Concrete Washout

Hello everyone, we are working on a project in an estate, part of this estate is the current LEED project, with its own LEED boundary. The estate has many projects going on and the construction company which works exclusively for the project owner, has its own concrete manufacturing plant within the estate, but outside de LEED boundary. The plant has a concrete washout facility that is well documented and that they use for all the projects they are working on. They fill the concrete mixer trucks with water inside the LEED boundary and do the washout when they arrive to the plant, a few miles away. Will this practice be OK with the ESC Plan requirements? A concrete washout area for small hand or electric mixers has been provided on site. Thanks