If you have an agent representing you, he or she should be able to give you advice. The advice will depend upon a lot of factors and what kind of contingencies you have (did you buy it As-is, did you have a home inspection etc?) You could try to get out someone like Mid-Atlantic (http://www.basements.com/) or someone that are experts on basements to get you an estimate for what it will cost to deal with the issue. You may or may not then be able to have the seller give credit and/or put money in escrow. They may or may not have any obligation to do that based on the contract though.
We have had a lot of rains lately so you'll have to figure out whether this is a one time thing , something that has happened in the past and/or is likely to happen again in the future.