Heavy metals in ag waste

Am trying to get a handle on what I need to worry about on high density dairy ag waste. Typical phosphate limits are 40 ppm before limits come into play for rate of application. State ag dept allowing 200 ppm without imposing restrictions.
Am not so concerned about phosphate, as they say it does not impact crop productivity. Am concerned about the heavy metals present (sodium, selenium, copper, etc) which may be also getting over concentrated and diminish the long term productivity of the ground. Any research available on this?
Thanks for the help.