I got that same email from the milk farmer this morning. And while its something to be aware of, the fact is that unless you have enough food stored to get you through the fallout period (which nobody can predict), everything grown in the Northern hemisphere and the pacific rim is going to be affected. That means all animals slaughtered after this point, all dairy products, all vegetation. Because even if its grown in a greenhouse, the soil and the water are contaminated, as are the people handling the produce, the boxes, the warehouses, etc.
Its not as simple as not joining a CSA and avoid contamination.
And yes, im part of a produce CSA, a meat CSA and a milk CSA.