Besides being cited and stopped from driving, what additional liability might a driver face for hauling liquid in a leaking tank?

AAMVA CDL Manual §8.1.1, p.8-8-1

Spill cleanup costs for hazardous or bulk liquids can run into the thousands of dollars, well beyond the cost of a citation. A driver who understands this personal financial exposure has a much stronger incentive to catch a leak before it ever leaves the yard.