Is contractual support capped at a percentage of the total grant?

No, there is no contractual cost cap. In fact, we commonly see projects that are 100% contractual.  Ensure that you describe in your project narrative, budget narrative, and workplan how the proposed contractor support clearly supports the overall accomplishment of listed goals and outputs and meets business or administrative needs.  Applicants should also explain how the management of that contact will be handled, in the case where 100% of grant funds will go towards a contract, as EPA will not be paying for personnel or fringe costs for a staff member to manage this contract.