Is impossibility of performance a defense to a state mandate compelling counties to contribute to welfare grants?

In Board of Supervisors v. McMahon (1990) 219 Cal.App.3d 286, 268 Cal.Rptr. 219, the court discussed impossibility of performance as a defense to a state mandate compelling counties to contribute to welfare grants. The appeal followed a hearing in the trial court where testimony was given and factual findings were made. Although the court concluded the county had not demonstrated it was impossible to comply with the law, it acknowledged that, consistent with Civil Code section 3531, "the law recognizes exceptions to statutory requirements for impossibility of performance." (Board of Supervisors v. McMahon , supra , at p. 300, 268 Cal.Rptr. 219.)