I harvested about 4lbs of wild watercress from my spring fed pond. When wash the watercress, I saw small black worms about 1/8 to 1/4 “. On a second wash, I put some potassium permanganate power and wash again. Then rinse with clear water.
My question is : is that safe to make soup out of this wild watercress?
According to the sefer Bedikas Hamazon Khilchoso (R’ M. Vaye shlit”a) pg. 365, watercress has two types of infestation, the small worms that you were describing, and also very small (1-2 mm- 1/10-1/20”) clear bugs from the water that are clear in color. Therefore it is very difficult to know with certainty that it is indeed clean. He also says that he doesn’t know of any certain way of cleaning the watercress. Therefore he recommends that we should not eat it.
Here is a link to what he says regarding watercress http://dinonline.org/Bedikat/watercress/