I believe this is goutweed, snow on the mountain, or bishops weed (Aegopodium podagraria).  It is in the same family as water hemlock and looks very similar.  I can tell the difference by looking at the leaf veins.  Also, water hemlock needs to be in the water to grow.  Goutweed is very common in ornamental flower beds.  Many people use it as a ground cover.  It can also be variegated.  It can be a pain to get rid of.  It forms colonies from underground rhizomes.  

Plants in the carrot or Apiaceae family can cause contact or photodermatitis if you get the sap on your skin.  If you plan to remove this plant I would long sleeves and gloves just to be safe.