In just five months, California will start to enroll millions of previously uninsured residents under the Affordable Care Act. Much uncertainty remains as the 58 counties of the most populous state in the nation take on the monumental task of reaching out to a new population.

As a county supervisor and a member of the California State Association of Counties Executive Committee, I see firsthand how this historic law will impact our whole community: working families, business owners and the uninsured.

By January 2014, uninsured residents must enroll in an expanded Medi-Cal program or purchase private insurance through California's new insurance marketplace, "Covered California." Nonprofit and for-profit health systems will compete for these new customers, who come with added revenue.

According to the California Healthcare Foundation, 7.1 million Californians under age 65 were uninsured in 2011.

Even with the expansion of coverage, 3 million to 4 million Californians will remain uninsured in 2019, according to a study by UC Berkeley and UCLA. If they get sick, these individuals will utilize services at public institutions like Highland Hospital, which served 82,060 emergency room patients in 2011-12.

Gov. Jerry Brown's May revise of his state budget proposal outlines the framework for a statewide implementation of the expansion of health insurance coverage, an approach I support.

But the governor erroneously claims that counties will realize savings as populations previously under our care transition onto Medi-Cal or private insurance. Pointing to the $1.5 billion annually that the state currently passes on to counties for health care, Gov. Brown says the state should not "pay for the same services twice."

Instead, he proposes that counties use the supposed savings to pay for health and human services programs that are now run by the state, including CalWorks, CalWorks-related child care programs and CalFresh (formerly Food Stamps).

This proposed "realignment" of human service programs was criticized by the nonpartisan Legislative Analyst's Office for adding "complexity to the already complicated issue of implementing" the expansion of coverage.

The gaping hole in the governor's argument is that the federal government will provide 100 percent of the funding for Medi-Cal expansion for the first three years of the program. It is actually the state that will be paid twice.

Under realignment, county health departments statewide would lose an estimated $300 million in 2013-14, $900 million in 2014-15 and $1.3 billion in 2016-17. On the chopping block would be public health prevention programs, preventive care, mental health services, and care for chronic conditions like asthma and diabetes.

It is difficult for Alameda County to predict any actual savings. We do not know how many people will enroll and stay in Medi-Cal, or how many people will buy insurance through Covered California, which will be too expensive for some.

Alameda County has already enrolled 90,000 people in HealthPAC, our program for those who will be eligible for Medi-Cal or Covered California in 2014. Despite those efforts, at least 100,000 people in our county are projected to remain uninsured in 2014.

Behind the numbers are undocumented residents, Medi-Cal eligible individuals who will miss enrollment, and those who can't afford private insurance. People of color and individuals with language barriers will be disproportionately affected.

County hospitals and clinics constitute the safety net, the place of last resort for uninsured patients most in need. As California prepares to implement the ACA, the state should not reduce funding to counties or realign programs until after the three years of full federal funding. In this time of uncertainty, the safety net must remain unharmed so counties can keep our doors open to all who need care.

Keith Carson is an Alameda County supervisor.