The documentation specifies:

readConcernLevel=String => :read_concern => Hash

This documentation should be expanded to explain how the hash is built from the string. Currently one has to read the :read_concern documentation to figure this out.

The :read_concern documentation states:

Specifies the read concern options. The only valid key is level, for which the valid values are :local, :majority, and :snapshot.

However, actual driver behavior is different:

irb(main):025:0> Mongo::URI.new('mongodb://test.host/?readConcernLevel=snapshot').uri_options
=> {"read_concern"=>:snapshot}

The expected result is {read_concern: {level: :snapshot}}.