Best practices include using leakproof, portable washout containers with liners, signage, and access controls

https://wiki-mixer.win/index.php/From_Slurry_to_Safety:_Concrete_Washout_Management_for_Small_Projects

Best practices include using leakproof, portable washout containers with liners, signage, and access controls, positioned away from drains and sensitive areas to contain sediment and caustic residues.