The 11,500 uninsured people in Marin will be eligible to enroll in California’s new health insurance exchange on Oct. 1 as a result of the Affordable Care Act, and some may be able to obtain free insurance through the expansion of Medi-Cal. Exchange enrollees can choose from four plans with varying levels of coverage, which will take effect at the beginning of next year. Marin residents can apply at coveredca.com, by calling (800) 300.1506 or…