Couple of points to consider.
(1) Did the sampling detect contaminated soil or contaminated soil gas. If the contamination is simply groundwater contamination, this may not be a significant issue for you. There are defenses to liability for property owners whose property is impacted by contaminated groundwater from an off-site source so long as you exercise what is known as "reasonable care".
(2) since you will be using this for self-storage, this will be considered a commercial, not residential use, and many states have less stringent standards for commercial properties since people are not occupying the property 24/7.
(3) If the levels in the groundwater would pose a risk of vapor intrusion (vapors migrating up from groundwater), you can install a passive sub-slab depressurization system (ssds) that acts like a radon system-capturing the vapors before they can get into the structures.
(4) if soil is contaminated and you plan to pave the surface, this will generally be sufficient remedy if the soil contamination is at depth. If is is shallow, you may want to develop a soil management plan to make sure soils are properly managed.
(5) much of the foregoing depends of the state where the site is located. what state is involved. It is usually best to go through a state voluntary cleanup program so you can get a no further action letter that will assuage concerns of any lender and also provide you with liability protection.
(6) if the concern is solely groundwater, you could also get an environmental insurance policy.