It is hard to find much written on the subject of Fed interdistrict imbalances. Virtually everything I know comes from this great blog http://jpkoning.blogspot.com/search?updated-max=2012-03-17T10:42:00-07:00&max-results=7 (see the Feb. 5th entry in particular). In short, according to the Fed's accounting manual, it settles imbalances that build up between the districts every in April by taking the average imbalance over the course of the year and then shifting SOMA securities from the debtor districts (biggest: Richmond) to the creditor districts (biggest: NY). There was some speculation last year that the Fed had not fully settled (hard to know for sure because cannot totally isolate the data) the imbalances because they were concerned about the Richmond Fed getting close to negative equity after transferring SOMA securities per the Fed's formula. In any event settlement just happened this year and as you can see the imbalances have almost entirely disappeared.
Richmond's balance sheet shows that post its roughly $90 billion SOMA transfer it still has over $186 billion in SOMA securities so its safe from dreaded negative equity for the time being.