Rhode Island’s substance abuse treatment system could be overwhelmed next year after tens of thousands of people with drug and alcohol problems become eligible for health insurance under the federal health overhaul and a state Medicaid expansion. Steven Costantino, the state’s health and human services secretary, said Wednesday that he worries whether the system can absorb the potential flood of patients. Costantino said he hopes treatment providers can expand their services. Federal statistics show that 17,600 people with substance abuse problems in Rhode Island would be newly eligible for insurance under the federal health care law. Another 29,000 would be newly eligible under the Medicaid expansion. The state’s treatment beds have a 95 percent occupancy rate.