A recent study by the Environmental Working Group found that the weedkiller glyphosate was present in all 28 cereal and cereal bars made with oats that that they tested. And they tested the most popular ones. In 26 of these samples the levels were really high.
The reason is that glyphosate is used to kill the plant and dry it out to make harvesting easier. The way to markedly diminish this exposure is to eat organic oats. And a recent study confirms that people who eat organic food get fewer cancers.
But if you or your neighbors use glyphosate on your yard and garden, it will wind up in our drinking water. The way to keep it out of our drinking water is by not using it. Why is it bad to have glyphosate in our drinking water (or its breakdown product which is just as biologically active)? Because it has been linked to numerous diseases including non-Hodgkins lymphoma. A recent verdict in California upheld the judgement against Monsanto in the case of a man who developed non-Hodgkins lymphoma following long-term exposure to glyphosate. The court found that the company knew how dangerous the chemical was and hid the information, much like tobacco. They however decreased the amount of the judgement to 78 million dollars.
If you have lots of dandelions, instead of spraying them with glyphosate, improve the soil and outcompete the weeds with grass or other plants. On a lawn the most common cause of lots of dandelions in an area is that the ground is compacted and also possibly deficient in calcium. Aerate now, in the fall, when weed seeds are not as much of a problem. Overseed with grass seed that will grow well in your area to outcompete the weeds. And test your soil, replace calcium if necessary. If you are just not going to test it, spread a thin layer of compost over the area. Good compost should replete the calcium and help support a healthy soil biota.