I asked my sister, who is wise in the ways of chemistry about the numbers in the article. She said that “Ug/L is roughly part per billion. So if they applied a million pounds of retardant they’d get (number * 0.001) pounds of that metal.”
So the middle of the road number from their report for, say, lead, would be 0.375 pounds of the heavy metal per million pounds of retardant. About half a bag of split-shot fishing weights, or a box of .308 ammo.
Some of the highest numbers on the chart are for manganese, but even 7 parts per million is much lower than the average amount in normal soil (20-3,000 ppm).