So I had them dug , they filled with ground water in 4 hours , inspector failed them .

I can't imagine you'd want to take it as-is with the tank, because if you try to remove it after buying and find the soil is contaminated, and possibly contaminated the ground water, you could be looking at a bank-breaker of a job.  

If you aren't afraid of the soil or groundwater being contaminated from chemicals that were spilled or dumped onto the land, or disposed of in a septic system, then it sounds like the deal of the century. 

I do know that there have been stricter rules and guidelines regarding water runoff and such since much of our ground water eventually impacts the Chesapeake Bay. 

He said the leak could be either from groundwater or from someone else's line. 

Most surface owners here make money through damage payments from the mineral owners, by leasing the property for cattle grazing, selling fresh water for oilfield operations (groundwater is tied to surface estate in TX), or building commercial facilities (man camp, RV parks, etc) depending on accessibility.

Oil tank removal and remediation should be the seller's responsibility, not yours, UNLESS you're a very experienced investor getting a tremendous deal sold strictly "as is" where you've waived all your inspection rights.The worst case could be significant soil contamination requiring thousands of dollars in remediation, followed by soil and ground water tests by the state of NJ.

Does it have area's that the rain water or ground water pools and doesn't drain off real well?

The issue if you buy without first having the tank removed is that you may own the environmental liability, including if the oil gets into the groundwater or contaminates the adjacent properties.

However, the concern should be if the tank has impacted soil and groundwater. are they proposing sampling in connection with the decommissioning?