|A dedicated fleet that just performs Hydro-Excavating is very important. Multi-functional equipment is not always good for this application. The problem with multi-functions is that those other jobs usually come with the truck, especially when material is to be left onsite.
Let us study this scenerio, a vacuum truck is out vacuuming oily sludges. When the job is complete, the truck goes to a waste treatment facility to dump the truck. After dumping, the worker and his truck are given a set amount of time to wash out the truck, otherwise they will be charged an extra washout fee.
Chances are good that the truck is not completely washed out, not to mention the interiors of the vacuum tubes and hoses--they're still contaminated. Now, this truck shows up on your site to Hydro-Excavate, and to leave not only the water and dirt slurry, but oily sludge from a different site. What do you do? Pay an extra fee to have the material disposed off site or dump it there on your site and hope nobody ever performs an environmental study on your property?
Protect yourself, when in doubt, ask the contractor to open the debris body and inspect for yourself. Also, look at the vacuum tubes, if they are grease- and oil- covered, chances are they are that way inside too.