

If you use a glyphosate product properly, it will never even touch the soil. You only need a light misting over the actively growing plants in order for the product to be translocated to the roots. Drenching to drip-off is not required and is considered improper use.

Although I know this is quite late, Glyphosate won't kill poison ivy, and at most would spot and maybe kill a couple leaves but do nothing to the vine. You specifically need a brush and poison ivy killer, but as was mentioned, only spray the leaves and branches so that it doesn't spill all around. Also don't spray if you are expecting heavy rains as it could wash off with the rain water.

You may want to stick with Rodeo or Accord, they are glyphosphate only products. However, you may need to do several applications on a woody vine like poison ivy in order for it to be effective. And if you take care to make sure the product is only being applied to the poision ivy and not hte ground around it, you are less likely to contaminate your ground water.

An good article about glyphosphate, gives some suggestions on how to apply it safely:

Here is a link that might be useful: Round up Article

Your child will get poison ivy, ticks and chiggers during its life if the child is a boy or very active. You can only minimize the problem.

One of the things you should consider is how deep is the well and what kind of casing does it have. Is the casing only at the top of the well or does it extend 25 ft or more. The deeper the casing the less likely the water will be severly contaminated. Do as Rizo suggests and spray a fine mist wait a week or so then hit the plants again. Do not spray when there is a breeze as this can spread the poison on things that you do not want injured like yourself. Wear protective gloves at least.