What is the statute of limitations on contract law?

My father was the contracted operator of a county contracted landfill. At the end of the contract period before the sight in question closed, he signed an agreed order saying he understood how to close the site. He proceeded to do the work necessary toward that end but soon realized that the site was not suited and continually eroded. Two years after his contract and his pay checks from the contracted landfill ended, he was ordered to court by the agreed order and forced into a Consent decree, finally ordering his complete responsibility for future care and maintenance of the site. Now 34 years later the environmental management agency of our state is forcing my 85 year old mother to prove she cannot afford to care for it, which will result in the county taking her property which has been in her family for 6 generations. If they don't actually take it from her they may offer her pennies on the dollar for it. Do you have any answers for this?