To me:
-The
safeness of BFWDCS car wash shampoo, or any other car wash shampoo for that matter,
depends on its
chemicals'-ability of breaking-down the contaminates that lay upon your vehicle's surface areas.
Of course...These contaminates will then be gently whisked away by a water source.
-Any of your car-wash sessions' effluents reaching the soil, where your arborvitae live, could possibly alter its ph.
-Do you know:
The proper ph recommended for the soil that your particular arborvitae are to be planted?
-Your arborvitae's current soil ph?
Also...Please keep in mind...
Besides car-washing sessions' run-off...
-The type of mulch used, if any.
-Airborn contaminates.
-Other waterborn contaminates
...Can also alter soil's ph.
OH...Less I forget...
Another arborvitae-hating contaminate:
Cat urine!!
Bob