A small submarine is exploring 1 mile from its base. It develops a slow leak, allowing water to enter at a rate of 7 liters per minute. The submarine has an emergency system that can pump out water, but it can only withstand a total accumulation of 40 liters of water before critical systems fail. The submarine navigates back to base at a constant speed of 3 miles per hour. What is the minimum rate, in liters per minute, that the emergency system must pump out water to prevent critical failure?